1.8. Loops

Loops in programming are used to repeat a block of code until the specified condition is met. A loop statement allows programmers to execute a statement or group of statements multiple times without repetition of code. There are mainly two types of loops in C Programming:

1. Entry Controlled loops are ones where the test condition is checked before entering the main body of the loop. for loop and while loop are Entry Controlled loops.

2. Exit Controlled loops are ones where the test condition is evaluated at the end of the loop body. The loop body will execute at least once, irrespective of whether the condition is true or false. do-while loop is an Exit Controlled loop.

1.8.1. for Loop

for loop in C programming is a repetition control structure that allows programmers to write a loop that will be executed a specific number of times. for loop enables programmers to perform n number of steps together in a single line. Following is the syntax of a for loop.

for (initialization expression; condition expression; update expression)
{
    //
    // body of for loop
    //
}

In for loop, a loop variable is used to control the loop. First, the loop variable is initialized with some value, then the condition is checked. If the statement is true then control will move to the body and the body of the for loop will be executed. The update expression is executed to update the value of the loop variable. Now the condition is checked again with the new value of the loop variable. These steps will be repeated till the condition expression results to false. Following code shows the usage of a for loop.

#include <stdio.h>

int main()
{
    int i = 0;
    for (i = 1; i <= 10; i++)
    {
        printf( "Hello World\n");  
    }
    return 0;
}

1.8.2. while Loop

A while loop does not depend upon the number of iterations. In for loop, the number of iterations were known in advance but in the while loop, the execution is terminated on the basis of the condition expression. If the condition results in a false then the while loop will break, otherwise the while loop body is executed. Following is the syntax of a while loop.

while (condition expression)
{
    // body of the while loop
}

Following code shows the use of a while loop.

#include <stdio.h>

int main()
{
    int i = 2;
    while(i < 10)
    {
        printf( "Hello World\n");
        i++;
    }

    return 0;
}

1.8.3. do-while Loop

The do-while loop is similar to a while loop but the only difference is that the do-while loop checks the condition at the end of the body. In do-while loop, the loop body will execute at least once irrespective of the condition result. Following is the syntax of a do-while loop.

do
{
    // body of do-while loop
} while (condition expression);

Following code shows the use of a do-while loop.

#include <stdio.h>

int main()
{
    int i = 2;
    do
    {
        printf( "Hello World\n");
        i++;
    } while (i < 1);

    return 0;
}

1.8.4. Loop Control Statements

Loop control statements in C programming are used to change execution from its normal sequence. C language supports the following control statements,

• break; terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.

• continue; causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

• goto transfers control to the labeled statement.

1.8.5. Infinite Loop

A loop becomes an infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since the three expressions that form the for loop are not required, you can make an endless loop by leaving the conditional expression empty.

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

   for( ; ; )
   {
      printf("This loop will run forever.\n");
   }

   return 0;
}

When the conditional expression is absent, it is assumed to be true. You can achieve the same result with while (true).