Chapter 2 - The Decision Control Structure

8/2/2019 Chapter 2 - The Decision Control Structure

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FUNDAMENTALSOF COMPUTER

PROGRAMMINGInstructor

Mr. Asad Ali Shah


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CHAPTER 2:

THE DECISION CONTROL STRUCTUREInstructor

Mr. Asad Ali Shah


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DECISIONS! DECISIONS!

Instructions were executed sequentially so far

Structures other than sequential structure

Decision Control Structure

You may want to execute an instruction in one situationand a different instruction for another situation

E.g. A/B where B should not be 0

In short, only divide the two variables if B is not 0


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CONTROL STRUCTURES

Control structure enable us to specify the order inwhich the various instructions in a program are tobe executed.

In short, they determine the flow of control

The three types of control structures are givenbelow

Sequence Control Instruction

Instructions are executed in order

Decision/Selection Control Instruction

Instructions execute if they meet the conditions

Repetition/Loop Control Instruction

The execution of instructions are repeated


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DECISION CONTROL STRUCTURE

Decide whether the instruction(s) should beexecuted or not.

A decision control instruction can be implementedin C using

The if statement

The if-else statement

The conditional operators


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DECISION CONTROL STRUCTURE

The different variations of Decision ControlStructure are given below

If Statement

If-else statement

Nested if-else statements

Else if statements

Case control


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THE IF STATEMENT

if keyword used for decision control instruction

if(this condition is true)

execute this statement;

If the condition within the parenthesis is true thenthe statement is executed

Otherwise, the compiler will skip past it

This is all good but how do we write thoseconditions anyway?


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CONDITIONAL EXPRESSIONS

The conditional expression are used with if

They are constructed using relational operators

Relational Operators allow us to compare twovalues.

This Expression Is true if

x==y x is equal to y

x!=y x is not equal to yxy x is greater than y

x=y x is greater than and equal to y


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EXERCISE

What are the results of the following relationalexpressions

9 < 25

9 < 3

9 > 14

9 = 25

9 == 13

9 != 13

9 !< 25


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SOLUTION

true

false

false

true false

false

true

Error, the "not less than" is not a valid operator.


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CAUTION!!!!!!!!!!!

X=Y IS NOT EQUIVALENT TO X==Y

Int x=3, y=3, z=4;

(1)X=Y assigns Ys value to X

(2)X==Y compares the two values

(3)X==Z compares the two values

The == comparison returns either true or false

Output of (1) is 3

Output of (2) is 1

Output of (3) is 0


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SIMPLE PROGRAMUSING IF STATEMENT

#include

void main(void)

{

int num;

printf(Enter a number less than 10);

scanf(%d,&num);

if(num


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WHATISHAPPENINGHERE?

#include

void main(void)

{

int num;

printf(Enter a number equal to10);

scanf(%d,&num);

if(num=10)

printf(What a obedient servant you are);

}


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THE ANSWERTO X=10 STATEMENT

The if conditions will be satisfied everytime.

All assignment operations like X=Y areconsidered as true.

All values other than zero areconsidered to be true


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FLOW CHARTS

A graphic representation of an algorithm, oftenused in the design phase of programming towork out the logical flow of a program.

Flow Charts are used to help programmers in theearly stages of programming.

A flow chart is a chart that flows from one stage tothe next and it will show what stage or event is first,second, third etc...


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FLOWCHART EXAMPLE


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SYMBOLSUSEDIN FLOW CHARTS


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EXERCISE

Draw a flow chart and write a program for theproblem given below

While purchasing certain items, a discount of 10%is offered if the quantity purchased is more than1000. If quantity and price per item are inputthrough the keyboards, write a program to calculatethe total expenses


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FLOW CHART


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PROGRAM

#include

void main(void)

{

int Discount=0, Quantity;float Rate, Total;

scanf(%d %f, &Quantity, &Rate);

if(quantity>1000)

Discount=10;

Total= Quantity*Rate (Discount/100 * Rate * Quantity)

printf(Total Expenses =%f,Total);

}


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THEIF-ELSE STATEMENT

IF expression executes instructions or set ofinstructions if the expressions is TRUE

If does nothing if the expression is FALSE

The else statement allows us to evaluateinstructions or set of instructions if the expression isfalse

If(expression)instruction;

else

instruction;


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EXAMPLE Write a program that tells takes an integer as input

and tells us whether the number entered is odd oreven.

#include

void main(void)

{

int a;

scanf(%d, &a);

if((a%2)==0)

printf(Number entered is even);

else

printf(Number entered is odd);

}


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FLOW CHART


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FEWPOINTSWORTH REMEMBERING

Groups of statements after the if up to elseis called an if block. Similarly, statementsafter else are in the else block

Else part always comes after an ifexpression

If an if block has only one statement thenbraces can be ignored. Same goes for theelse block


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FEWPOINTSWORTH REMEMBERING

if(expressions)

{ // if block starts

statement1;

statement2;} //if block ends

else

{ //else block starts

statement1;

statement2;

} //else block ends


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NESTED IF-ELSES

You can define an entire if-else construct within either thebody of the if statement or the body of an else statement. E.g.

If(expression1)

statement;

else{

if(expression2)

statement;

else

statement;}

NOTE: Indenting is VERY IMPORTANT. Improves readability


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SOLUTION

#include

Void main(void)

{

int m1,m2,m3,m4,m5,percent;

printf(Enter marks in five subjects);

scanf(%d %d %d %d

%d,&m1,&m2,&m3,&m4,&m5);percent=(m1+m2+m3+m4+m5)/500 * 100;


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SOLUTION

If(percent>=60)

printf(First Division);

Else

{

If(percent>=50)

printf(Second Division);

Else

{

If(percent>=40)

printf(Third Division);Else

printf(Fail);

}

}


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DISADVANTAGESINTHEAPPROACH

Else blocks are moving towards the right

Can become difficult to match the corresponding ifand else blocks

Can become difficult to match braces of if and elseblocks


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USEOF LOGICAL OPERATORS

C allows usage of three logical operators

AND &&

OR ||

NOT !

The logical operators contain two symbols note 1

& and | have different meanings as compared to &&and ||

Logical operators can be used to combine two ormore conditions. E.g.

If(Marks>90 && Marks


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AND OPERATOR

AND operator is used when we want both theconditions to be true

Here the condition will become true if and only if thenumber is between 50 and 60

A>50 Operator A


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OR OPERATOR

OR operator is used when we want either of theconditions to be TRUE.

Here the condition will be true if either A or B isgreater than 50.

A>50 Operator B>50 Result

FALSE || FALSE FALSE

FALSE || TRUE TRUE

TRUE || FALSE TRUETRUE || TRUE TRUE


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NOT OPERATOR

The not operator i.e. ! reverses the result of the

expression.

E.g. 2==2 is true

However, !(2==2) if false

Similarly, 1==2 is false

But !(1==2) is true

If(!flag) is equivalent to if(flag==0)


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PROGRAM USING LOGICAL OPERATORS

#include

Void main(void)

{

int m1,m2,m3,m4,m5,percent;

printf(Enter marks in five subjects);

scanf(%d %d %d %d

%d,&m1,&m2,&m3,&m4,&m5);percent=(m1+m2+m3+m4+m5)/500 * 100;


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PROGRAM USING LOGICAL OPERATORS

If(percent>=60)


if(percent>=50 && percent=40 && percent


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ADVANTAGESINTHEAPPROACH

No else Blocks

No need to match the corresponding if and elseblocks

Matching braces of if blocks becomes easy


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THE ELSEIF CLAUSE

There is one more way of writing if-else statements

In this we use the else if clause

If(percent>=60)


else if(percent>=50)

printf(Second Division);

else if(percent>=40)

printf(Third Division);

else if(percent


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THE ELSEIF CLAUSE

In this approach the second else if statement is linkwith the first if statement.

The second else if part only works if the firstcondition becomes false.

The same pattern is followed as we go down

In short, the else part will only work if all theconditions above have failed.

Similarly, other conditions are not checked if thefirst statement is true.


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COMPARISONBETWEEN ELSE-IF AND NESTED IF

i=2;

If(i==2)

printf(true);

else{

If(j==2)

printf(true again);

}


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COMPARISONBETWEEN ELSE-IF AND NESTED IF

i=2;

If(i==2)

printf(true);

else if(i==2)

printf(true again);


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THECONDITIONAL OPERATOR

If(a%2==0)

b=t

else

b=f

Can be written as

a%2==0 ? b=t : b=f ;Condition if block else block

Chapter 2 - The Decision Control Structure

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