control structuresin c

Control Structures in C

Dept. of CS&E and IT, DBCET, Guwahati1

ConditionOperator Meaning Example

== Equal to count == 10

!= Not equal to flag != DONE

< Less than a < b

<= Less than or equal to <= LIMIT

> Greater than pointer > end_of_list

>= Greater than or equal to lap >= start

Dept. of CS&E and IT, DBCET,

Guwahati2

Control Structures in C� These include

� if else,

� while,

� do-while,

� for, and a selection statement called


� for, and a selection statement called

� switch.

if-else

� The if-else statement can exist in two forms: with or without the else. The two forms are:

if(expression)statement


statement

� orif(expression)statement1

elsestatement2

if-else� If

if (condition)

statement1;

else

statement2;


int a, b;

// ...

if(a < b) a = 0;

else b = 0;

Nested ifNested ifNested ifNested if

if (i == 10) {

if (j < 20) a = b;

if (k > 100) c = d; // this if is

else a = c; // associated with this else

}


}

else a = d; // this else refers to if(i == 10)

ifififif----elseelseelseelse----if Ladderif Ladderif Ladderif Ladder

if(condition)statement;

else if(condition)statement;

else if(condition)statement;


statement;...elsestatement;

ifififif----elseelseelseelse----if Ladderif Ladderif Ladderif Ladder// Demonstrate if-else-if statements (IfElse.c).#include <stdio.h>main () {int no; printf(“\nEnter any number from 1 to 12 to know the month”);scanf(“%d”,&no);


scanf(“%d”,&no);

If (no==1)

printf(“\n Month is January”);

else if (no==2)

printf(“\n Month is February”);

else if (no==3)

printf(“\n Month is March”);

else if (no==4)

printf(“\n Month is April”);

else if (no==5)

printf(“\n Month is May”);

else if (no==6)

printf(“\n Month is June”);

else if (no==7)

printf(“\n Month is July”);


printf(“\n Month is July”);

else if (no==8)

printf(“\n Month is August”);

else if (no==9)

printf(“\n Month is September”);

else if (no==10)

printf(“\n Month is October”);

else if (no==11)

printf(“\n Month is November”);

else if (no==12)

printf(“\n Month is December”);

else printf(“ \n Invalid Entry”);

}

switchswitchswitchswitch

The control statement that allows us to make a decision

from the number of choices is called a switch, or more

correctly a switch-case-default, since these three

keywords go together to make up the control statement.


keywords go together to make up the control statement.

They most often appear as follows:

switchswitchswitchswitch

switch ( integer expression) {case value1:

// statement sequencebreak;

case value2:// statement sequencebreak;


break;...case valueN:

// statement sequencebreak;

default:// default statement sequence

}

switch� The integer expression following the keyword switch is anyC expression that will yield an integer value.

� It could be an integer constant like 1, 2 or 3, or anexpression that evaluates to integer.

� The keyword case is followed by an integer or a character


� The keyword case is followed by an integer or a characterconstant.

� Each constant in each case must be different from all theothers.

switchswitchswitchswitch// A simple example of the switch(switch.c)#include <stdio.h>main() {int i;for(i=0; i<6; i++)switch(i) {

case 0:printf("i is zero.\n");

break;case 1:

printf("i is one.\n");break;


break;case 2:

printf("i is two.\n");break;

case 3:printf("i is three.\n");break;

default:printf("i is greater than 3.\n");

} // switch} // main

main()

{

char c = 'x' ;

switch ( c )

{

case 'v' :

printf ( "I am in case v \n" ) ;

break ;

case 'a' :

printf ( "I am in case a \n" ) ;


printf ( "I am in case a \n" ) ;

break ;

case 'x' :

printf ( "I am in case x \n" ) ;

break ;

default :

printf ( "I am in default \n" ) ;

}

}

Nested switchNested switchNested switchNested switch

switch(count) {case 1:switch(target) { // nested switchcase 0:

printf("target is zero");


printf("target is zero");break;

case 1: // no conflicts with outer switchprintf("target is one");break;

} // switch(target)break;case 2: // ...

switch Versus ifswitch Versus ifswitch Versus ifswitch Versus if----else Ladder else Ladder else Ladder else Ladder

� There are some things that we simply cannot dowith a switch. These are:�A float expression cannot be tested using aswitch


switch

�Cases can never have variable expressions (forexample it is wrong to say case a +3 :

�Multiple cases cannot use same expressions.

Loop structure� The repetitive operation in a program is done through loop-

� They are: � (a) Using a for statement

� (b) Using a while statement

� (c) Using a do-while statement


� (c) Using a do-while statement

for loopfor loopfor loopfor loop� for loop allows us to specify the following three things in a single line-

� Setting a loop counter to an initial value.

� Testing the loop counter to determine whether its value has reached the number of iterations desired.


� Increasing the value of loop counter each time the program segment within the loop has been executed.

� General Form:for(initialization; condition; iteration){// body

}

for// Using the comma (comma.c)#include <stdio.h>main() {

int a, b;for(a=1, b=4; a<b; a++, b--) {printf("a = %d \n", a);


printf("a = %d \n", a);printf("b = %d \n", b);

}}

for// Demonstrate the for loop (loop.c).

#include <stdio.h>

main() {

int n;

for(n=10; n>0; n--)


for(n=10; n>0; n--)

printf("tick %d \n",n);

}

While loopWhile loopWhile loopWhile loop� While loop� The general form of while is as shown below:

initialize loop counterwhile(test loop counter using a condition) {statement block;increment/decrement loop counter;


increment/decrement loop counter;}

� The statements within the while loop would keep on getting executed till the condition being tested remains true.

While loop


while// Demonstrate the while loop (while.c).#include <stdio.h>main() {

int n = 10;while(n > 0) {



printf("tick %d \n",n);n--;

} // while} // main

do-while loop The general form of do-while

initialize loop counter

do

{ statement block;


statement block;increment/decrement loop counter;

} while(test loop counter using a condition);

dodododo----while loopwhile loopwhile loopwhile loop// Demonstrate the do-while loop (dowhile.c).#include <stdio.h>main() {

int n = 10;do {



printf("tick %d \n",n);n--;

} while(n > 0);} // main

While Vs do-while loops� There is a minor difference between the working ofwhile and do-while loops.

� This difference is the place where the condition is tested. � The while tests the condition before executing any of the statements within the while loop.


statements within the while loop. � The do-while tests the condition after having executed the statements within the loop.

� Statements under do-while loop will get executed atleast once and not so incase of while loop

Nested LoopsNested LoopsNested LoopsNested Loops� Like all other programming languages, C allows loops to be nested. That is, one

loop may be inside another. For example, here is a program that nests for loops:

// Loops may be nested (nestedfor.c).#include <stdio.h>main() {int i, j;for(i=0; i<10; i++) {


for(i=0; i<10; i++) {for(j=i; j<10; j++)

printf(".");printf("\n");

}}

JumpJumpJumpJump

� C supports four jump statements: � break,

� continue,

� return


� goto.

� These statements transfer control to another part of your program.

breakbreakbreakbreak

�In C, the break statement has two uses. �First, it terminates a statement sequence in a switch statement.

�Second, it can be used to exit a loop.


�Second, it can be used to exit a loop.

break statement� We often come across situations where we want to jump out of a loop instantly, without waiting to get back to the conditional test.

� The keyword break allows us to do this.

� When break is encountered inside any loop, control


� When break is encountered inside any loop, controlautomatically passes to the first statement after the loop.

break

// Using break to exit a loop (break.c).

#include <stdio.h>

main() {

int i;

for(i=0; i<100; i++) {


for(i=0; i<100; i++) {

if(i == 10) break; // terminate loop if i is 10

printf("i: %d \n", i);

}

printf("Loop complete.");

}

break// Using break to exit a while loop (break2.c).#include <stdio.h>

main() {int i = 0;while(i < 100) {if(i == 10) break; // terminate loop if i is 10printf("i: %d \n", i);


printf("i: %d \n", i);i++;

}printf("Loop complete.");

}

break statement example

main( ) { int i = 1 , j = 1 ;

while ( i++ <= 100 )

{

while ( j++ <= 200 )

{ In this program when j equals 150,

break takes the control outside


if ( j == 150 )

break ;

else

printf ( "%d %d\n", i, j ) ;

}

}

}

break takes the control outside

the inner while only, since it is

placed inside the inner while.

continuecontinuecontinuecontinue� continue go immediately to next iteration of loop� In while and do-while loops, a continue statement causes control to be transferred directly to the conditional expression that controls the loop.

� In a for loop, control goes first to the iteration portion of the for statement and then to the conditional expression.


In a for loop, control goes first to the iteration portion of the for statement and then to the conditional expression.

continue// Demonstrate continue (continue.c).#include <stdio.h>main() {

int i;for(i=0; i<10; i++) {

printf("%d ", i);


printf("%d ", i);if (i%2 == 0) continue;printf("\n");

}}

returnreturnreturnreturn� The return statement is used to explicitly return from a method. That is, it causes program control to transfer back to the caller of the method.

� The following example illustrates this point. Here, return causes execution to return to the C, since it is the


return causes execution to return to the C, since it is the run-time system that calls main( ).

return// Demonstrate return (return.c).

#include <stdio.h>

main() {

int t = 1;

printf("Before the return.");


printf("Before the return.");

if(t==1) return; // return to caller

printf("This won't execute.");

}

goto� It is possible to jump to any statement within the same function using goto.

� A label is used to mark the destination of the jump.

goto label1;


::label1:

goto// Using continue with a label (goto.c).

#include <stdio.h>

main() {

int i,j;

for (i=0; i<10; i++) {

for(j=0; j<10; j++) {

if(j > i) {

printf("\n");


printf("\n");

goto outer;

}

printf(" %d", (i * j));

}

outer: printf(".. outer ..\n");

}

}

control structuresin c

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