为什么这个 c 程序无法编译阶乘

Question

#include <stdio.h>

int FAC(int a)
{
    if (a >= 1, a--)
    {
        return a + 1 * a;
        FAC(a);
    }
}

int main() 
{
    int a = 0;
    int ret = 0;
    scanf("%d", &a);
    ret = FAC(a);
    printf("%d\n", ret);
}

if I input 5 the outcome is 8

But in the first function should't it be

5>=1 5-1 return 5*4 4>=1...

Answer 1

First of all, you're returning a value before you're using recursion. The return keyword takes effect immediately, so all statements following this won't be executed.

Also, your factorial function does not actually calculate the factorial of a .

Examples for factorial functions:

Recursive

int factorial(int n) {
  if(n > 0) {
    return n * factorial(n - 1); // n! = n * (n-1) * (n-2) * ... * 1
  } else {
    return 1;
  }
}

Iterative

// Does exactly the same, just an iterative function
int factorial(int n) {
  int fac = 1;
  for(; n > 0; n--) {
    fac *= n;
  }
  return fac;
}

Answer 2

#include <stdio.h>  

int FAC(int a)
{
    if (a < 0) {
        return -1;
    } else {
        if (a == 0) {
            return 1;
        } else {
            return (a * FAC(a-1));
        }
    }
} 

int main()  
{  
  int a = 0;  
  int ret = 0;  
  scanf("%d", &a);   
  ret = FAC(a);
  if (ret == -1) {
    printf("%s\n", "Input was a negative integer.");
  } else {    
    printf("%d\n", ret); 
  }
}  

Answer 3

Your factorial function ( FAC ) should ideally be something like:

unsigned int FAC(unsigned int a)
{
    // base condition - break out of recursion
    if (a <= 1)
        return 1;

    return a * FAC(a - 1);
}

unsigned int restricts range of argument to be in [0, UINT_MAX] .

Do note that FAC returns a unsigned int , so you may be able to provide argument value up to 12, else there will be an overflow and you can see weird output.

BTW, UINT_MAX is defined in limits.h