While vs Do-While Loop in C Programming – Key Differences Explained

Introduction

In this tutorial section, we will learn the difference between the while loop and the do-while loop in the C programming language.

Both while and do-while loops are types of iterative (looping) statements used in C. However, there are some key differences between them. In this tutorial, we will compare them in a tabular form. But first, let's briefly understand what each loop does.

What is a while Loop?

A while loop in C is a control flow statement that repeatedly executes a block of code as long as a given Boolean condition is true. In other words, a while loop is a type of iterative or repeating statement.

What is a do-while Loop?

A do-while loop is known as an Exit-Controlled Loop. It is similar to a while loop, except that it guarantees the loop body will execute at least once.

The main difference is that a do-while loop checks the condition after executing the loop body, whereas a while loop checks the condition before execution.

Key Differences Between while and do-while Loop

Flowchart Comparison

The diagram below shows the difference between while and do-while loops in C programming.

Let's break it down and understand how these two loops differ — one step at a time.

1. Syntax:

while(condition) {
    // code inside the loop
}

do {
    // code inside the loop
} while(condition);

2. When the condition is checked:

• while loop: The condition is checked before the loop starts. So if the condition is false right away, the loop won't run at all.
• do-while loop: Here, the condition is checked after the loop runs. That means the code inside the loop will run at least once, no matter what.

3. How iteration works:

• while loop: If the condition is false at the beginning, the loop body won't run.
• do-while loop: The loop body runs once before checking the condition — so it always runs at least once.

4. Flow of control:

In a while loop, control goes straight to the condition check. In a do-while loop, it first goes to the loop body, then checks the condition. That's the key difference in how they work behind the scenes.

C Program to Illustrate while Loop

The program below displays numbers from 1 to 10 using a while loop.

#include <stdio.h>

int main() {
    int i = 1;
    
    while(i <= 10) {
        printf("\n%d", i);
        i++;
    }
    
    return 0;
}

Sample Output:

1
2
3
4
5
6
7
8
9
10

Explanation:

1. We declare an integer variable i and initialize it to 1 using int i = 1;.
2. The program checks the condition in the while loop: i <= 10. Since 1 is less than or equal to 10, the condition is true and the loop begins executing.
3. Inside the loop, printf("\n%d", i); prints the current value of i, which is 1.
4. Then, i++; increases the value of i by 1, so i becomes 2.
5. The control goes back to the top of the while loop and checks the condition again: 2 <= 10. Since it's still true, the loop continues.
6. This process repeats until i becomes 11. At that point, the condition i <= 10 is false, and the loop stops.

C Program to Illustrate do-while Loop

The program below displays numbers from 1 to 10 using a do-while loop.

#include <stdio.h>

int main() {
    int i = 1;
    
    do {
        printf("\n%d", i);
        i += 1;
    } while(i <= 10);
    
    return 0;
}

Sample Output:

1
2
3
4
5
6
7
8
9
10

Explanation:

1. The variable i is declared as an integer and initialized to 1 using int i = 1;.
2. Since this is a do-while loop, the program begins by executing the loop body first — without checking the condition.
3. The printf("\n%d", i); statement prints 1 to the screen.
4. Then i += 1; increments the value of i by 1, so now i becomes 2.
5. After that, the program checks the condition in the while part: i <= 10. Since 2 is less than or equal to 10, the condition is true, and the loop runs again.
6. This process continues — printing each value of i from 1 to 10 — until i becomes 11. At that point, the condition i <= 10 becomes false, and the loop terminates.
7. Note: The do-while loop always executes its body at least once, even if the condition is false at the start.

#include <stdio.h>

int main() {
    int num;
    
    // Using do-while loop (guarantees at least one execution)
    do {
        printf("Enter a number greater than 100: ");
        scanf("%d", &num);
    } while(num <= 100);
    
    printf("You entered: %d\n", num);
    return 0;
}

Frequently Asked Questions

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