C Loops: while and do...while

Introduction

Loops let you repeat actions in your program. They’re essential for tasks like counting, reading input, or processing data until a condition is met.

In C, the two main loop types for condition-based repetition are while and do...while.

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Why Loops Matter

Loops help you:

• Process multiple items (e.g., array elements)
• Retry operations until success
• Validate input
• Perform repeated calculations

They reduce repetition and make your code cleaner and more flexible.

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while loop

Syntax

while (condition) {
  // code that does cool stuff
}

• The condition is checked first.
• If the condition is true, the body runs; then the condition is checked again.
• If the condition is false at the start, the body never runs.

Example 1 — Count from 1 to 5

#include <stdio.h>

int main(void) {
  int i = 1;
  while (i <= 5) {
    printf("%d\n", i);
    i++;
  }
  return 0;
}

Example 2 — Sum numbers until zero

Often a special value is used to signal the end of the loop, known as the sentinel value. For example we can read input numbers until the user inputs 0 (the sentinel value):

#include <stdio.h>

int main(void) {
  int n = 0;
  int sum = 0;

  printf("Enter numbers (0 to stop): ");
  if (scanf("%d", &n) != 1) return 1;  // basic input check

  while (n != 0) { // 0 is the sentinel value to indicate "stop looping"
    sum += n;
    if (scanf("%d", &n) != 1) return 1;
  }

  printf("Sum = %d\n", sum);
  return 0;
}

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do...while loop

A variation of a while loop is the do ... while loop, where the condition check happens after the body is run.

Syntax

do {
  // statements
} while (condition);

• The body runs first, then the condition is checked.
• The loop always runs at least once, even if the condition is false initially.

We can use this to ensure that code runs at least once, in a more convenient way than using just while loops. For example:

Example 1 — Ask for a positive number (input validation)

#include <stdio.h>

int main(void) {
  int n;
  do {
    printf("Enter a positive number: ");
    if (scanf("%d", &n) != 1) return 1;
  } while (n <= 0);

  printf("You entered: %d\n", n);
  return 0;
}

Example 2 — Simple menu-driven program

#include <stdio.h>

int main(void) {
  int choice;
  do {
    printf("\nMenu:\n");
    printf("1. Say Hello\n");
    printf("2. Say Goodbye\n");
    printf("0. Exit\n");
    printf("Enter choice: ");

    if (scanf("%d", &choice) != 1) return 1;

    if (choice == 1) {
      printf("Hello!\n");
    } else if (choice == 2) {
      printf("Goodbye!\n");
    } else if (choice != 0) {
      printf("Invalid choice.\n");
    }
  } while (choice != 0);

  printf("Program ended.\n");
  return 0;
}

continue and break

We can also have more control over how the loop runs using break and continue:

• break immediately stops the loop and moves to the next line after the loop.
• continue skips the rest of the current loop iteration and starts the next iteration right away.

With while loops be careful that you don’t accidentally create an infinite loop when using continue, what would happen if we incremented the counter j++ after the line if (j % 2 ==0)?

#include <stdio.h>
// print only odd numbers
int main(void) {
    int j = 0;
    while (j < 5) {
        j++;                 // Progress the counter first
        if (j % 2 == 0)      // If even, skip printing
            continue;
        printf("odd: %d\n", j);
    }
    return 0;
}

An example of how we can use break to stop a loop early:

#include <stdio.h>

int main(void) {
    int num;
    while (1) {  // Infinite loop
        printf("Enter a number (0 to stop): ");
        scanf("%d", &num);
        if (num == 0) {
            break;  // Exit the loop when user enters 0
        }
        printf("You entered: %d\n", num);
    }
    // program will jump to here on break
    printf("Loop ended.\n");
    return 0;
}

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When to use which loop?

• Use while when you may not need the loop body to run at all e.g., reading data from a file until you reach the end (sometimes called End Of File EOF).
• Use do...while when the body must run at least once (e.g., input validation, menus).

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Summary

Feature	while	do...while
Condition checked	Before first iteration	After first iteration
Runs at least once?	No	Yes
Common use	Loops that might skip body	Input prompts, menus

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Quick tips

• Always ensure the loop condition changes inside the loop, or you’ll create an infinite loop. Everyone has done this at least once :D
• Use braces {} even for single statements for clarity.
• For input loops, validate user input inside the loop body and handle scanf failures.
• Compile with warnings: -Wall -Wextra -Werror to catch common mistakes.

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