4. Write a C program function called ‘factorial’ that calculates and returns the factorial of a number.

// Online C compiler to run C program online
#include <stdio.h>

int main() {
    int  n;
    int factorial = 1;
    printf("Enter a number: ");
    scanf("%d", &n);
    if(n<0)
    printf("Error!");
    else{
        for(int i = 1; i <= n; ++i){
            factorial *= i;
        }
        printf("Factorial of %d = %d", n,factorial);
    }

    return 0;
}