CS-1301

Lecture 2

Algorithms and flowcharts Department of Computer Science

College of Arts and Science

Lecturer : Mr. Asad Abd Elrashid

a.Khalil@psau.edu.sa

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Content

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Introduction.

Pseudocode.

Algorithm.

The Flowchart.

Control structure.

If structure.

Introduction

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Computer programming can be divided into two

phases:

Problem solving phase

–Make an ordered sequence of steps that

solves a problem

– these sequence of steps is called an

algorithm

Implementation phase :

– implement using a programming language

Pseudocode

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Artificial, informal language that helps us

develop algorithms

Similar to everyday English

Not actually executed on computers

Helps us “think out” a program before writing

it.

Algorithm

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Step 1: input x,y

Step 2: sum x+y.

Step 3: print sum value.

The Flowchart

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Graphical representation of an algorithm.

Drawn using certain special-purpose symbols connected by arrows called flow lines.

–A flowchart must have a start and stop

–A steps in a flowchart must connect. Can’t leave a step “hanging” with no connection.

Flow chart symbols

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Example 1

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Input two numbers.

Multiply your numbers together.

Output Multiplication result.

Cont..

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Step 1: Input x,y.

Step 2 : z = x*y.

step 3 : print z.

Flow chart

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Start

z = x*y

Enter x,y

Print z

End

Control Structures

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Sequential execution :

Statements executed one after the other in the

order written.

All programs written in terms of three control

structures

Sequence structures: Built into C. Programs

executed sequentially by default.

Cont..

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Selection structures:

C has three types: if, if…else, and switch.

Repetition structures:

C has three types: while, do…while and for.

Sequence structures

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Start

z = a+b

Enter a,b

Print z

End

Selection structures:

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Used to choose among alternative options of

actions.

Step 1: start

Step 2: if(a=0) then

Step 3: print a.

Step 4: stop

If structure

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if

A=0

Print A

Y N

Print A

End

If else structure

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Specifies an action to be performed both when

the condition is true and when it is false.

Challenge :

Think about any example of if else structure.

Example (1)

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Step 1: Start.

Step 2: Input M1,M2,M3,M4

Step 3: GRADE (M1+M2+M3+M4)/4

Step 4: if (GRADE < 60) then

Print “FAIL”

else

Print “PASS”

endif

Cont..

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PRINT

“PASS”

START

Input

M1,M2,M3,M4

GRADE(M1+M2+M3+M4)/4

GRADE<60

PRINT

“FAIL”

STOP

YN

Print “Fail”Print “Pass”

Example

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Input two numbers.

Check which number is greater.

Output greater number.

Cont..

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Step 1 : start.

Step 2 : input A and B.

Step 3 : if A is greater than B.

Step 4 : if “true” print A.

Step 5 : else print B.

Step 6 : stop.

Challenge

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Change previous algorithm to flow chart?

Thank you for your attention

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Lecturer : Aasd Abd Elrashid