Looping tricky questions in C language

Looping questions and answers with explanation for written test exam and interview in c programming language. Following are the some basic tricky questions in C language.

1.  What will be output of following c code?

#include<stdio.h>

extern int x;

int main(){

do{

do{

printf(“%o”,x);

}

while(!-2);

}

while(0);

return 0;

}

int x=8;
Solution: Output: 10

Explanation:

Here variable x is extern type. So it will search the definition of variable x. which is present at the end of the code. So value of variable x =8

There are two do-while loops in the above code.  AS we know do-while executes at least one time even that condition is false.  So program control will reach  at printf statement at it will print octal number 10 which is equal to decimal number 8.

Note: %o is used to print the number in octal format.

In inner do- while loop while condition is ! -2 = 0

In C zero means false.  Hence program control will come out of the inner do-while loop.   In outer do-while loop while condition is 0. That is again false. So program control will also come out of the outer do-while loop.

 

(2) What will be output of following c code?

#include<stdio.h>

int main(){

int i=2,j=2;

while(i+1?–i:j++)

printf(“%d”,i);

return 0;

}

Solution: 

Output: 1

Explanation:

Consider the while loop condition: i + 1 ? — i : ++j

In first iteration:

i + 1 = 3 (True)

So ternary operator will return -–i i.e. 1

In c 1 means true so while condition is true. Hence printf statement will print 1

In second iteration:

i+ 1 = 2 (True)

So ternary operator will return -–i i.e. 0

In c zero means false so while condition is false. Hence program control will come out of the while loop.

 

(3) What will be output of following c code?

#include<stdio.h>

int main(){

int x=011,i;

for(i=0;i<x;i+=3){

printf(“Start “);

continue;

printf(“End”);

}

return 0;

}

Solution: 

Output: Start Start Start

Explantion:

011 is octal number. Its equivalent decimal value is 9.

So, x = 9

First iteration:

i = 0

i < x i.e. 0 < 9  i.e. if loop condition is true.

Hence printf statement will print: Start

Due to continue keyword program control will come at the beginning of the for loop and value of variable i will be:

i += 3

i = i + 3 = 3

Second iteration:

i = 3

i < x i.e. 3 < 9 i.e. if loop condition is true.

Hence printf statement will print: Start

Due to continue keyword program control will come at the beginning of the for loop and value of variable i will be:

i += 3

i = i + 3 = 6

Third iteration:

i = 3

i < x i.e. 6 < 9 i.e. if loop condition is true.

Hence printf statement will print: Start

Due to continue keyword program control will come at the beginning of the for loop and value of variable i will be:

i += 3

i = i + 3 = 9

fourth iteration:

i = 6

i < x i.e. 9 < 9 i.e. if loop condition is false.

Hence program control will come out of the for loop.

 

(4)What will be output of following c code?

#include<stdio.h>

int main(){

int i,j;

i=j=2,3;

while(–i&&j++)

printf(“%d %d”,i,j);

return 0;

}

Solution: 

Output: 13

Explanation:

Initial value of variable

i = 2

j = 2

Consider the while condition : –i && j++

In first iteration:

–i && j++

= 1 && 2 //In c any non-zero number represents true.

= 1 (True)

So while loop condition is true. Hence printf function will print value of i = 1 and j = 3 (Due to post increment operator)

In second iteration:

–i && j++

= 0 && 3  //In c zero represents false

= 0  //False

So while loop condition is false. Hence program control will come out of the for loop.

 

(5) What will be output of following c code?
#include<stdio.h>

int main(){

static int i;

for(++i;++i;++i) {

printf(“%d “,i);

if(i==4) break;

}

return 0;

}

Solution:

Output: 24

Explanation:

Default value of static int variable in c is zero. So, initial value of variable i = 0

First iteration:

For loop starts value: ++i i.e. i = 0 + 1 = 1

For loop condition: ++i i.e. i = 1 + 1 = 2 i.e. loop condition is true. Hence printf statement will print 2

Loop incrimination: ++I i.e. i = 2 + 1 =3

Second iteration:

For loop condition: ++i i.e. i = 3 + 1 = 4 i.e. loop condition is true. Hence printf statement will print 4.

Since is equal to for so if condition is also true. But due to break keyword program control will come out of the for loop.

 

(6) What will be output of following c code?

#include<stdio.h>

int main(){

int i=1;

for(i=0;i=-1;i=1) {

printf(“%d “,i);

if(i!=1) break;

}

return 0;

}

Solution: 

Output: -1

Explanation:

Initial value of variable i is 1.

First iteration:

For loop initial value: i = 0

For loop condition: i = -1 . Since -1 is non- zero number. So loop condition true. Hence printf function will print value of variable i i.e. -1

Since variable i is not equal to 1. So, if condition is true. Due to break keyword program control will come out of the for loop.

 

(7) What will be output of following c code?

#include<stdio.h>

int main(){

for(;;) {

printf(“%d “,10);

}

return 0;

}

Solution: 

Output: Infinite loop

Explanation:

In for loop each part is optional.