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.

Factorial of a Number Using Recursion

#include<stdio.h>
long int multiplyNumbers(int n);
int main(){
    int n;
    printf("Enter a positive integer: ");
    scanf("%d",&n);
    printf("Factorial of %d = %ld", n, multiplyNumbers(n));
    return 0;
}

long int multiplyNumbers(int n){
    if (n>=1)
        return n*multiplyNumbers(n-1);
    else
        return 1;
}

Output

Enter a positive integer: 6
Factorial of 6 = 720

Suppose the user entered 6.

Initially, multiplyNumbers() is called from main() with 6 passed as an argument.

Then, 5 is passed to multiplyNumbers() from the same function (recursive call). In each recursive call, the value of argument n is decreased by 1.

When the value of n is less than 1, there is no recursive call and the factorial is returned ultimately to the main() function.