lec_4_data_structures_and_algorithm_analysis.ppt

Some slides are borrowed from Mr. Mohammad Alqahtani
•Overview.
•Algorithm Analysis.

 Be familiar with problem solving.
 Be able to develop (and implement) algorithms.
 Be able to trace algorithms.
 Be able to select appropriate data structures and
algorithms for given problems.

 Prerequisites:
◦ Variables and expressions.
◦ Methods (functions or procedures ).
◦ Decision structures( like if-statements and
switch-statements.
◦ Iteration structures (for-loops and while-loops).
◦ Classes and objects

 Data
◦ A collection of facts from which conclusion may be drawn.
◦ e.g. Data: Temperature 35°C; Conclusion: It is hot.
 Types of data
◦ Textual: For example, your name (Alya)
◦ Numeric: For example, your ID (090254)
◦ Audio: For example, your voice
◦ Video: For example, your voice and picture
◦ (...)

5
 Applications/programs read data, store data
temporarily, process it and finally output
results.
 What is data? Numbers, Characters, etc.
Application/
Program
Data Data

6
 Data is classified into data types. e.g. char, float, int, etc.
 A data type is (i) a domain of allowed values and (ii) a set
of operations on these values.
 Compiler signals an error if wrong operation is performed
on data of a certain type. For example, char x,y,z; z =
x*y is not allowed.

7
 Examples
Data Type Domain Operations
boolean 0,1 and, or, =, etc.
char ASCII =, <>, <, etc.
integer -maxint to
+maxint
+, _, =, ==,
<>, <, etc.

8
 Simple Data types: also known as atomic data
types  have no component parts. E.g. int,
char, float, etc.
21 3.14 ‘a’

 A particular way of storing and organizing data in a
computer so that it can be used efficiently and
effectively.
 Data structure is the logical or mathematical model of
a particular organization of data.
 A group of data elements grouped together under one
name.
◦ For example, an array of integers

10
 Structured Data types: can be broken into
component parts. E.g. an object, array, set,
file, etc. Example: a student object.
A H M A D
20
C S C
Name
Age
Branch
A Component part

There are many, but we named a few. We’ll learn these
data structures in great detail!
Array
Linked List
Tree
Queue Stack
Types of Data Structures

 Goal: to organize data
 Criteria: to facilitate efficient
◦ storage of data
◦ retrieval of data
◦ manipulation of data
 Design Issue:
◦ select and design appropriate data types
(This is the main motivation to learn and
understand data structures)

(Demonstrate using class room
example!)
 Navigating
Accessing each data element exactly once so
that certain items in the data may be processed
 Searching
Finding the location of the data element (key)
in the structure
 Insertion
Adding a new data element to the structure

 Deletion
◦ Removing a data element from the structure
 Sorting
◦ Arrange the data elements in a logical order
(ascending/descending)
 Merging
◦ Combining data elements from two or more
data structures into one

 A finite set of instructions which accomplish a particular
task
 A method or process to solve a problem
 Transforms input of a problem to output
Algorithm = Input + Process + Output
Algorithm development is an art – it needs practice, practice
and only practice!

 It must be correct
 It must be finite (in terms of time and size)
 It must terminate
 It must be unambiguous
 Which step is next?
 It must be space and time efficient
A program is an instance of an algorithm, written in some
specific programming language

 Problem: Find maximum of a, b, c
 Algorithm
 Input = a, b, c
 Output = max
 Process
o Let max = a
o If b > max then
max = b
o If c > max then
max = c
o Display max
Order is very important!!!

 Clearly identify:
 what output is required?
 what is the input?
 What steps are required to transform input into
output
o The most crucial bit
o Needs problem solving skills
o A problem can be solved in many different ways
o Which solution, amongst the different possible
solutions is optimal?

 A sequence of steps to solve a problem
 We need a way to express this sequence of steps
1. Natural language (NL) is an obvious choice, but not
a good choice. Why?
o NLs are notoriously ambiguous (unclear)
2. Programming language (PL) is another choice, but
again not a good choice. Why?
o Algorithm should be PL independent
 We need some balance
o We need PL independence
o We need clarity
o Pseudo-code provides the right balance

 Pseudo-code is a short hand way of describing a
computer program
 Rather than using the specific syntax of a
computer language, more general wording is used
 It is a mixture of NL and PL expressions, in a
systematic way
 Using pseudo-code, it is easier for a non-
programmer to understand the general workings of
the program

 Use PLs construct that are consistent with
modern high level languages, e.g. C++,
Java, ...
 Use appropriate comments for clarity
 Be simple and precise

 Expressions
 Standard mathematical symbols are used
o Left arrow sign (←) as the assignment operator in assignment statements
(equivalent to the = operator in Java)
o Equal sign (=) as the equality relation in Boolean expressions (equivalent to the "=
=" relation in Java)
o For example
Sum ← 0
Sum ← Sum + 5
What is the final value of sum?

 Decision structures (if-then-else logic)
 if condition then
true-actions
[else
false-actions]
 We use indentation to indicate what actions should be included in the true-
actions and false-actions
 For example
if marks > 50 then
print “Congratulation, you are passed!”
else
print “Sorry, you are failed!”
end if
What will be the output if marks are equal to 75?

 Loops (Repetition)
◦ Pre-condition loops
o While loops
 while condition do actions
 We use indentation to indicate what actions should be included in
the loop actions
 For example
while counter < 5 do
print “Welcome to CS204!”
counter ← counter + 1
end while
What will be the output if counter is initialised to 0, 7?

◦ Pre-condition loops
o For loops
 for variable-increment-definition do actions
 For example
for counter ← 0; counter < 5; counter ← counter + 2 do
end for
What will be the output?

 Post-condition loops
o Do loops
 do actions while condition
 For example
do
counter ← counter + 1
while counter < 5
What will be the output, if counter was initialised to 10?
The body of a post-condition loop must execute at least once

 Method declarations
◦ Return_type method_name (parameter_list)
method_body
◦ For example
 integer sum ( integer num1, integer num2)
 start
 result ← num1 + num2
 end
 Method calls
 object.method (args)
 For example
mycalculator.sum(num1, num2)

 Method returns
 return value
 For example
integer sum ( integer num1, integer num2)
start
result ← num1 + num2
return result
end

 Comments
◦ /* Multiple line comments go here. */
◦ // Single line comments go here
◦ Some people prefer braces {}, for comments
 Arrays
 A[i] represents the ith cell in the array A.
 The cells of an n-celled array A are indexed from A[0] to A[n −
1] (consistent with Java).

 Example 1: Determining even/odd number
 A number divisible by 2 is considered an even number, while a
number which is not divisible by 2 is considered an odd number.
Write pseudo-code to display first N odd/even numbers.
 Example 2: Computing Weekly Wages
 Gross pay depends on the pay rate and the number of hours
worked per week. However, if you work more than 40 hours, you
get paid time-and-a-half for all hours worked over 40. Write the
pseudo-code to compute gross pay given pay rate and hours
worked

Input range
for num←0; num<=range; num←num+1 do
if num % 2 = 0 then
print num is even
else
print num is odd
endif
endfor

 Example 1: Determining even/odd number
◦ A number divisible by 2 is considered an even number, while
a number which is not divisible by 2 is considered an odd
number. Write pseudo-code to display first N odd/even
numbers.
 Example 2: Computing Weekly Wages
 Gross pay depends on the pay rate and the number of hours
worked per week. However, if you work more than 40 hours,
you get paid time-and-a-half for all hours worked over 40.
Write the pseudo-code to compute gross pay given pay rate and
hours worked

Input hours_worked, pay_rate
if hours_worked <= 40 then
gross_pay ← pay_rate x hours_worked
else
basic_pay ← pay_rate x 40
over_time ← hours_worked – 40
over_time_pay ← 1.5 x pay_rate x over_time
gross_pay ← basic_pay + over_time_pay
endfor
print gross_pay

1. Write an algorithm to find the largest
of a set of numbers. You do not know
the number of numbers.
2. Write an algorithm in pseudocode
that finds the average of (n) numbers.
For example numbers are [4,5,14,20,3,6]s

lec_4_data_structures_and_algorithm_analysis.ppt

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