1、

#include <stdio.h>

int fac(int x)
{
    int i, fac = 1;
    if(x > 0)
    {
        for(i = x; i > 0; i--)
             fac *= i;
    }
    else
        fac = 1;
    return fac;
}

int main(void)
{
    int a;
    printf("a = "); scanf("%d", &a);
    
    printf("result: %d\n", fac(a));
    
    return 0;
}

 

 

2、

#include <stdio.h>

int fac(int x)
{
    int i, fac = 1;
    if(x > 0)
    {
        for(i = 1; i <= x; i++)
        {
            fac *= i;
        }
    }
    else
        fac = 1;
    return fac;
}

int main(void)
{
    int a;
    puts("please input an integer.");
    printf("a = "); scanf("%d", &a);
    
    printf("the result: %d\n", fac(a));
    
    return 0; 
}