程序代码：

#include<stdio.h>
void main()
{
    float fac(int n);
    int n;
    float y;
    printf("input an number:");
    scanf("%d\n", &n);
    y = fac( n );
    printf("%d! = %10.0f\n",n , y);
}
float fac(int n)
{
    float f;
    if(n < 0);
    {
        printf("n < 0,dataerror!");
    }
    else
       if(n == 0 || n== 1)
       f=1;
    else
       f = fac(n - 1) * n;
        return ( f );
}