The factorial of a positive integer n is equal to 1*2*3*...n. You will learn to calculate the factorial of a number using for loop in this example.

The factorial of a positive number n is given by:

factorial of n (n!) = 1*2*3*4....n

The factorial of a negative number doesn't exist. And, the factorial of 0 is 1, 0! = 1

Example: Factorial of a Number

#include <stdio.h>
int main()
{
    int n, i;
    unsigned long long factorial = 1;

    printf("Enter an integer: ");
    scanf("%d",&n);

    // show error if the user enters a negative integer
    if (n < 0)
        printf("Error! Factorial of a negative number doesn't exist.");

    else
    {
        for(i=1; i<=n; ++i)
        {
            factorial *= i;              // factorial = factorial*i;
        }
        printf("Factorial of %d = %llu", n, factorial);
    }

    return 0;
}

Output

Enter an integer: 10
Factorial of 10 = 3628800

This program takes a positive integer from the user and computes factorial using for loop.Since the factorial of a number may be very large, the type of factorial variable is declared as unsigned long long.

If the user enters negative number, the program displays error message.