Cs1123 2 comp_prog

Introduction to Programming
(CS1123)
By
Dr. Muhammad Aleem,
Department of Computer Science,
Mohammad Ali Jinnah University, Islamabad
Fall 2013

What is a Computer?
• A computer is a electro-mechanical device that
works semi-automatically to process input data
according to the stored set of instructions and
produces output or resultant data.
A Computer
System
Instructions
Data
Results

Components of a Computer System
Main Memory
CPU
Arithmetic and Logic Unit
Control Unit
Computer
Peripheral Devices / Connected devices
Keyboard Mouse Display CD Rom Hard Disk

Computer Instructions and Programs
• Instruction: A computer instruction is a command
or directive given to a computer to perform
specific task.
Examples: Add 2 and 5, Print “Hello World”
• Program: A program is sequence of instructions
written in programming language that directs a
computer to solve a problem
Examples: Draw a square, etc.

Computer Instructions and Programs
• Program “Draw a square”
1 – Draw a vertical line of length n inches
2 – Draw a horizontal line of n inches
3– Draw a vertical line of length n inches
4 – Draw a horizontal line of n inches

Computer Software System
Operating Systems
(Windows, Linux, MAC, Solaris)
Compilers / Libraries
(C++, C, Java)
Application Programs
(.cpp, .c, .java,)
Computer Hardware

Programming Languages
• Classification of programming languages:
1. Machine language
2. Low-level languages
3. High-level languages

1. Machine level languages
• A computer understands only sequence of bits or 1’s
and 0’s (the smallest piece of information)
• A computer program can be written using machine
languages (01001101010010010….)
• Very fast execution
• Very difficult to write and debug
• Machine specific (different codes on different
machines)

2. Low level languages
• English encrypted words instead of codes (1’s and
0’s)
• More understandable (for humans)
• Example: Assembly language
• Requires: “Translation” from Assembly code to
Machine code
compare:
cmpl #oxa,n
cgt end_of_loop
acddl #0x1,n
end_of_loop:
1001010101001101
1110010110010100
0101010111010010
0110100110111011
1101100101010101
Assembler
Assembly Code Machine Code

3. High level languages
• Mostly machine independent
• Close to natural language (English like language
keywords)
• Easy to write and understand programs
• Easy to debug and maintain code
• Requires compilers to translate to machine code
• Slower than low-level languages

3. High level languages
• Some Popular High-Level languages
– COBOL (COmmon Business Oriented Language)
– FORTRAN (FORmula TRANslation)
– BASIC (Beginner All-purpose Symbolic Instructional Code)
– Pascal (named for Blaise Pascal)
– Ada (named for Ada Lovelace)
– C (whose developer designed B first)
– Visual Basic (Basic-like visual language by Microsoft)
– C++ (an object-oriented language, based on C)
– Java

Programming Paradigms
• Programming paradigm is the fundamental
style of computer programming
1. Imperative
2. Functional
3. Logical
4. Object Oriented

Imperative Paradigm
• Machine based modes (Sequence of steps
required to compute)
• Statements executed sequentially step-wise
• Emphasis on How is to compute
• How to obtain the required results or output
• Example languages: C, Pascal, Ada, etc.

Functional/Logical Paradigm
• Specify what is to be computed using
programming abstractions.
• Responsibility of a programmer is to specify What
is to compute
• How to compute is implementation dependent
and is managed by language compiler
• Example languages: Lisp, Scheme, Erlang, Haskell,
Scala etc.

Object-Oriented Paradigm
• Programming with Abstract Data Types, to model
real world
• Basic Program Unit: Class (a programmer defined
type)
– A entity that contains data and methods which work
on the data
• Basic Run-time Unit: Object
– Instance of a class during execution

Problem Solving Steps
1. Understand the problem
2. Plan the logic
3. Code the program
4. Test the program
5. Deploy the program into production

1. Understanding the Problem
• Problems are often described in natural language
like English.
• Users may not be able to specify needs well, and
the needs may changing frequently
• Identify the requirements
1. Inputs or given data-items
2. Required output(s) or desired results
3. Indirect inputs (may not be given directly, you
have to calculate or assume)

1. Understanding the Problem
– Example: Calculate the area of a circle having
the radius of 3 cm
• Inputs:
– Radius=3
• Output:
– Area
• Indirect Inputs:
– Pi=3.14
– Area = 3.14 * (3*3) = 28.27

2. Plan the Logic
• Identify/Outline small steps in sequence, to
achieve the goal (or desired results)
• Tools such as flowcharts and pseudocode can be
used:
1. Flowchart: a pictorial representation of the logic
steps
2. Pseudocode: English-like representation of the logic
• Walk through the logic before coding

3. Code the Program
• Code the program:
–Select the programming language
–Write the program instructions in the selected
programming language
–Use the compiler software to translate the
program into machine understandable code or
executable file
–Syntax errors (Error in program instructions) are
identified by the compiler during compilation
and can be corrected.

4. Test the Program
• Testing the program
–Execute it with sample data and check the
results
–Identify logic errors if any (undesired results
or output) and correct them
–Choose test data carefully to exercise all
branches of the logic (Important)

5. Deploy the Program
• Putting the program into production
–Do this after testing is complete and all known
errors have been corrected

Introduction to Pseudocode
• One of the popular representation based on
natural language
• Widely used
–Easy to read and write
–Allow the programmer to concentrate on the
logic of the problem
• Structured in English language (Syntax/grammar)

What is Pseudocode (continued...)
• English like statements
• Each instruction is written on a separate line
• Keywords and indentation are used to signify
particular control structures.
• Written from top to bottom, with only one
entry and one exit
• Groups of statements may be formed into
modules

Some Basic Constructs
• Print  Output is to be sent to the Printer
• Write  Output is to be written to a file
• Display  Output is to be written to the screen
• Prompt  required before an input instruction
Get, causes the message to be sent to the screen
• Compute /Calculate  Calculation performed by
computer
• Calculation operations: +, -, *, /, ()

Some Basic Constructs
• Set  Used to set inital value,
E.g., Set Marks to 0
• =  Place values from right hand-side item to left
hand side
E.g., Marks = 67
• Save  Save the variable in file or disk
E.g., Save Marks

Comparison/Selection
IF student_Marks are above 50 THEN
Result = “Pass“
ELSE
Result = “Fail“
ENDIF 
• Making decision by comparing values
•
• Keywords used:
•IF, THEN, ELSE
IF student_Marks are above 50 THEN
Result = “Pass“
ELSE
Result = “Fail“
ENDIF

CASE of <condition-variable>
Case <valueA>
Do Step1
Case <valueB>
Do Step2
Case <valueC>
Do Step3
ENDCASE
• Case Strtucture
 Multiple banching based on value of condition
• Three keywords used:
• CASE of, Case, ENDCASE

• Case Strtucture
 Example...

Repeat a group of Statements
DOWHILE student_total < 50
Read student record
Print student name
Add 1 to student_total
ENDDO
• Repeating set of instruction base on some condition
• Keyword used:
•DOWHILE, ENDDO

Example Pseudocode Program
• A program is required to read three
numbers, add them together and print their
total.
Inputc Processing Output
Number1
Number2
Number3
total

Solution Algorithm
Add_three_numbers
Set total to 0
Read number1
Read number2
Read number3
Total = number1 + number2 + number3
Print total
END

Repeat Until (Loop)
• Repeat-Until statement (loop)
REPEAT
Do StepA
Do StepB
UNTIL conditionN is True
- What is the Difference between (While & Repeat) ?
Example:…

Flowcharts
• “A graphic representation of a sequence of
operations to represent a computer program”
• Flowcharts show the sequence of instructions in a
single program or subroutine.

A Flowchart
• A Flowchart
– Shows logic of an algorithm or problem
– Shows individual steps and their interconnections
– E.g., control flow from one action to the next

Name Symbol Description
Oval Beginning or End of the Program
Parallelogram Input / Output Operations
Rectangle Processing for example, Addition,
Multiplication, Division, etc.
Diamond
Denotes a Decision (or branching)
for example IF-Then-Else
Arrow
Denotes the Direction of logic
flow
Flowchart Symbols

Example 1
Step 1: Input M1,M2,M3,M4
Step 2: GRADE = (M1+M2+M3+M4)/4
Step 3: if (GRADE < 50) then
Print “FAIL”
else
Print “PASS”
endif
START
Input
M1,M2,M3,M4
GRADE(M1+M2+M3+M4)/4
IS
GRADE<50
STOP
YN
Print
“FAIL”
Print
“PASS”

Example 2
• Write an algorithm and draw a flowchart to
convert the length in feet to centimeter.

Example 2
Algorithm
• Step 1: Read Lft
• Step 2: Lcm = Lft x 30
• Step 3: Print Lcm
Flowchart
START
Read Lft
Lcm = Lft x 30
STOP
Print LCM

Example 3
• Write an algorithm and draw a flowchart that will read
the two sides of a rectangle and calculate its area.

Example 3
Algorithm
• Step 1: Read W,L
• Step 2: A = L x W
• Step 3: Print A
START
Read
W, L
A  L x W
STOP
Print
A

DECISION STRUCTURES
• The expression A>B is a logical expression
• It describes a condition we want to test
• if A>B is true (if A is greater than B) we take the action
on left
• Print the value of A
• if A>B is false (if A is not greater than B) we take the
action on right
• Print the value of B

IF–THEN–ELSE STRUCTURE
• The algorithm for the flowchart is as follows:
If A>B then
print A
Else
print B
endif
is
A>B
Y N
Print A Print B

• Multiple branching based on a single condition
CASE STRUCTURE
CASE
Condition
Do Step A Do Step B Do Step C Do Step D

Relational / Logical Operators
Relational Operators
Operator Description
> Greater than
< Less than
= Equal to
 Greater than or equal to
 Less than or equal to
 Not equal to

Example 4
• Write an algorithm that reads two values, finds largest
value and then prints the largest value.
ALGORITHM
Step 1: Read VALUE1, VALUE2
Step 2: if (VALUE1 > VALUE2) then
MAX  VALUE1
else
MAX  VALUE2
endif
Step 3: Print “The largest value is”, MAX
-- DRAW the Flow Chart for the Program

Selection Structure
• A Selection structure can be based on
1. Dual-Alternative (two code paths)
2. Single Alternative (one code path)
is
A>B
Y N
Print A Print B
Dual Alternative Example 

Selection Structure
• Single Alternative Example
Pseudocode: IF GPA is greater than 2.0 Then
Print “Promoted ”
End IF
GPA
> 2.0
N Y
Print
“Promoted”

Loop Structure
• Repetition (WHILE structure)
– Repeats a set of actions based on the answer to a
question/condition
pseudocode: DoWHILE <Some-True-Condition>
Do Something
ENDDO

Loop Structure
• REPEAT-UNTIL structure
– Repeats a set of actions until a condition remains True
– pseudocode: REPEAT
Do-Something
UNTIL <Some True Condition>

Cs1123 2 comp_prog

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