C Program to Find Factorial of a Number Using While and Do-While Loops

In this tutorial, we will explore how to calculate the factorial of a number in C using two fundamental loop structures: the while loop and the do-while loop. Mastering these iterative techniques is essential for understanding control flow and handling repeated mathematical operations in C programming.

The factorial of a non-negative integer n (n!) is the product of all positive integers less than or equal to n. The architectural goal of our programs is to use a counter-controlled loop to accumulate this product. We compare an entry-controlled while loop, which evaluates the counter condition before each multiplication, and an exit-controlled do-while loop, which ensures the logic executes at least once.

Factorial Using a While Loop

In our first implementation, we define an integer n to store user input, fact to hold the running product (initialized to 1), and i as our loop counter. The while loop remains active as long as i <= n. Inside the loop body, we update the product by multiplying the current fact by i, then increment the counter. This entry-controlled structure ensures that if the condition is not met initially, the calculation logic is bypassed entirely, providing a clean execution flow.

C Program: Factorial Using a While Loop

#include <stdio.h>

int main() {
    int n, fact = 1, i = 1;
    printf("\nEnter Any Number: ");
    scanf("%d", &n);

    while(i <= n) {
        fact = fact * i;
        i++;
    }
    printf("\nFactorial of %d = %d", n, fact);
    return 0;
}

Output

Enter Any Number: 5
Factorial of 5 = 120
    

How This Program Works

The program begins by allocating memory for three integer variables: n to store the target number, fact to accumulate the result (initialized to 1), and i as the loop counter (initialized to 1). Once the user provides an input, the program enters the while loop. This structure is "entry-controlled," meaning it verifies the condition i <= n before every iteration.

Inside the loop, the program performs the multiplication fact = fact * i, which builds the factorial value step by step. Following the multiplication, the counter i is incremented by 1. The loop repeats this process until i exceeds the user-defined number n. Upon exit, the final calculated value is displayed.

Factorial Using a Do-While Loop

Our second approach utilizes an exit-controlled do-while loop, which is ideal for operations where the logic must execute at least once regardless of the initial input. The program enters the do block immediately, performs the multiplication of fact by i, and updates the counter. Only at the bottom of the block does the program evaluate the constraint while(i <= n). This design is robust for iterative math, ensuring the cycle continues seamlessly until the counter exceeds the input threshold.

C Program to Find Factorial Using a Do-While Loop

#include <stdio.h>

int main() {
    int n, fact = 1, i = 1;
    printf("\nEnter Any Number: ");
    scanf("%d", &n);

    do {
        fact = fact * i;
        i++;
    } while(i <= n);

    printf("\nFactorial of %d = %d", n, fact);
    return 0;
}

Output

Enter any Number: 5
Factorial of 5 = 120
    

How This Program Works

This implementation serves as a functional alternative to the while loop by using an "exit-controlled" structure. The logic within the do block is executed immediately upon entry, regardless of the condition. This ensures that the factorial calculation code block runs at least once before the program checks the loop constraint.

After the code inside the do block completes (multiplying the current product and incrementing the counter), the program evaluates the while(i <= n) condition at the end. If the condition remains true, it loops back to the start of the do block. This design is highly effective for mathematical iterations where you want to guarantee that the operation is performed at least one time before validation occurs.

Practice Challenges

Now that you have mastered calculating factorials using while and do-while loops, try expanding your programs with these exercises to deepen your understanding:

• Input Validation: Modify your programs to handle negative numbers. If a user enters a negative integer, display an error message and prompt them to enter a non-negative number instead of proceeding with the calculation.
• Continuous Execution: Wrap the existing factorial logic inside a parent loop so that the user can calculate the factorial of multiple numbers in a single session without needing to restart the program.
• Factorial Limit: Determine the largest integer whose factorial can be stored in a standard int variable in C before "overflow" occurs, and add a check to warn the user if they enter a number larger than that limit.