cpp/CPrimerPlus6E/Ch09/factor.c

// factor.c -- uses loops and recursion to calculate factorials
#include <stdio.h>
long fact(int n);
long rfact(int n);
int main(void)
{
    int num;
    
    printf("This program calculates factorials.\n");
    printf("Enter a value in the range 0-12 (q to quit):\n");
    while (scanf("%d", &num) == 1)
    {
        if (num < 0)
            printf("No negative numbers, please.\n");
        else if (num > 12)
            printf("Keep input under 13.\n");
        else
        {
            printf("loop: %d factorial = %ld\n",
                   num, fact(num));
            printf("recursion: %d factorial = %ld\n",
                   num, rfact(num));
        }
        printf("Enter a value in the range 0-12 (q to quit):\n");
    }
    printf("Bye.\n");
    
    return 0;
}

long fact(int n)     // loop-based function
{
    long ans;
    
    for (ans = 1; n > 1; n--)
        ans *= n;
    
    return ans;
}

long rfact(int n)    // recursive version
{
    long ans;
    
    if (n > 0)
        ans= n * rfact(n-1);
    else
        ans = 1;
    
    return ans;
}