C looping statement

Definition

A loop is a way to repeat a block of code again and again until a certain condition is false.

In C, looping (or iteration) statements let you execute a block of code repeatedly as long as a condition holds.

Looping allows a set of instructions to be executed repeatedly until a specific condition is met. C provides three distinct looping constructs: `for`, `while`, and `do...while`, each suited for different control flow scenarios.

1. for Loop

A for loop is a control structure that allows code to be executed repeatedly based on a condition. The `for` loop combines initialization, condition evaluation, and update expressions all within a single statement, offering compact control over iteration.

Syntax:

for (initialization; condition; update) {     
          // body: runs each iteration 
} 

• Initialization runs once before the loop starts (e.g. int i = 0;).
• Before each iteration, the loop evaluates the condition—executing the body only if the condition holds true.
• Update executes after each iteration (e.g. i++).

Example

#include <stdio.h>  

int main(void) {     
      for (int i = 1; i <= 5; i++) {         
              printf("%d ", i);     
       }    
       // Output: 1 2 3 4 5     
      return 0; 
} 

2. while Loop

A while loop is a control structure that executes a block of code as long as the condition is true. It checks the condition before each iteration.

Syntax:

while (condition) {     
       // body: runs as long as condition is true 
}

• The condition is checked before each iteration.
• If the condition is false at the start, the body may never execute.

Example

#include <stdio.h>  

int main(void) {     
      int n = 5;     
      while (n > 0) {         
               printf("%d ", n);         
               n--;    
       }     
       // Output: 5 4 3 2 1     
       return 0; 
} 

3. do...while Loop

The do...while loop differs by executing the code block first, then deciding whether to continue based on the condition—ensuring one guaranteed pass.

Syntax:

do {     
       // body: always runs at least once 
} while (condition); 

• The loop body runs initially without any condition, and only then the condition is evaluated for subsequent iterations.
• Useful when you need the loop body to run at least one time.

Example

#include <stdio.h>  

int main(void) {     
         int n = 1;     
         do {         
                 printf("Enter a number (0 to quit): ");         
                 scanf("%d", &n);         
                 printf("You entered: %d\n", n);     
           } while (n != 0);     
         return 0; 
} 

4. Controlling Loop Execution

• break: immediately exits the nearest enclosing loop.
• continue: skips the rest of the current iteration and jumps to the next.

Example with break and continue

#include <stdio.h>  

int main(void) {     
      for (int i = 1; i <= 10; i++) {         
              if (i == 5) {            
                     continue;   // skip printing 5         
              }            
              if (i == 8) {             
                       break;      // exit loop when i == 8         
             }         
             printf("%d ", i);     
        }     
       // Output: 1 2 3 4 6 7     
       return 0; 
} 

5. Nested Loops

You can place one loop inside another. This is common for working with multi-dimensional data.

for (int i = 1; i <= 3; i++) {     
         for (int j = 1; j <= 3; j++) {         
               printf("(%d,%d) ", i, j);     
          }     
        printf("\n"); 
} 

Notes

• Always ensure loops terminate (avoid infinite loops) unless intentionally infinite (e.g., event loops).
• Keep loop bodies simple; move complex logic into functions.
• For performance, minimize work inside the condition or update expressions.