C Program: Calculate Factorial with while loop and User-entered positive integer

Last update on July 29 2025 12:58:16 (UTC/GMT +8 hours)

6. Factorial Calculation Using a While Loop

Write a C program that prompts the user to enter a positive integer. It then calculates and prints the factorial of that number using a while loop.

Sample Solution:

C Code:

#include <stdio.h>

int main() {
    int number;        // Variable to store the user input
    long long factorial = 1;  // Variable to store the factorial, initialized to 1

    // Prompt the user to enter a positive integer
    printf("Input a positive integer: ");
    scanf("%d", &number);  // Read the user input

    // Check if the entered number is negative
    if (number < 0) {
        printf("Error: Input enter a non-negative integer.\n");
        return 1;  // Exit the program with an error code
    }

    // Calculate the factorial using a while loop
    while (number > 0) {
        factorial *= number;  // Multiply the current factorial by the current number
        number--;  // Decrement the number for the next iteration
    }

    // Print the calculated factorial
    printf("Factorial: %lld\n", factorial);

    return 0;  // Indicate successful program execution
}

Sample Output:

Input a positive integer: 5
Factorial: 120

Input a positive integer: -6
Error: Input enter a non-negative integer.

Explanation:

Here are key parts of the above code step by step:

#include <stdio.h>: Include the standard input-output library for functions like "printf()" and "scanf()".
int main() {: Start the main function.
int number;: Declare an integer variable 'number' to store user input.
long long factorial = 1;: Declare a long long variable 'factorial' and initialize it to 1 to store the factorial.
printf("Enter a positive integer: ");: Prompt the user to enter a positive integer.
scanf("%d", &number);: Read the user input and store it in the variable 'number'.
if (number < 0) {: Check if the entered number is negative.
printf("Error: Please input a non-negative integer.\n");: Print an error message if the number is negative.
return 1;: Exit the program with an error code.
while (number > 0) {: Start a while loop that continues as long as the 'number' is greater than 0.
factorial *= number;: Multiply the current 'factorial' by the current 'number'.
number--;: Decrement the 'number' for the next iteration.
}: Close the while loop.
printf("Factorial: %lld\n", factorial);: Print the calculated factorial using the %lld format specifier for long long.
return 0;: Indicate successful program execution.
}: Close the main function.

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to calculate the factorial of a positive integer using a while loop and then count the trailing zeros in the result.
Write a C program to compute the factorial of a number using a while loop and determine whether the factorial is even or odd.
Write a C program to calculate the factorial of a positive integer using a while loop and then compute the sum of the digits of the factorial.
Write a C program to calculate the factorial using a while loop, but prompt the user for confirmation if the input number is greater than 10.

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