While loop in C

Introduction

A while loop helps you repeat a set of instructions as long as a condition is true. It saves you from writing the same code many times.

When you want to keep asking a user for input until they give a valid answer.

When you want to count up or down until a certain number is reached.

When you want to keep checking if a task is done before moving on.

When you want to repeat an action but don't know in advance how many times.

Syntax

C

while (condition) {
    // code to repeat
}

The condition is checked before each repetition.

If the condition is false at the start, the code inside the loop will not run at all.

Examples

This prints numbers 0 to 4 by increasing i each time.

C

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

This counts down from 10 to 1, printing each number.

C

int count = 10;
while (count > 0) {
    printf("Countdown: %d\n", count);
    count--;
}

Sample Program

This program prints numbers from 1 to 5 using a while loop. It starts with number at 1 and keeps printing and increasing it until it reaches 5.

C

#include <stdio.h>

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

Important Notes

Make sure to change something inside the loop that affects the condition, or the loop will run forever.

Use break; inside the loop if you want to stop it early.

Summary

A while loop repeats code while a condition is true.

The condition is checked before each repetition.

Remember to update variables inside the loop to avoid infinite loops.

Practice

1. What does a while loop do in C programming?

easy

A. Repeats a block of code while a condition is true

B. Executes code only once regardless of condition

C. Runs code a fixed number of times without checking condition

D. Stops the program immediately

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly as long as its condition remains true.

2. Step 2: Compare options with the definition

   Only Repeats a block of code while a condition is true correctly describes this behavior.

3. Final Answer:

   Repeats a block of code while a condition is true -> Option A

4. Quick Check:

   While loop = repeat while condition true [OK]

Hint: While loops run only if condition is true before each repeat [OK]

Common Mistakes:

• Thinking while loop runs at least once even if condition is false
• Confusing while loop with for loop fixed iterations
• Believing while loop stops program immediately

2. Which of the following is the correct syntax for a while loop in C?

easy

A. while (condition) { /* code */ }

B. while condition { /* code */ }

C. while [condition] { /* code */ }

D. while: (condition) { /* code */ }

Solution

1. Step 1: Recall C while loop syntax

   The condition must be inside parentheses and the code block inside braces.

2. Step 2: Check each option

   Only while (condition) { /* code */ } uses correct parentheses and braces syntax.

3. Final Answer:

   while (condition) { /* code */ } -> Option A

4. Quick Check:

   Correct syntax uses parentheses and braces [OK]

Hint: Remember: while(condition) with parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using square brackets instead of parentheses
• Adding colon after while keyword

3. What is the output of this C code?

int i = 1;
while (i <= 3) {
    printf("%d ", i);
    i++;
}

medium

A. 0 1 2 3

B. 1 2 3 4

C. 3 2 1

D. 1 2 3

Solution

1. Step 1: Trace the loop variable values

   i starts at 1, prints 1, then increments to 2, prints 2, then 3, and stops after printing 3 because i becomes 4 and condition fails.

2. Step 2: Confirm printed output

   The printed numbers are "1 2 3 " with spaces after each number.

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   Prints 1 to 3 with spaces [OK]

Hint: Check loop start, end condition, and increments carefully [OK]

Common Mistakes:

• Including 4 in output by mistake
• Starting from 0 instead of 1
• Printing numbers in reverse order

4. Identify the error in this code snippet:

int i = 0;
while (i < 5)
    printf("%d ", i);

medium

A. Syntax error due to missing braces

B. Missing semicolon after while condition

C. Infinite loop because i is never updated

D. Variable i is not declared

Solution

1. Step 1: Analyze loop behavior

   Variable i starts at 0 and condition is i < 5, but i is never incremented inside the loop.

2. Step 2: Understand consequence

   Since i never changes, the condition stays true forever, causing an infinite loop.

3. Final Answer:

   Infinite loop because i is never updated -> Option C

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Always update loop variable inside while loop [OK]

Common Mistakes:

• Assuming loop ends without variable update
• Thinking missing braces cause syntax error here
• Ignoring infinite loop possibility

5. How can you modify this code to print all even numbers from 2 to 10 using a while loop?

int num = 1;
while (num <= 10) {
    printf("%d ", num);
    num++;
}

hard

A. Use a for loop instead of while loop

B. Initialize num to 2 and increment by 2 inside the loop

C. Initialize num to 0 and increment by 1 inside the loop

D. Keep num at 1 and print only if num % 2 == 0 without incrementing

Solution

1. Step 1: Set starting point and increment for even numbers

   Start num at 2 (first even number) and increase by 2 each time to get only even numbers.

2. Step 2: Modify loop accordingly

   Change initialization to num = 2 and inside loop do num += 2 instead of num++.

3. Final Answer:

   Initialize num to 2 and increment by 2 inside the loop -> Option B

4. Quick Check:

   Start at 2, add 2 each time for evens [OK]

Hint: Start at first even, add 2 each loop to get evens [OK]

Common Mistakes:

• Not incrementing num causing infinite loop
• Printing odd numbers by starting at 1
• Using for loop when question asks for while loop