Downloaded 35 times
![9
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Error-Correcting Compilers
•!Issue an error message
•!Fix the problem
•!Produce an executable
Example
Error on line 23: “myVarr” undefined.
“myVar” was used.
Is this a good idea???
Compiler guesses the programmer’s intent
A shifting notion of what constitutes a correct / legal / valid program
May encourage programmers to get sloppy
Declarations provide redundancy
! Increased reliability
10
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Error Avalanche
One error generates a cascade of messages
Example
x := 5 while ( a == b ) do
^
Expecting ;
^
Expecting ;
^
Expecting ;
The real messages may be buried under the avalanche.
Missing #include or import will also cause an avalanche.
Approaches:
Only print 1 message per token [ or per line of source ]
Only print a particular message once
Error: Variable “myVarr” is undeclared
All future notices for this ID have been suppressed
Abort the compiler after 50 errors.](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-5-2048.jpg)
![97
Syntax Analysis - Part 1
© Harry H. Porter, 2005
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
Nonterminals
Input Symbols, plus $
Input String
( a * 4 ) 3 $
Algorithm Output
+
Y
Z
$
X
Stack of
grammar symbols
Blank entries
indicate ERRORPre-Computed Table:
Table-Driven Predictive Parsing Algorithm
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
98
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Predictive Parsing Algorithm
Set input ptr to first symbol; Place $ after last input symbol
Push $
Push S
repeat
X = stack top
a = current input symbol
if X is a terminal or X = $ then
if X == a then
Pop stack
Advance input ptr
else
Error
endIf
elseIf Table[X,a] contains a rule then // call it X " Y1 Y2 ... YK
Pop stack
Push YK
...
Push Y2
Push Y1
Print (“X " Y1 Y2 ... YK”)
else // Table[X,a] is blank
Syntax Error
endIf
until X == $
X
A
$
YK
A
$
...
Y2
Y1](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-49-2048.jpg)
![101
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
Add $ to end of input
Push $
Push E
E
102
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
Look at Table [ E, ‘(’ ]
Use rule E"TE'
Pop E
Push E'
Push T
Print E"TE'
E](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-51-2048.jpg)
![103
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T
Look at Table [ E, ‘(’ ]
Use rule E"TE'
Pop E
Push E'
Push T
Print E"TE'
104
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
Table [ T, ‘(’ ] = T"FT'
Pop T
Push T’
Push F
Print T"FT'
E’
T](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-52-2048.jpg)
![105
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
F Table [ T, ‘(’ ] = T"FT'
Pop T
Push T’
Push F
Print T"FT'
106
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
Table [ F, ‘(’ ] = F"(E)
Pop F
Push (
Push E
Push )
Print F"(E)
E’
T’
F](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-53-2048.jpg)
![107
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E
(
Table [ F, ‘(’ ] = F"(E)
Pop F
Push )
Push E
Push (
Print F"(E)
108
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E
(
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-54-2048.jpg)
![109
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E Top of Stack matches next input
Pop and Scan
110
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E Table [ E, id ] = E"TE'
Pop E
Push E'
Push T
Print E"TE'](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-55-2048.jpg)
![111
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T
Table [ E, id ] = E"TE'
Pop E
Push E'
Push T
Print E"TE'
112
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T
Table [ T, id ] = T"FT'
Pop T
Push T'
Push F
Print T"FT'](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-56-2048.jpg)
![113
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
F
Table [ T, id ] = T"FT'
Pop T
Push T'
Push F
Print T"FT'
114
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
F
Table [ F, id ] = F"id
Pop F
Push id
Print F"id](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-57-2048.jpg)
![115
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
id
Table [ F, id ] = F"id
Pop F
Push id
Print F"id
116
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
id
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-58-2048.jpg)
![117
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
Top of Stack matches next input
Pop and Scan
118
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
Table [ T', ‘*’ ] = T'"*FT'
Pop T'
Push T'
Push F
Push ‘*’
Print T'"*FT'](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-59-2048.jpg)
![119
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
Table [ T', ‘*’ ] = T'"*FT'
Pop T'
Push T'
Push F
Push ‘*’
Print T'"*FT'
F
*
120
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
F
*
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-60-2048.jpg)
![121
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
F
Top of Stack matches next input
Pop and Scan
122
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
F
Table [ F, id ] = F"id
Pop F
Push id
Print F"id](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-61-2048.jpg)
![123
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
id
Table [ F, id ] = F"id
Pop F
Push id
Print F"id
124
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
id
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-62-2048.jpg)
![125
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
Top of Stack matches next input
Pop and Scan
126
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’
T’
Table [ T', ‘)’ ] = T'"&
Pop T'
Push <nothing>
Print T'"&](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-63-2048.jpg)
![127
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’ Table [ T', ‘)’ ] = T'"&
Pop T'
Push <nothing>
Print T'"&
128
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
E’ Table [ E', ‘)’ ] = E'"&
Pop E'
Push <nothing>
Print E'"&](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-64-2048.jpg)
![129
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
Table [ E', ‘)’ ] = E'"&
Pop E'
Push <nothing>
Print E'"&
130
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
)
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-65-2048.jpg)
![131
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Top of Stack matches next input
Pop and Scan
132
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Table [ T', ‘+’ ] = T'"&
Pop T'
Push <nothing>
Print T'"&](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-66-2048.jpg)
![133
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
Table [ T', ‘+’ ] = T'"&
Pop T'
Push <nothing>
Print T'"&
134
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
Table [ E', ‘+’ ] = E'"+TE'
Pop E'
Push E'
Push T
Push ‘+’
Print E'"+TE'](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-67-2048.jpg)
![135
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
Table [ E', ‘+’ ] = E'"+TE'
Pop E'
Push E'
Push T
Push ‘+’
Print E'"+TE'
T
+
136
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T
+
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-68-2048.jpg)
![137
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T
Top of Stack matches next input
Pop and Scan
138
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T
Table [ T, id ] = T"FT'
Pop T
Push T'
Push F
Print T"FT'](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-69-2048.jpg)
![139
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Table [ T, id ] = T"FT'
Pop T
Push T'
Push F
Print T"FT'
F
140
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Table [ F, id ] = F"id
Pop F
Push id
Print F"id
F](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-70-2048.jpg)
![141
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Table [ F, id ] = F"id
Pop F
Push id
Print F"id
id
142
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
id
Top of Stack matches next input
Pop and Scan](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-71-2048.jpg)
![143
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Top of Stack matches next input
Pop and Scan
144
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
T’
Table [ T', $ ] = T'"&
Pop T'
Push <nothing>
Print T'"&](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-72-2048.jpg)
![145
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
T' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
Table [ T', $ ] = T'"&
Pop T'
Push <nothing>
Print T'"&
146
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
T' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
E’
Table [ E', $ ] = E'"&
Pop E'
Push <nothing>
Print E'"&](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-73-2048.jpg)
![147
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
T' " &
E' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
Table [ E', $ ] = E'"&
Pop E'
Push <nothing>
Print E'"&
148
Syntax Analysis - Part 1
© Harry H. Porter, 2005
ExampleInput:
(id*id)+id
Output:
E " T E'
T " F T'
F " ( E )
E " T E'
T " F T'
F " id
T' " * F T'
F " id
T' " &
E' " &
T' " &
E' " + T E'
T " F T'
F " id
T' " &
E' " &
E " T E'
E' " + T E' | &
T " F T'
T' " * F T' | &
F " ( E ) | id
T"FT'T"FT'T
T'"&T'"&T'"*FT'T'"&T'
*
F"(E)
E"TE'
(
E'"&
)
F"idF
E'"&
$+id
E'"+TE'E'
E"TE'E
( id * id ) id $+
$
Input symbol == $
Top of stack == $
Loop terminates with success](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-74-2048.jpg)
![201
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Building the Predictive Parsing Table
The Main Idea:
Assume we’re looking for an A
i.e., A is on the stack top.
Assume b is the current input symbol.
If A"# is a rule and b is in FIRST(#)
then expand A using the A"# rule!
What if & is in FIRST(#)? [i.e., # !* & ]
If b is in FOLLOW(A)
then expand A using the A"# rule!
202
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Building the Predictive Parsing Table
The Main Idea:
Assume we’re looking for an A
i.e., A is on the stack top.
Assume b is the current input symbol.
If A"# is a rule and b is in FIRST(#)
then expand A using the A"# rule!
What if & is in FIRST(#)? [i.e., # !* & ]
If b is in FOLLOW(A)
then expand A using the A"# rule!
If & is in FIRST(#) and $ is the current input symbol
then if $ is in FOLLOW(A)
then expand A using the A"# rule!](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-101-2048.jpg)
![221
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
222
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
Compute FIRST and FOLLOW sets](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-111-2048.jpg)
![223
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
Compute FIRST and FOLLOW sets
for each rule A"# do
for each terminal b in FIRST(#) do
add A"# to TABLE[A,b]
endFor
endFor
224
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
Compute FIRST and FOLLOW sets
for each rule A"# do
for each terminal b in FIRST(#) do
add A"# to TABLE[A,b]
endFor
if & is in FIRST(#) then
for each terminal b in FOLLOW(A) do
add A"# to TABLE[A,b]
endFor
endIf
endFor](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-112-2048.jpg)
![225
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
Compute FIRST and FOLLOW sets
for each rule A"# do
for each terminal b in FIRST(#) do
add A"# to TABLE[A,b]
endFor
if & is in FIRST(#) then
for each terminal b in FOLLOW(A) do
add A"# to TABLE[A,b]
endFor
if $ is in FOLLOW(A) then
add A"# to TABLE[A,$]
endIf
endIf
endFor
226
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
Compute FIRST and FOLLOW sets
for each rule A"# do
for each terminal b in FIRST(#) do
add A"# to TABLE[A,b]
endFor
if & is in FIRST(#) then
for each terminal b in FOLLOW(A) do
add A"# to TABLE[A,b]
endFor
if $ is in FOLLOW(A) then
add A"# to TABLE[A,$]
endIf
endIf
endFor
TABLE[A,b] is undefined? Then set TABLE[A,b] to “error”](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-113-2048.jpg)
![227
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Algorithm to Build the Table
Input: Grammar G
Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank”
Compute FIRST and FOLLOW sets
for each rule A"# do
for each terminal b in FIRST(#) do
add A"# to TABLE[A,b]
endFor
if & is in FIRST(#) then
for each terminal b in FOLLOW(A) do
add A"# to TABLE[A,b]
endFor
if $ is in FOLLOW(A) then
add A"# to TABLE[A,$]
endIf
endIf
endFor
TABLE[A,b] is undefined? Then set TABLE[A,b] to “error”
TABLE[A,b] is multiply defined?
The algorithm fails!!! Grammar G is not LL(1)!!!
228
Syntax Analysis - Part 1
© Harry H. Porter, 2005
LL(1) Grammars
LL(1) grammars
• Are never ambiguous.
•! Will never have left recursion.
Furthermore...
If we are looking for an “A” and the next symbol is “b”,
Then only one production must be possible.
More Precisely...
If A"# and A"$ are two rules
If # !* a... and $ !* b...
then we require a - b
(i.e., FIRST(#) and FIRST($) must not intersect)
If # !* &
then $ !* & must not be possible.
(i.e., only one alternative can derive &.)
If # !* & and $ !* b...
then b must not be in FOLLOW(A)
Scanning input left-to-right
Find Leftmost derivation
Using only one symbol of look-ahead](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-114-2048.jpg)
![229
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Error Recovery
We have an error whenever...
•!Stacktop is a terminal, but stacktop - input symbol
•!Stacktop is a nonterminal but TABLE[A,b] is empty
Options
1. Skip over input symbols, until we can resume parsing
Corresponds to ignoring tokens
2. Pop stack, until we can resume parsing
Corresponds to inserting missing material
3. Some combination of 1 and 2
4. “Panic Mode” - Use Synchronizing tokens
•!Identify a set of synchronizing tokens.
•!Skip over tokens until we are positioned on a synchronizing token.
•!Pop stack until we can resume parsing.
230
Syntax Analysis - Part 1
© Harry H. Porter, 2005
Option 1: Skip Input Symbols
Example:
Decided to use rule
S " IF E THEN S ELSE S END
Stack tells us what we are expecting next in the input.
We’ve already gotten IF and E
Assume there are extra tokens in the input.
if (x<5))) then y = 7; ...
A syntax error occurs here.
< 5 ) ) ) y =TH
$
...
END
S
ELSE
S
THEN
We want to skip tokens until
we can resume parsing.
... ...](https://image.slidesharecdn.com/syntaxpart1-130804111658-phpapp02/75/Syntax-part1-115-2048.jpg)
Uploaded byYophi Gimbal
Syntax part1
This document discusses syntax analysis and parsing techniques for programming languages. It covers: - Top-down and bottom-up parsing using techniques like recursive descent, table-driven, and LR parsing. - Grammar issues like ambiguity and left-recursion that must be addressed for parsing programming languages. - Error handling challenges for parsers including producing clear error messages, recovering from errors, and avoiding cascading error messages. - Representing syntax with context-free grammars and translating between regular expressions, NFAs/DFAs, and grammars.
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Syntax part1
- 1. 1 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Outline Context-Free Grammars (CFGs) Parsing Top-Down Recursive Descent Table-Driven Bottom-Up LR Parsing Algorithm How to Build LR Tables Parser Generators Grammar Issues for Programming Languages Syntax Analysis 2 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Top-Down Parsing •!LL Grammars - A subclass of all CFGs •!Recursive-Descent Parsers - Programmed “by hand” •!Non-Recursive Predictive Parsers - Table Driven •!Simple, Easy to Build, Better Error Handling Bottom-Up Parsing •!LR Grammars - A larger subclass of CFGs •!Complex Parsing Algorithms - Table Driven •!Table Construction Techniques •!Parser Generators use this technique •!Faster Parsing, Larger Set of Grammars •!Complex •!Error Reporting is Tricky
- 2. 3 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Output of Parser? Succeed is string is recognized ... and fail if syntax errors Syntax Errors? Good, descriptive, helpful message! Recover and continue parsing! Build a “Parse Tree” (also called “derivation tree”) Build Abstract Syntax Tree (AST) In memory (with objects, pointers) Output to a file Execute Semantic Actions Build AST Type Checking Generate Code Don’t build a tree at all! * y 1 x + 4 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Errors in Programs Lexical if x<1 thenn y = 5: “Typos” Syntactic if ((x<1) & (y>5))) ... { ... { ... ... } Semantic if (x+5) then ... Type Errors Undefined IDs, etc. Logical Errors if (i<9) then ... Should be <= not < Bugs Compiler cannot detect Logical Errors
- 3. 5 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Compiler Always halts Any checks guaranteed to terminate “Decidable” Other Program Checking Techniques Debugging Testing Correctness Proofs “Partially Decidable” Okay? ! The test terminates. Not Okay? ! The test may not terminate! You may need to run some programs to see if they are okay. 6 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Requirements Detect All Errors (Except Logical!) Messages should be helpful. Difficult to produce clear messages! Example: Syntax Error Example: Line 23: Unmatched Paren if ((x == 1) then ^ Compiler Should Recover Keep going to find more errors Example: x := (a + 5)) * (b + 7)); We’re in the middle of a statement Skip tokens until we see a “;” Resume Parsing Misses a second error... Oh, well... Checks most of the source Error detected here This error missed
- 4. 7 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Difficult to generate clear and accurate error messages. Example function foo () { ... if (...) { ... } else { ... ... } <eof> Example var myVarr: Integer; ... x := myVar; ... Missing } here Not detected until here Misspelled ID here Detected here as “Undeclared ID” 8 Syntax Analysis - Part 1 © Harry H. Porter, 2005 For Mature Languages Catalog common errors Statistical studies Tailor compiler to handle common errors well Statement terminators versus separators Terminators: C, Java, PCAT {A;B;C;} Separators: Pascal, Smalltalk, Haskell Pascal Examples begin var t: Integer; t := x; x := y; y := t end if (...) then x := 1 else y := 2; z := 3; function foo (x: Integer; y: Integer)... Tend to put a comma here Tend to insert a ; here Tend to insert a ; here
- 5. 9 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Error-Correcting Compilers •!Issue an error message •!Fix the problem •!Produce an executable Example Error on line 23: “myVarr” undefined. “myVar” was used. Is this a good idea??? Compiler guesses the programmer’s intent A shifting notion of what constitutes a correct / legal / valid program May encourage programmers to get sloppy Declarations provide redundancy ! Increased reliability 10 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Error Avalanche One error generates a cascade of messages Example x := 5 while ( a == b ) do ^ Expecting ; ^ Expecting ; ^ Expecting ; The real messages may be buried under the avalanche. Missing #include or import will also cause an avalanche. Approaches: Only print 1 message per token [ or per line of source ] Only print a particular message once Error: Variable “myVarr” is undeclared All future notices for this ID have been suppressed Abort the compiler after 50 errors.
- 6. 11 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Error Recovery Approaches: Panic Mode Discard tokens until we see a “synchronizing” token. •!Simple to implement •!Commonly used •!The key... Good set of synchronizing tokens Knowing what to do then •!May skip over large sections of source Example Skip to next occurrence of } end ; Resume by parsing the next statement 12 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Error Recovery Approaches: Phrase-Level Recovery Compiler corrects the program by deleting or inserting tokens ...so it can proceed to parse from where it was. •!The key... Don’t get into an infinite loop ...constantly inserting tokens ...and never scanning the actual source Example while (x = 4) y := a+b; ... Insert do to “fix” the statement.
- 7. 13 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Error Recovery Approaches: Error Productions Augment the CFG with “Error Productions” Now the CFG accepts anything! If “error productions” are used... Their actions: { print (“Error...”) } Used with... • LR (Bottom-up) parsing •!Parser Generators Theoretical Approach Find the minimum change to the source to yield a valid program (Insert tokens, delete tokens, swap adjacent tokens) Impractical algorithms - too time consuming Error Recovery Approaches: Global Correction 14 Syntax Analysis - Part 1 © Harry H. Porter, 2005 CFG: Context Free Grammars Example Rule: Stmt " if Expr then Stmt else Stmt Terminals Keywords else “else” Token Classes ID INTEGER REAL Punctuation ; “;” ; Non-terminals Any symbol appearing on the lefthand side of any rule Start Symbol Usually the non-terminal on the lefthand side of the first rule Rules (or “Productions”) BNF: Backus-Naur Form / Backus-Normal Form Stmt ::= if Expr then Stmt else Stmt
- 8. 15 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Rule Alternatives E " E + E E " ( E ) E " - E E " ID E " E + E " ( E ) " - E " ID E " E + E | ( E ) | - E | ID E " E + E | ( E ) | - E | ID All Notations are Equivalent 16 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Notational Conventions Terminals a b c ... Nonterminals A B C ... S Expr Grammar Symbols (Terminals or Nonterminals) X Y Z U V W ... Strings of Symbols # $ % ... Strings of Terminals x y z u v w ... Examples A " # B A rule whose righthand side ends with a nonterminal A " x # A rule whose righthand side begins with a string of terminals (call it “x”) A sequence of zero Or more terminals And nonterminals Including &
- 9. 17 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Derivations A “Derivation” of “(id*id)” E ! (E) ! (E*E) ! (id*E) ! (id*id) “Sentential Forms” A sequence of terminals and nonterminals in a derivation (id*E) 1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID 18 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Derives in one step ! If A " $ is a rule, then we can write #A% ! #$% Any sentential form containing a nonterminal (call it A) ... such that A matches the nonterminal in some rule. Derives in zero-or-more steps !* E !* (id*id) If # !* $ and $ ! %, then # !* % Derives in one-or-more steps !+
- 10. 19 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Given G A grammar S The Start Symbol Define L(G) The language generated L(G) = { w | S !+ w} “Equivalence” of CFG’s If two CFG’s generate the same language, we say they are “equivalent.” G1 ' G2 whenever L(G1) = L(G2) In making a derivation... Choose which nonterminal to expand Choose which rule to apply 20 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Leftmost Derivations In a derivation... always expand the leftmost nonterminal. E ! E+E ! (E)+E ! (E*E)+E ! (id*E)+E ! (id*id)+E ! (id*id)+id Let !LM denote a step in a leftmost derivation (!LM* means zero-or-more steps ) At each step in a leftmost derivation, we have wA% !LM w$% where A " $ is a rule (Recall that w is a string of terminals.) Each sentential form in a leftmost derivation is called a “left-sentential form.” If S !LM* # then we say # is a “left-sentential form.” 1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID
- 11. 21 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Rightmost Derivations In a derivation... always expand the rightmost nonterminal. E ! E+E ! E+id ! (E)+id ! (E*E)+id ! (E*id)+id ! (id*id)+id Let !RM denote a step in a rightmost derivation (!RM* means zero-or-more steps ) At each step in a rightmost derivation, we have #Aw !RM #$w where A " $ is a rule (Recall that w is a string of terminals.) Each sentential form in a rightmost derivation is called a “right-sentential form.” If S !RM* # then we say # is a “right-sentential form.” 1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID 22 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Bottom-Up Parsing Bottom-up parsers discover rightmost derivations! Parser moves from input string back to S. Follow S !RM* w in reverse. At each step in a rightmost derivation, we have #Aw !RM #$w where A " $ is a rule String of terminals (i.e., the rest of the input, which we have not yet seen)
- 12. 23 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Parse Trees Two choices at each step in a derivation... •!Which non-terminal to expand •!Which rule to use in replacing it The parse tree remembers only this Leftmost Derivation: E ! E+E ! (E)+E ! (E*E)+E ! (id*E)+E ! (id*id)+E ! (id*id)+id E E EE EE id id id + * )(1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID 24 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Parse Trees Two choices at each step in a derivation... •!Which non-terminal to expand •!Which rule to use in replacing it The parse tree remembers only this E E EE EE id id id + * )( Rightmost Derivation: E ! E+E ! E+id ! (E)+id ! (E*E)+id ! (E*id)+id ! (id*id)+id 1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID
- 13. 25 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Parse Trees Two choices at each step in a derivation... •!Which non-terminal to expand •!Which rule to use in replacing it The parse tree remembers only this Leftmost Derivation: E ! E+E ! (E)+E ! (E*E)+E ! (id*E)+E ! (id*id)+E ! (id*id)+id Rightmost Derivation: E ! E+E ! E+id ! (E)+id ! (E*E)+id ! (E*id)+id ! (id*id)+id E E EE EE id id id + * )(1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID 26 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Given a leftmost derivation, we can build a parse tree. Given a rightmost derivation, we can build a parse tree. Every parse tree corresponds to... • A single, unique leftmost derivation • A single, unique rightmost derivation Ambiguity: However, one input string may have several parse trees!!! Therefore: •!Several leftmost derivations •!Several rightmost derivations Rightmost Derivation of (id*id)+id Lefttmost Derivation of (id*id)+id Same Parse Tree
- 14. 27 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Ambuiguous Grammars Leftmost Derivation #1 E ! E+E ! id+E ! id+E*E ! id+id*E ! id+id*id Leftmost Derivation #2 E ! E*E ! E+E*E ! id+E*E ! id+id*E ! id+id*id 1. E " E + E 2. " E * E 3. " ( E ) 4. " - E 5. " ID E EE EE id id id * + E EE E E id id id + * Input: id+id*id 28 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Ambiguous Grammar More than one Parse Tree for some sentence. The grammar for a programming language may be ambiguous Need to modify it for parsing. Also: Grammar may be left recursive. Need to modify it for parsing.
- 15. 29 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Translating a Regular Expression into a CFG First build the NFA. For every state in the NFA... Make a nonterminal in the grammar For every edge labeled c from A to B... Add the rule A " cB For every edge labeled & from A to B... Add the rule A " B For every final state B... Add the rule B " & 30 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Recursive Transition Networks Regular Expressions ( NFA ( DFA Context-Free Grammar ( Recursive Transition Networks Exactly as expressive a CFGs... But clearer for humans! ; elseIf if then else end then Expr Expr Stmt Stmt Stmt IfStmt Stmt Expr ... ... ... ... Terminal Symbols: Nonterminal Symbols:
- 16. 31 Syntax Analysis -Part 1 © Harry H. Porter, 2005 The Dangling “Else” Problem This grammar is ambiguous! Stmt " if Expr then Stmt " if Expr then Stmt else Stmt " ...Other Stmt Forms... Example String: if E1 then if E2 then S1 else S2 Interpretation #1: if E1 then (if E2 then S1) else S2 Interpretation #2: if E1 then (if E2 then S1 else S2) if then S2elseE1 S if thenE2 S1 S if then S2elseE2 S if thenE1 S1 S 32 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Goal: “Match else-clause to the closest if without an else-clause already.” Solution: Stmt " if Expr then Stmt " if Expr then WithElse else Stmt " ...Other Stmt Forms... WithElse " if Expr then WithElse else WithElse " ...Other Stmt Forms... Any Stmt occurring between then and else must have an else. i.e., the Stmt must not end with “then Stmt”. Interpretation #2: if E1 then (if E2 then S1 else S2) if then WithElseelseE2 Stmt if thenE1 WithElse Stmt S1 S2 The Dangling “Else” Problem
- 17. 33 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Goal: “Match else-clause to the closest if without an else-clause already.” Solution: Stmt " if Expr then Stmt " if Expr then WithElse else Stmt " ...Other Stmt Forms... WithElse " if Expr then WithElse else WithElse " ...Other Stmt Forms... Any Stmt occurring between then and else must have an else. i.e., the Stmt must not end with “then Stmt”. Interpretation #1: if E1 then (if E2 then S1) else S2 Stmt if thenE1 Stmt The Dangling “Else” Problem elseWithElse 34 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Goal: “Match else-clause to the closest if without an else-clause already.” Solution: Stmt " if Expr then Stmt " if Expr then WithElse else Stmt " ...Other Stmt Forms... WithElse " if Expr then WithElse else WithElse " ...Other Stmt Forms... Any Stmt occurring between then and else must have an else. i.e., the Stmt must not end with “then Stmt”. Interpretation #1: if E1 then (if E2 then S1) else S2 if then WithElseelseE2 Stmt if thenE1 WithElse Stmt The Dangling “Else” Problem elseWithElse
- 18. 35 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Goal: “Match else-clause to the closest if without an else-clause already.” Solution: Stmt " if Expr then Stmt " if Expr then WithElse else Stmt " ...Other Stmt Forms... WithElse " if Expr then WithElse else WithElse " ...Other Stmt Forms... Any Stmt occurring between then and else must have an else. i.e., the Stmt must not end with “then Stmt”. Interpretation #1: if E1 then (if E2 then S1) else S2 if then WithElseelseE2 Stmt if thenE1 WithElse Stmt The Dangling “Else” Problem elseWithElse Oops, No Match! 36 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Top-Down Parsing Find a left-most derivation Find (build) a parse tree Start building from the root and work down... As we search for a derivation... Must make choices: •!Which rule to use •!Where to use it May run into problems! Option 1: “Backtracking” Made a bad decision Back up and try another choice Option 2: Always make the right choice. Never have to backtrack: “Predictive Parser” Possible for some grammars (LL Grammars) May be able to fix some grammars (but not others) •!Eliminate Left Recursion •!Left-Factor the Rules
- 19. 37 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S 38 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a
- 20. 39 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a Ba a 40 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a Ba a
- 21. 41 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a Ba a 42 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a B b a a bb
- 22. 43 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a B b a a bb 44 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a B b a a bb
- 23. 45 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a B b a a bb Failure Occurs Here!!! 46 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a Ba a We need an ability to back up in the input!!!
- 24. 47 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a 48 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a a a b a
- 25. 49 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a a a b a 50 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a a a b a
- 26. 51 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a a a b a 52 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a a a b a Failure Occurs Here!!!
- 27. 53 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S A a 54 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S
- 28. 55 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S C e 56 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S C e a a D
- 29. 57 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S C e a b a D b d 58 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Backtracking Input: aabbde 1. S " Aa 2. " Ce 3. A " aaB 4. " aaba 5. B " bbb 6. C " aaD 7. D " bbd S C e a b a D b d
- 30. 59 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Predictive Parsing Will never backtrack! Requirement: For every rule: A " #1 | #2 | #3 | ... | #N We must be able to choose the correct alternative by looking only at the next symbol May peek ahead to the next symbol (token). Example A " aB " cD " E Assuming a,c ) FIRST (E) Example Stmt " if Expr ... " for LValue ... " while Expr ... " return Expr ... " ID ... 60 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Predictive Parsing LL(1) Grammars Can do predictive parsing Can select the right rule Looking at only the next 1 input symbol
- 31. 61 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Predictive Parsing LL(1) Grammars Can do predictive parsing Can select the right rule Looking at only the next 1 input symbol LL(k) Grammars Can do predictive parsing Can select the right rule Looking at only the next k input symbols 62 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Predictive Parsing LL(1) Grammars Can do predictive parsing Can select the right rule Looking at only the next 1 input symbol LL(k) Grammars Can do predictive parsing Can select the right rule Looking at only the next k input symbols Techniques to modify the grammar: •!Left Factoring •!Removal of Left Recursion
- 32. 63 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Predictive Parsing LL(1) Grammars Can do predictive parsing Can select the right rule Looking at only the next 1 input symbol LL(k) Grammars Can do predictive parsing Can select the right rule Looking at only the next k input symbols Techniques to modify the grammar: •!Left Factoring •!Removal of Left Recursion But these may not be enough! 64 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Predictive Parsing LL(1) Grammars Can do predictive parsing Can select the right rule Looking at only the next 1 input symbol LL(k) Grammars Can do predictive parsing Can select the right rule Looking at only the next k input symbols Techniques to modify the grammar: •!Left Factoring •!Removal of Left Recursion But these may not be enough! LL(k) Language Can be described with an LL(k) grammar.
- 33. 65 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left-Factoring Problem: Stmt " if Expr then Stmt else Stmt " if Expr then Stmt " OtherStmt With predictive parsing, we need to know which rule to use! (While looking at just the next token) 66 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left-Factoring Problem: Stmt " if Expr then Stmt else Stmt " if Expr then Stmt " OtherStmt With predictive parsing, we need to know which rule to use! (While looking at just the next token) Solution: Stmt " if Expr then Stmt ElsePart " OtherStmt ElsePart " else Stmt | &
- 34. 67 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left-Factoring Problem: Stmt " if Expr then Stmt else Stmt " if Expr then Stmt " OtherStmt With predictive parsing, we need to know which rule to use! (While looking at just the next token) Solution: Stmt " if Expr then Stmt ElsePart " OtherStmt ElsePart " else Stmt | & General Approach: Before: A " #$1 | #$2 | #$3 | ... | *1 | *2 | *3 | ... After: A " #C | *1 | *2 | *3 | ... C " $1 | $2 | $3 | ... 68 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left-Factoring Problem: Stmt " if Expr then Stmt else Stmt " if Expr then Stmt & " OtherStmt With predictive parsing, we need to know which rule to use! (While looking at just the next token) Solution: Stmt " if Expr then Stmt ElsePart " OtherStmt ElsePart " else Stmt | & General Approach: Before: A " #$1 | #$2 | #$3 | ... | *1 | *2 | *3 | ... After: A " #C | *1 | *2 | *3 | ... C " $1 | $2 | $3 | ... # # $1 $2 *1 A
- 35. 69 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left-Factoring Problem: Stmt " if Expr then Stmt else Stmt " if Expr then Stmt & " OtherStmt With predictive parsing, we need to know which rule to use! (While looking at just the next token) Solution: Stmt " if Expr then Stmt ElsePart " OtherStmt ElsePart " else Stmt | & General Approach: Before: A " #$1 | #$2 | #$3 | ... | *1 | *2 | *3 | ... After: A " #C | *1 | *2 | *3 | ... C " $1 | $2 | $3 | ... C *1 $1 # # # $1 $2 $2 *1 A A C 70 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion Whenever A !+ A# Simplest Case: Immediate Left Recursion Given: A " A# | $ Transform into: A " $A' A' " #A' | & where A' is a new nonterminal More General (but still immediate): A " A#1 | A#2 | A#3 | ... | $1 | $2 | $3 | ... Transform into: A " $1A' | $2A' | $3A' | ... A' " #1A' | #2A' | #3A' | ... | &
- 36. 71 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. 72 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive?
- 37. 73 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf 74 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf Approach: Look at the rules for S only (ignoring other rules)... No left recursion.
- 38. 75 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf Approach: Look at the rules for S only (ignoring other rules)... No left recursion. Look at the rules for A... 76 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf Approach: Look at the rules for S only (ignoring other rules)... No left recursion. Look at the rules for A... Do any of A’s rules start with S? Yes. A " Sd
- 39. 77 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf Approach: Look at the rules for S only (ignoring other rules)... No left recursion. Look at the rules for A... Do any of A’s rules start with S? Yes. A " Sd Get rid of the S. Substitute in the righthand sides of S. A " Afd | bd 78 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf Approach: Look at the rules for S only (ignoring other rules)... No left recursion. Look at the rules for A... Do any of A’s rules start with S? Yes. A " Sd Get rid of the S. Substitute in the righthand sides of S. A " Afd | bd The modified grammar: S " Af | b A " Ac | Afd | bd | e
- 40. 79 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step Example: S " Af | b A " Ac | Sd | e Is A left recursive? Yes. Is S left recursive? Yes, but not immediate left recursion. S ! Af ! Sdf Approach: Look at the rules for S only (ignoring other rules)... No left recursion. Look at the rules for A... Do any of A’s rules start with S? Yes. A " Sd Get rid of the S. Substitute in the righthand sides of S. A " Afd | bd The modified grammar: S " Af | b A " Ac | Afd | bd | e Now eliminate immediate left recursion involving A. S " Af | b A " bdA' | eA' A' " cA' | fdA' | & 80 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | e
- 41. 81 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k Assume there are still more nonterminals; Look at the next one... 82 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | &
- 42. 83 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " Ag | Sh | k Look at the B rules next; Does any righthand side start with “S”? 84 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " Ag | Afh | bh | k Substitute, using the rules for “S” Af... | b...
- 43. 85 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " Ag | Afh | bh | k Does any righthand side start with “A”? 86 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " Ag | Afh | bh | k Do this one first.
- 44. 87 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " bdA'g | BeA'g | Afh | bh | k Substitute, using the rules for “A” bdA'... | BeA'... 88 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " bdA'g | BeA'g | Afh | bh | k Do this one next.
- 45. 89 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " bdA'g | BeA'g | bdA'fh | BeA'fh | bh | k Substitute, using the rules for “A” bdA'... | BeA'... 90 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " bdA'g | BeA'g | bdA'fh | BeA'fh | bh | k Finally, eliminate any immediate Left recursion involving “B”
- 46. 91 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be B " Ag | Sh | k So Far: S " Af | b A " bdA' | BeA' A' " cA' | fdA' | & B " bdA'gB' | bdA'fhB' | bhB' | kB' B' " eA'gB' | eA'fhB' | & Finally, eliminate any immediate Left recursion involving “B” 92 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Left Recursion in More Than One Step The Original Grammar: S " Af | b A " Ac | Sd | Be | C B " Ag | Sh | k C " BkmA | AS | j So Far: S " Af | b A " bdA' | BeA' | CA' A' " cA' | fdA' | & B " bdA'gB' | bdA'fhB' | bhB' | kB' | CA'gB' | CA'fhB' B' " eA'gB' | eA'fhB' | & If there is another nonterminal, then do it next.
- 47. 93 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Algorithm to Eliminate Left Recursion Assume the nonterminals are ordered A1, A2, A3,... (In the example: S, A, B) for each nonterminal Ai (for i = 1 to N) do for each nonterminal Aj (for j = 1 to i-1) do Let Aj " $1 | $2 | $3 | ... | $N be all the rules for Aj if there is a rule of the form Ai " Aj# then replace it by Ai " $1# | $2# | $3# | ... | $N# endIf endFor Eliminate immediate left recursion among the Ai rules endFor A1 A2 A3 ... Aj ... A7 Ai Inner Loop Outer Loop 94 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Table-Driven Predictive Parsing Algorithm Assume that the grammar is LL(1) i.e., Backtracking will never be needed Always know which righthand side to choose (with one look-ahead) • !No Left Recursion •! Grammar is Left-Factored. Step 1: From grammar, construct table. Step 2: Use table to parse strings. Example E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Term ... +Term +Term + ... +Term * Factor * Factor * ... * Factor Factor ...
- 48. 95 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Table-Driven Predictive Parsing Algorithm Input String ( a * 4 ) 3 $ Algorithm Output + Y Z $ X Stack of grammar symbols Pre-Computed Table: E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id 96 Syntax Analysis - Part 1 © Harry H. Porter, 2005 T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E Nonterminals Input Symbols, plus $ Input String ( a * 4 ) 3 $ Algorithm Output + Y Z $ X Stack of grammar symbols Pre-Computed Table: Table-Driven Predictive Parsing Algorithm E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id
- 49. 97 Syntax Analysis -Part 1 © Harry H. Porter, 2005 T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E Nonterminals Input Symbols, plus $ Input String ( a * 4 ) 3 $ Algorithm Output + Y Z $ X Stack of grammar symbols Blank entries indicate ERRORPre-Computed Table: Table-Driven Predictive Parsing Algorithm E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id 98 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Predictive Parsing Algorithm Set input ptr to first symbol; Place $ after last input symbol Push $ Push S repeat X = stack top a = current input symbol if X is a terminal or X = $ then if X == a then Pop stack Advance input ptr else Error endIf elseIf Table[X,a] contains a rule then // call it X " Y1 Y2 ... YK Pop stack Push YK ... Push Y2 Push Y1 Print (“X " Y1 Y2 ... YK”) else // Table[X,a] is blank Syntax Error endIf until X == $ X A $ YK A $ ... Y2 Y1
- 50. 99 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id+ 100 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id+ Add $ to end of input Push $ Push E
- 51. 101 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ Add $ to end of input Push $ Push E E 102 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ Look at Table [ E, ‘(’ ] Use rule E"TE' Pop E Push E' Push T Print E"TE' E
- 52. 103 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T Look at Table [ E, ‘(’ ] Use rule E"TE' Pop E Push E' Push T Print E"TE' 104 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ Table [ T, ‘(’ ] = T"FT' Pop T Push T’ Push F Print T"FT' E’ T
- 53. 105 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ F Table [ T, ‘(’ ] = T"FT' Pop T Push T’ Push F Print T"FT' 106 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ Table [ F, ‘(’ ] = F"(E) Pop F Push ( Push E Push ) Print F"(E) E’ T’ F
- 54. 107 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E ( Table [ F, ‘(’ ] = F"(E) Pop F Push ) Push E Push ( Print F"(E) 108 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E ( Top of Stack matches next input Pop and Scan
- 55. 109 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E Top of Stack matches next input Pop and Scan 110 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E Table [ E, id ] = E"TE' Pop E Push E' Push T Print E"TE'
- 56. 111 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T Table [ E, id ] = E"TE' Pop E Push E' Push T Print E"TE' 112 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T Table [ T, id ] = T"FT' Pop T Push T' Push F Print T"FT'
- 57. 113 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ F Table [ T, id ] = T"FT' Pop T Push T' Push F Print T"FT' 114 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ F Table [ F, id ] = F"id Pop F Push id Print F"id
- 58. 115 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ id Table [ F, id ] = F"id Pop F Push id Print F"id 116 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ id Top of Stack matches next input Pop and Scan
- 59. 117 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ Top of Stack matches next input Pop and Scan 118 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ Table [ T', ‘*’ ] = T'"*FT' Pop T' Push T' Push F Push ‘*’ Print T'"*FT'
- 60. 119 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ Table [ T', ‘*’ ] = T'"*FT' Pop T' Push T' Push F Push ‘*’ Print T'"*FT' F * 120 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ F * Top of Stack matches next input Pop and Scan
- 61. 121 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ F Top of Stack matches next input Pop and Scan 122 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ F Table [ F, id ] = F"id Pop F Push id Print F"id
- 62. 123 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ id Table [ F, id ] = F"id Pop F Push id Print F"id 124 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ id Top of Stack matches next input Pop and Scan
- 63. 125 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ Top of Stack matches next input Pop and Scan 126 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ T’ Table [ T', ‘)’ ] = T'"& Pop T' Push <nothing> Print T'"&
- 64. 127 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ Table [ T', ‘)’ ] = T'"& Pop T' Push <nothing> Print T'"& 128 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) E’ Table [ E', ‘)’ ] = E'"& Pop E' Push <nothing> Print E'"&
- 65. 129 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) Table [ E', ‘)’ ] = E'"& Pop E' Push <nothing> Print E'"& 130 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ ) Top of Stack matches next input Pop and Scan
- 66. 131 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Top of Stack matches next input Pop and Scan 132 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Table [ T', ‘+’ ] = T'"& Pop T' Push <nothing> Print T'"&
- 67. 133 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ Table [ T', ‘+’ ] = T'"& Pop T' Push <nothing> Print T'"& 134 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ Table [ E', ‘+’ ] = E'"+TE' Pop E' Push E' Push T Push ‘+’ Print E'"+TE'
- 68. 135 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ Table [ E', ‘+’ ] = E'"+TE' Pop E' Push E' Push T Push ‘+’ Print E'"+TE' T + 136 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T + Top of Stack matches next input Pop and Scan
- 69. 137 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T Top of Stack matches next input Pop and Scan 138 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T Table [ T, id ] = T"FT' Pop T Push T' Push F Print T"FT'
- 70. 139 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Table [ T, id ] = T"FT' Pop T Push T' Push F Print T"FT' F 140 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Table [ F, id ] = F"id Pop F Push id Print F"id F
- 71. 141 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Table [ F, id ] = F"id Pop F Push id Print F"id id 142 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ id Top of Stack matches next input Pop and Scan
- 72. 143 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Top of Stack matches next input Pop and Scan 144 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ T’ Table [ T', $ ] = T'"& Pop T' Push <nothing> Print T'"&
- 73. 145 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ Table [ T', $ ] = T'"& Pop T' Push <nothing> Print T'"& 146 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ E’ Table [ E', $ ] = E'"& Pop E' Push <nothing> Print E'"&
- 74. 147 Syntax Analysis -Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ Table [ E', $ ] = E'"& Pop E' Push <nothing> Print E'"& 148 Syntax Analysis - Part 1 © Harry H. Porter, 2005 ExampleInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T"FT'T"FT'T T'"&T'"&T'"*FT'T'"&T' * F"(E) E"TE' ( E'"& ) F"idF E'"& $+id E'"+TE'E' E"TE'E ( id * id ) id $+ $ Input symbol == $ Top of stack == $ Loop terminates with success
- 75. 149 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T E E' 150 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T E E' T'F
- 76. 151 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T E E E' T'F )( 152 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T'F )(
- 77. 153 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' F F )( 154 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' F F id )(
- 78. 155 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' T' F F Fid * )( 156 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' T' F F Fid id * )(
- 79. 157 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' T' F F Fid id * )( & 158 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' T' F F Fid id * )( & &
- 80. 159 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T E E E' E' T' T' T' F F Fid id * )( & & & 160 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T T E E E' E' E'T' T' T' F F Fid id + * )( & & &
- 81. 161 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T T E E E' E' E'T' T' T' T' F F F F id id + * )( & & & 162 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T T E E E' E' E'T' T' T' T' F F F F id id id + * )( & & &
- 82. 163 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T T E E E' E' E'T' T' T' T' F F F F id id id + * )( & & & & 164 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id T T T E E E' E' E'T' T' T' T' F F F F id id id + * )( && & & &
- 83. 165 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Reconstructing the Parse TreeInput: (id*id)+id Output: E " T E' T " F T' F " ( E ) E " T E' T " F T' F " id T' " * F T' F " id T' " & E' " & T' " & E' " + T E' T " F T' F " id T' " & E' " & E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | idLeftmost Derivation: E T E' F T' E' ( E ) T' E' ( T E' ) T' E' ( F T' E' ) T' E' ( id T' E' ) T' E' ( id * F T' E' ) T' E' ( id * id T' E' ) T' E' ( id * id E' ) T' E' ( id * id ) T' E' ( id * id ) E' ( id * id ) + T E' ( id * id ) + F T' E' ( id * id ) + id T' E' ( id * id ) + id E' ( id * id ) + id 166 Syntax Analysis - Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & }
- 84. 167 Syntax Analysis -Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & } Example: FIRST (F) = ? E " T E' E'" + T E' | & T " F T' T'" * F T' | & F " ( E ) | id 168 Syntax Analysis - Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & } Example: FIRST (F) = { (, id } FIRST (T') = ? E " T E' E'" + T E' | & T " F T' T'" * F T' | & F " ( E ) | id
- 85. 169 Syntax Analysis -Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & } Example: FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = ? E " T E' E'" + T E' | & T " F T' T'" * F T' | & F " ( E ) | id 170 Syntax Analysis - Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & } Example: FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = ? E " T E' E'" + T E' | & T " F T' T'" * F T' | & F " ( E ) | id
- 86. 171 Syntax Analysis -Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & } Example: FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = ? E " T E' E'" + T E' | & T " F T' T'" * F T' | & F " ( E ) | id 172 Syntax Analysis - Part 1 © Harry H. Porter, 2005 “FIRST” Function Let # be a string of symbols (terminals and nonterminals) Define: FIRST (#) = The set of terminals that could occur first in any string derivable from # = { a | # !* aw, plus & if # !* & } Example: FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } E " T E' E'" + T E' | & T " F T' T'" * F T' | & F " ( E ) | id
- 87. 173 Syntax Analysis -Part 1 © Harry H. Porter, 2005 To Compute the “FIRST” Function For all symbols X in the grammar... if X is a terminal then FIRST(X) = { X } if X " & is a rule then add & to FIRST(X) if X " Y1 Y2 Y3 ... YK is a rule then if a + FIRST(Y1) then add a to FIRST(X) if & + FIRST(Y1) and a + FIRST(Y2) then add a to FIRST(X) if & + FIRST(Y1) and & + FIRST(Y2) and a + FIRST(Y3) then add a to FIRST(X) ... if & + FIRST(Yi) for all Yi then add & to FIRST(X) Repeat until nothing more can be added to any sets. 174 Syntax Analysis - Part 1 © Harry H. Porter, 2005 To Compute the FIRST(X1X2X3...XN) Result = {} Add everything in FIRST(X1), except &, to result
- 88. 175 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Result = {} Add everything in FIRST(X1), except &, to result if & + FIRST(X1) then Add everything in FIRST(X2), except &, to result endIf To Compute the FIRST(X1X2X3...XN) 176 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Result = {} Add everything in FIRST(X1), except &, to result if & + FIRST(X1) then Add everything in FIRST(X2), except &, to result if & + FIRST(X2) then Add everything in FIRST(X3), except &, to result endIf endIf To Compute the FIRST(X1X2X3...XN)
- 89. 177 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Result = {} Add everything in FIRST(X1), except &, to result if & + FIRST(X1) then Add everything in FIRST(X2), except &, to result if & + FIRST(X2) then Add everything in FIRST(X3), except &, to result if & + FIRST(X3) then Add everything in FIRST(X4), except &, to result endIf endIf endIf To Compute the FIRST(X1X2X3...XN) 178 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Result = {} Add everything in FIRST(X1), except &, to result if & + FIRST(X1) then Add everything in FIRST(X2), except &, to result if & + FIRST(X2) then Add everything in FIRST(X3), except &, to result if & + FIRST(X3) then Add everything in FIRST(X4), except &, to result ... if & + FIRST(XN-1) then Add everything in FIRST(XN), except &, to result endIf ... endIf endIf endIf To Compute the FIRST(X1X2X3...XN)
- 90. 179 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Result = {} Add everything in FIRST(X1), except &, to result if & + FIRST(X1) then Add everything in FIRST(X2), except &, to result if & + FIRST(X2) then Add everything in FIRST(X3), except &, to result if & + FIRST(X3) then Add everything in FIRST(X4), except &, to result ... if & + FIRST(XN-1) then Add everything in FIRST(XN), except &, to result if & + FIRST(XN) then // Then X1 !* &, X2 !* &, X3 !* &, ... XN !* & Add & to result endIf endIf ... endIf endIf endIf To Compute the FIRST(X1X2X3...XN) 180 Syntax Analysis - Part 1 © Harry H. Porter, 2005 add $ to FOLLOW(S) repeat if A"#B$ is a rule then add every terminal in FIRST($) except & to FOLLOW(B) if FIRST($) contains & then add everything in FOLLOW(A) to FOLLOW(B) endIf endIf if A"#B is a rule then add everything in FOLLOW(A) to FOLLOW(B) endIf until We cannot add anything more To Compute FOLLOW(Ai) for all Nonterminals in the Grammar
- 91. 181 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { ? FOLLOW (E') = { ? FOLLOW (T) = { ? FOLLOW (T') = { ? FOLLOW (F) = { ? E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id 182 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { FOLLOW (E') = { FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Add $ to FOLLOW(S)
- 92. 183 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, FOLLOW (E') = { FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Add $ to FOLLOW(S) 184 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, FOLLOW (E') = { FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule F " ( E ) | id What can follow E?
- 93. 185 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule F " ( E ) | id What can follow E? 186 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule E " T E' Whatever can follow E can also follow E'
- 94. 187 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule E " T E' Whatever can follow E can also follow E' 188 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule E'0 " + T E'1 Whatever is in FIRST(E'1) can follow T
- 95. 189 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule E'0 " + T E'1 Whatever is in FIRST(E'1) can follow T 190 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, FOLLOW (T') = { FOLLOW (F) = { E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule T'0 " * F T'1 Whatever is in FIRST(T'1) can follow F
- 96. 191 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, FOLLOW (T') = { FOLLOW (F) = { *, E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule T'0 " * F T'1 Whatever is in FIRST(T'1) can follow F 192 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, FOLLOW (T') = { FOLLOW (F) = { *, E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule E'0 " + T E'1 Since E'1 can go to & i.e., & + FIRST(E') Everything in FOLLOW(E'0) can follow T
- 97. 193 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, $, ) FOLLOW (T') = { FOLLOW (F) = { *, E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule E'0 " + T E'1 Since E'1 can go to & i.e., & + FIRST(E') Everything in FOLLOW(E'0) can follow T 194 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, $, ) FOLLOW (T') = { FOLLOW (F) = { *, E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule T " F T' Whatever can follow T can also follow T'
- 98. 195 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, $, ) FOLLOW (T') = { +, $, ) FOLLOW (F) = { *, E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule T " F T' Whatever can follow T can also follow T' 196 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, $, ) FOLLOW (T') = { +, $, ) FOLLOW (F) = { *, E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule T'0 " * F T'1 Since T'1 can go to & i.e., & + FIRST(T') Everything in FOLLOW(T'0) can follow F
- 99. 197 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) FOLLOW (E') = { $, ) FOLLOW (T) = { +, $, ) FOLLOW (T') = { +, $, ) FOLLOW (F) = { *, +, $, ) E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Look at rule T'0 " * F T'1 Since T'1 can go to & i.e., & + FIRST(T') Everything in FOLLOW(T'0) can follow F 198 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example of FOLLOW Computation Previously computed FIRST sets... FIRST (F) = { (, id } FIRST (T') = { *, &} FIRST (T) = { (, id } FIRST (E') = { +, &} FIRST (E) = { (, id } The FOLLOW sets... FOLLOW (E) = { $, ) } FOLLOW (E') = { $, ) } FOLLOW (T) = { +, $, ) } FOLLOW (T') = { +, $, ) } FOLLOW (F) = { *, +, $, ) } E " T E' E' " + T E' | & T " F T' T' " * F T' | & F " ( E ) | id Nothing more can be added.
- 100. 199 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Building the Predictive Parsing Table The Main Idea: Assume we’re looking for an A i.e., A is on the stack top. Assume b is the current input symbol. 200 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Building the Predictive Parsing Table The Main Idea: Assume we’re looking for an A i.e., A is on the stack top. Assume b is the current input symbol. If A"# is a rule and b is in FIRST(#) then expand A using the A"# rule!
- 101. 201 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Building the Predictive Parsing Table The Main Idea: Assume we’re looking for an A i.e., A is on the stack top. Assume b is the current input symbol. If A"# is a rule and b is in FIRST(#) then expand A using the A"# rule! What if & is in FIRST(#)? [i.e., # !* & ] If b is in FOLLOW(A) then expand A using the A"# rule! 202 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Building the Predictive Parsing Table The Main Idea: Assume we’re looking for an A i.e., A is on the stack top. Assume b is the current input symbol. If A"# is a rule and b is in FIRST(#) then expand A using the A"# rule! What if & is in FIRST(#)? [i.e., # !* & ] If b is in FOLLOW(A) then expand A using the A"# rule! If & is in FIRST(#) and $ is the current input symbol then if $ is in FOLLOW(A) then expand A using the A"# rule!
- 102. 203 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar “if b then if b then otherStmt else otherStmt” 1. S " if E then S S' 2. S " otherStmt 3. S' " else S 4. S' " & 5. E " boolExpr 204 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o , “if b then if b then otherStmt else otherStmt” 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b 1. S " if E then S S' 2. S " otherStmt 3. S' " else S 4. S' " & 5. E " boolExpr
- 103. 205 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b 206 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b
- 104. 207 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E e i t $bo S' S Look at Rule 1: S " i E t S S' If we are looking for an S and the next symbol is in FIRST(i E t S S' )... Add that rule to the table 208 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E e S " iEtSS' i t $bo S' S Look at Rule 1: S " i E t S S' If we are looking for an S and the next symbol is in FIRST(i E t S S' )... Add that rule to the table
- 105. 209 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E e S " iEtSS' i t $bo S' S Look at Rule 2: S " o If we are looking for an S and the next symbol is in FIRST(o)... Add that rule to the table 210 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E e S " iEtSS' i t $bo S' S " oS Look at Rule 2: S " o If we are looking for an S and the next symbol is in FIRST(o)... Add that rule to the table
- 106. 211 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E e S " iEtSS' i t $bo S' S " oS Look at Rule 5: E " b If we are looking for an E and the next symbol is in FIRST(b)... Add that rule to the table 212 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E " bE e S " iEtSS' i t $bo S' S " oS Look at Rule 5: E " b If we are looking for an E and the next symbol is in FIRST(b)... Add that rule to the table
- 107. 213 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E " bE e S " iEtSS' i t $bo S' S " oS Look at Rule 3: S' " e S If we are looking for an S' and the next symbol is in FIRST(e S )... Add that rule to the table 214 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b Look at Rule 3: S' " e S If we are looking for an S' and the next symbol is in FIRST(e S )... Add that rule to the table E " bE S' " e S e S " iEtSS' i t $bo S' S " oS
- 108. 215 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b Look at Rule 4: S' " & If we are looking for an S' and & + FIRST(rhs)... Then if $ + FOLLOW(S')... Add that rule under $ E " bE S' " e S e S " iEtSS' i t $bo S' S " oS 216 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b Look at Rule 4: S' " & If we are looking for an S' and & + FIRST(rhs)... Then if $ + FOLLOW(S')... Add that rule under $ E " bE S' " e S e S " iEtSS' i t S' " & $bo S' S " oS
- 109. 217 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b Look at Rule 4: S' " & If we are looking for an S' and & + FIRST(rhs)... Then if e + FOLLOW(S')... Add that rule under e E " bE S' " e S e S " iEtSS' i t S' " & $bo S' S " oS 218 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E " bE S' " e S S' " & e S " iEtSS' i t S' " & $bo S' S " oS Look at Rule 4: S' " & If we are looking for an S' and & + FIRST(rhs)... Then if e + FOLLOW(S')... Add that rule under e
- 110. 219 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E " bE S' " e S S' " & e S " iEtSS' i t S' " & $bo S' S " oS CONFLICT! Two rules in one table entry. 220 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Example: The “Dangling Else” Grammar i b t i b t o e o FIRST(S) = { i, o } FOLLOW(S) = { e, $ } FIRST(S') = { e, & } FOLLOW(S') = { e, $ } FIRST(E) = { b } FOLLOW(E) = { t } 1. S " i E t S S' 2. S " o 3. S' " e S 4. S' " & 5. E " b E " bE S' " e S S' " & e S " iEtSS' i t S' " & $bo S' S " oS CONFLICT! Two rules in one table entry. The grammar is not LL(1)!
- 111. 221 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” 222 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” Compute FIRST and FOLLOW sets
- 112. 223 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” Compute FIRST and FOLLOW sets for each rule A"# do for each terminal b in FIRST(#) do add A"# to TABLE[A,b] endFor endFor 224 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” Compute FIRST and FOLLOW sets for each rule A"# do for each terminal b in FIRST(#) do add A"# to TABLE[A,b] endFor if & is in FIRST(#) then for each terminal b in FOLLOW(A) do add A"# to TABLE[A,b] endFor endIf endFor
- 113. 225 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” Compute FIRST and FOLLOW sets for each rule A"# do for each terminal b in FIRST(#) do add A"# to TABLE[A,b] endFor if & is in FIRST(#) then for each terminal b in FOLLOW(A) do add A"# to TABLE[A,b] endFor if $ is in FOLLOW(A) then add A"# to TABLE[A,$] endIf endIf endFor 226 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” Compute FIRST and FOLLOW sets for each rule A"# do for each terminal b in FIRST(#) do add A"# to TABLE[A,b] endFor if & is in FIRST(#) then for each terminal b in FOLLOW(A) do add A"# to TABLE[A,b] endFor if $ is in FOLLOW(A) then add A"# to TABLE[A,$] endIf endIf endFor TABLE[A,b] is undefined? Then set TABLE[A,b] to “error”
- 114. 227 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Algorithm to Build the Table Input: Grammar G Output: Parsing Table, such that TABLE[A,b]= Rule to use or “ERROR/Blank” Compute FIRST and FOLLOW sets for each rule A"# do for each terminal b in FIRST(#) do add A"# to TABLE[A,b] endFor if & is in FIRST(#) then for each terminal b in FOLLOW(A) do add A"# to TABLE[A,b] endFor if $ is in FOLLOW(A) then add A"# to TABLE[A,$] endIf endIf endFor TABLE[A,b] is undefined? Then set TABLE[A,b] to “error” TABLE[A,b] is multiply defined? The algorithm fails!!! Grammar G is not LL(1)!!! 228 Syntax Analysis - Part 1 © Harry H. Porter, 2005 LL(1) Grammars LL(1) grammars • Are never ambiguous. •! Will never have left recursion. Furthermore... If we are looking for an “A” and the next symbol is “b”, Then only one production must be possible. More Precisely... If A"# and A"$ are two rules If # !* a... and $ !* b... then we require a - b (i.e., FIRST(#) and FIRST($) must not intersect) If # !* & then $ !* & must not be possible. (i.e., only one alternative can derive &.) If # !* & and $ !* b... then b must not be in FOLLOW(A) Scanning input left-to-right Find Leftmost derivation Using only one symbol of look-ahead
- 115. 229 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Error Recovery We have an error whenever... •!Stacktop is a terminal, but stacktop - input symbol •!Stacktop is a nonterminal but TABLE[A,b] is empty Options 1. Skip over input symbols, until we can resume parsing Corresponds to ignoring tokens 2. Pop stack, until we can resume parsing Corresponds to inserting missing material 3. Some combination of 1 and 2 4. “Panic Mode” - Use Synchronizing tokens •!Identify a set of synchronizing tokens. •!Skip over tokens until we are positioned on a synchronizing token. •!Pop stack until we can resume parsing. 230 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Option 1: Skip Input Symbols Example: Decided to use rule S " IF E THEN S ELSE S END Stack tells us what we are expecting next in the input. We’ve already gotten IF and E Assume there are extra tokens in the input. if (x<5))) then y = 7; ... A syntax error occurs here. < 5 ) ) ) y =TH $ ... END S ELSE S THEN We want to skip tokens until we can resume parsing. ... ...
- 116. 231 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Option 2: Pop The Stack Example: Decided to use rules S " IF E THEN S ELSE S END E " ( E ) We’ve already gotten if ( E Assume there are missing tokens. if (x < 5 then y = 7;... IF ( x < 5 y =TH We want to pop the stack until we can resume parsing. $ ... END S ELSE S THEN ) A syntax error occurs here. ... ... 232 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Panic Mode Recovery The “Synchronizing Set” of tokens ... is determined by the compiler writer beforehand Example: { SEMI-COLON, RIGHT-BRACE } Skip input symbols until we find something in the synchronizing set. Idea: Look at the non-terminal on the stack top. Choose the synchronizing set based on this non-terminal. Assume A is on the stack top Let SynchSet = FOLLOW(A) Skip tokens until we see something in FOLLOW(A) Pop A from the stack. Should be able to keep going. Idea: Look at the non-terminals in the stack (e.g., A, B, C, ...) Include FIRST(A), FIRST(B), FIRST(C), ... in the SynchSet. Skip tokens until we see something in FIRST(A), FIRST(B), FIRST(C), ... Pop stack until NextToken + FIRST(NonTerminalOnStackTop)
- 117. 233 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Error Recovery - Table Entries Each blank entry in the table indicates an error. Tailor the error recovery for each possible error. Fill the blank entry with an error routine. The error routine will tell what to do. 234 Syntax Analysis - Part 1 © Harry H. Porter, 2005 Error Recovery - Table Entries Each blank entry in the table indicates an error. Tailor the error recovery for each possible error. Fill the blank entry with an error routine. The error routine will tell what to do. Example: ... E5 E4 RPAREN LPAREN ... $SEMIid E' E
- 118. 235 Syntax Analysis -Part 1 © Harry H. Porter, 2005 Error Recovery - Table Entries Each blank entry in the table indicates an error. Tailor the error recovery for each possible error. Fill the blank entry with an error routine. The error routine will tell what to do. Example: ... E4: SynchSet = { SEMI, IF, THEN } SkipTokensTo (SynchSet) Print (“Unexpected right paren”) Pop stack break E5: ... ... ... E5 E4 RPAREN LPAREN ... $SEMIid E' E Choose the SynchSet based on the particular error Error-Handling Code