Loop Type & Description

while loop in C

A while loop in C programming repeatedly executes a target statement as long as a given condition is true.

Syntax

The syntax of a while loop in C programming language is −

while(condition) {
   statement(s);
}

Here, statement(s) may be a single statement or a block of statements. Thecondition may be any expression, and true is any nonzero value. The loop iterates while the condition is true.

When the condition becomes false, the program control passes to the line immediately following the loop.

Flow Diagram

Here, the key point to note is that a while loop might not execute at all. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.

Example

#include <stdio.h>
 
int main () {

   /* local variable definition */
   int a = 10;

   /* while loop execution */
   while( a < 20 ) {
      printf("value of a: %d\n", a);
      a++;
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result −

value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

for loop in C


A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.

Syntax

The syntax of a for loop in C programming language is −
for ( init; condition; increment ) {
   statement(s);
}


Here is the flow of control in a 'for' loop −


The init step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.


Next, the condition is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and the flow of control jumps to the next statement just after the 'for' loop.


After the body of the 'for' loop executes, the flow of control jumps back up to the increment statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the condition.


The condition is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then increment step, and then again condition). After the condition becomes false, the 'for' loop terminates.



Flow Diagram

Example
#include <stdio.h>
 
int main () {

   int a;
 
   /* for loop execution */
   for( a = 10; a < 20; a = a + 1 ){
      printf("value of a: %d\n", a);
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result −
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19


do...while loop in C


Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop in C programming checks its condition at the bottom of the loop.

A do...while loop is similar to a while loop, except the fact that it is guaranteed to execute at least one time.

Syntax

The syntax of a do...while loop in C programming language is −
do {
   statement(s);
} while( condition );


Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop executes once before the condition is tested.

If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop executes again. This process repeats until the given condition becomes false.

Flow Diagram

Example
#include <stdio.h>
 
int main () {

   /* local variable definition */
   int a = 10;

   /* do loop execution */
   do {
      printf("value of a: %d\n", a);
      a = a + 1;
   }while( a < 20 );
 
   return 0;
}

When the above code is compiled and executed, it produces the following result −
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19

nested loops in C


C programming allows to use one loop inside another loop. The following section shows a few examples to illustrate the concept.

Syntax

The syntax for a nested for loop statement in C is as follows −
for ( init; condition; increment ) {

   for ( init; condition; increment ) {
      statement(s);
   }
 
   statement(s);
}


The syntax for a nested while loop statement in C programming language is as follows −
while(condition) {

   while(condition) {
      statement(s);
   }
 
   statement(s);
}


The syntax for a nested do...while loop statement in C programming language is as follows −
do {

   statement(s);
 
   do {
      statement(s);
   }while( condition );

}while( condition );


A final note on loop nesting is that you can put any type of loop inside any other type of loop. For example, a 'for' loop can be inside a 'while' loop or vice versa.

Example

The following program uses a nested for loop to find the prime numbers from 2 to 100 −
#include <stdio.h>
 
int main () {

   /* local variable definition */
   int i, j;
   
   for(i = 2; i<100; i++) {
   
      for(j = 2; j <= (i/j); j++) 
         if(!(i%j)) break; // if factor found, not prime
         if(j > (i/j)) printf("%d is prime\n", i);
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result −
2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime