Decision Control Statements

8/12/2019 Decision Control Statements

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Decision Control Statements

Conditional control statements Unconditional control statement

Simple if statement

if-else statement

Nested if-else statement

switch statement

goto statement



F It is used to execute only one action.

F It is called one way branching.F Here the logical condition is tested which results either

TRUE or FALSE.

The syntax is:if (condition)

statement;

condition true

Execute this

Otherwise it just skip the

statement



Is

condition

?

statement

Next executable statement

True

False



main( )

{

int prize, dis;

printf(“Enter the prize of book \n”);

scanf(“%d”,&prize); if(prize>=500)

{

dis=10;

prize=prize-(prize*dis/100);}

printf(“Amount of book=%d”, prize);

getch( );

}



if(prize>=500)

{

dis=10;

prize=prize-(prize*dis/100);

} printf(“Amount of book=%d”, prize);

Suppose prize is 600

The condition is true



if(prize>=500)

{

dis=10;



Suppose prize is 600 Execute this

dis=10



if(prize>=500)

{

dis=10;




prize=540



if(prize>=500)

{

dis=10;




Amount of book=540



Isprize>=500

?dis=10

prize=prize-(prize*dis/100)

print “Amount of book=prize”

True

False

Read prize

STOP

START



Isprize>=500

?dis=10

prize=prize-(prize*dis/100)

print “Amount of book=prize”

True

False

Read prize

STOP

START



Adding a semicolon at the end of an if statement is a common

mistake.if (prize>= 500);

{

dis=10;


}

printf(“Amount of book=%d”, prize);

This mistake is hard to find, because it is not a compilation error

or a runtime error, it is a logic error.

Wrong



The if statement does nothing when the expression evaluates to

false. So to execute another group of statements, we use if-elsestatement.

This is used to execute two statements alternatively.

It is called a two way branching.

The syntax is:

if (condition)statement 1;

else

statement 2;







The syntax is:


else

statement 2;

condition true

Execute this







The syntax is:


else

statement 2;

condition false

Execute this



Is

condition

?

Statement 1

Next executable statement

TrueFalse

Statement 2



main( )

{

int n;

printf(“Enter a number \n”);

scanf(“%d”,&n); if((n%2)==0)

printf(“The number is even”);

else

printf(“The number is odd”); getch( );

}



if((n%2)==0)


else

printf(“The number is odd”);

Suppose n is 17 The condition is false



if((n%2)==0)


else


Suppose n is 17 Execute this

The number is odd



Do not put assignment operator in the place of equal operator.

if((n%2)=0)


else


Wrong

Condition is False

Hence the else condition is executed



If there are more than two alternative statements to be executed, then we use

nested if-else statement.

Here we use if-else statement within the other if-else statement.

The syntax is:

if (condition 1)

{

if(condition 2)

statement 1;

else

statement 2;

}

else

{

if(condition 3)

statement 3;

else

statement 4;

}statement x;






The syntax is:

if (condition 1)

{

if(condition 2)

statement 1;

else

statement 2;

}

else

{

if(condition 3)

statement 3;

else

statement 4;

}statement x;

If both condition1 and

condition 2 are true Execute this






The syntax is:

if (condition 1)

{

if(condition 2)

statement 1;

else

statement 2;

}

else

{

if(condition 3)

statement 3;

else

statement 4;

}statement x;

Condition1 true and

condition 2 is false Execute this






The syntax is:

if (condition 1)

{

if(condition 2)

statement 1;

else

statement 2;

}

else

{

if(condition 3)

statement 3;

else

statement 4;

}statement x;

Condition1 is falseExecute this

Condition 2 is true






The syntax is:

if (condition 1)

{

if(condition 2)

statement 1;

else

statement 2;

}

else

{

if(condition 3)

statement 3;

else

statement 4;

}statement x;

Condition1 is falseExecute this

Condition 2 is false



Is

condition2

?

statement x

True False

Statement 1

Is

condition1?

Is

condition3

?

Statement 2 Statement 3 Statement 4

True False FalseTrue



main( )

{ int n;

printf(“Enter a number \n”);

scanf(“%d”, &n);

if (n==0) printf(“Neither +ve nor –ve”);

else

{

if (n>0)

printf(“No. is +ve”);

else

printf(“No. is -ve”);

}

}



if (n==0)

printf(“Neither +ve nor –ve”);

else

{if (n>0)


else


}

Suppose n is -14The condition is false



if (n==0)


else

{if (n>0)


else


}




if (n==0)


else

{if (n>0)


else


}


Execute this

No. is -ve

Fl h t



Flow chart:

True FalseIs

n==0

?

Is

n>0

?

No. is +ve No. is -ve

FalseTrue

Read n

START

Neither +ve nor -ve

STOP

Flow chart:



Flow chart:

True FalseIs

n==0

?

Is

n>0

?

No. is +ve No. is -ve

FalseTrue

Read n

START

Neither +ve nor -ve

STOP



The control statement that allows us to make a decision from

the number of choices is called switch. The switch statement tests the value of expression against a list

of case values.

When the match is found, a block of statements associated with

that case is executed.



The keyword break is

used at the end of each

case to terminate fromthe switch statement.

If the break statement is

not present, the next case

statement will beexecuted.

When none of the

specified cases matchesthe switch-expression,

then the statement

following default are

executed.

switch (expression)

{

case value1: block 1;

break;

case value2: block 2;

break;…

case value N: block N;

break;

default: default-block;}



main( )

{ char op;

float a, b; printf(“Enter any two no. & the operator +, -, * or / \n”);

scanf(“%f%f%c”, &a, &b, &op);

switch(op){

case „+‟: printf(“%f+%f=%f”, a, b, a+b);

break;

case „-‟: printf(“%f -%f=%f”, a, b, a-b);

break;

case „*‟: printf(“%f*%f=%f ”, a, b, a*b);

break;case „/‟: printf(“%f / %f=%f”, a, b, a/b);

break;

default: printf(“INVALID OPERATOR”);

}

}

Example: program to preform any one of the arithmetic operation.



switch(op){


break;


break;case „*‟: printf(“%f*%f=%f ”, a, b, a*b);

break;

case „/‟: printf(“%f / %f=%f”, a, b, a/b);

break;default: printf(“INVALID OPERATOR”);

}

getch( );

Suppose operator is „*‟:



switch(op){

case „+': printf(“%f+%f=%f”, a, b, a+b);

break;



break;



}

getch( );

operator is not „*‟:



switch(op){


break;

case „-': printf(“%f -%f=%f”, a, b, a-b);


break;



}

getch( );

operator is not „*‟:



switch(op){


break;


break;case „*': printf(“%f*%f=%f ”, a, b, a*b);

break;



}

getch( );

operator is „*‟:



switch(op){


break;



break;



}

getch( );

Execute this line



switch(op){


break;



break;



}

getch( );

Execute this line



switch(op){


break;



break;



}

getch( );

Execute this line



The goto statement is used to take control where we want,unconditionally from one point to another in the program.

The goto requires a label in order to identify the place where the

control is to be made.

A label is any valid variable name, and must be followed by a

colon( : ).

The label is placed immediately before the statements where the

control is to be transferred.



goto label;

- - - - -

- - - - -- - - - -

label:

statement;

label:

Statement;

- - - - -- - - - -

- - - - -

goto label;

Forward jump Backward jump

If the label: is placed after the goto label; some statements will be

skipped and jump is known as forward jump.

If the label: is placed before the goto label; a loop will be formed and

some statements will be executed repeatedly. Such a jump is known as

a backward jump.

Example: program to calculate factorial of a non-



p p g

negative number

main( )

{ int n, fact=1, i=1;start:

clrscr( );

printf(“Enter a non-negative number”);

scanf(“%d”, &n); if(n<0) goto start;

find:

fact=fact*i;

i++;if(i<=n) goto find;

printf(“Factorial of %d=%d”, n , fact);

getch( );

}

Do not dis-

order

because ittakes

infinite loop

So avoid

goto

statement

as far as

possible



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Decision Control Statements