为什么我的 FirstFactorial 程序即使在不满足条件后仍然循环回到 while 条件

Question

Here's the code snippet, this when run with number 4 outputs 2424242448484848288288288288576576576576 . Not sure as to why would the execution would jump back to while loop after exiting the function code. Any help will be appreciated. Thank you in advance.

#include <stdio.h>
#include <string.h>

int result = 1;
void FirstFactorial(int);

void FirstFactorial(int num) {
    // code goes here
    while (num > 0) {
        result = result * num;
        num--;
        FirstFactorial(num);
    }
    printf("%d", result);
}

int main(void) {
    int var;
    // keep this function call here
    printf ("Enter your no.\n");
    scanf("%d", &var);
    FirstFactorial(var);
    return 0;
}

Answer 1

Within the function

void
FirstFactorial(int num)
{
  // code goes here
  while(num > 0)
    {
      result = result * num;
      num--;
      FirstFactorial(num);
    }
  printf("%d", result);
}

each its iteration calls itself num times and all iterations together output the global variable result.

So for example in the first call of the function the function calls itself in the while loop for the range of values [num, 1].

Remove the while loop and do not use the global variable.

Here is a demonstrative program.

#include <stdio.h>

unsigned long long int factorial( unsigned long long int n )
{
    return n < 2 ? 1 : n * factorial( n - 1 );
}

int main(void) 
{
    printf( "%llu! = %llu\n", 4llu, factorial( 4 ) );
    printf( "%llu! = %llu\n", 20llu, factorial( 20 ) );

    return 0;
}

The program output is

4! = 24
20! = 2432902008176640000

Pay attention that the maximum value you may specify is 20.

Answer 2

Either you implement the factorial with a loop or you do it recursively.

Both ways are feasible but your code mixes it up.

Answer 3

Your function mixes iterative and recursive approaches. You can correct it by removing the useless recursion which causes multiple intermediary results to be computed and printed. Defining result as a global variable is also a mistake, especially since you do not reinitialize it before the loop. Using type long long will allow for larger factorials to be computed. Adding a trailing \\n after the printf conversion specifier is advisable too.

Here is a corrected version:

#include <stdio.h>

void FirstFactorial(int num) {
    long long result = 1;

    while (num > 1) {
        result = result * num;
        num--;
    }
    printf("%lld\n", result);
}

int main(void) {
    int var;
    // keep this function call here
    printf("Enter your number\n");
    if (scanf("%d", &var) == 1)
        FirstFactorial(var);
    return 0;
}