Function Call in C Language: Calculating the Difference of Factorials of Two Numbers

It has been a long time since I wroteC language, and my memory has become quite vague. After recalling and flipping through books, I managed to write a runnable program. The last program was from September 15. [C Language Program 10] Calculation of Factorial of Positive Integers

After further corrections and improvements byKimi, the following program code was produced. I have looked at the code several times, and I can still understand what functions, formal parameters, and actual parameters are.

#define _CRT_SECURE_NO_WARNINGS#include <stdio.h>          // Standard Input Output Library#include <stdlib.h>         // General Utility Library (includes abs, malloc, etc.)#include <math.h>           // Declaration of Mathematical Functions
// Calculate the factorial of n, returning unsigned long long typeunsigned long long fac(int n){    int i;    unsigned long long f = 1;    n = abs(n);             // Allow negative input, treat as positive    for (i = 1; i <= n; ++i)        f *= i;    return f;}
int main(void){    int x, y;    unsigned long long c1, c2;
    printf("Please enter numbers x, y (negative numbers will be treated as positive): ");    scanf("%d%d", &x, &y);
    c1 = fac(x);            // Call factorial function    c2 = fac(y);            // Call factorial function
    printf("The difference of the factorials of the two numbers is: %llu\n", c1 - c2);    return 0;}

Running Result:

请输入数字 x, y（负数将按正数处理）: 53两个数阶乘之差是: 114

请输入数字 x, y（负数将按正数处理）: 2019两个数阶乘之差是: 2311256907767808000

When calculating 100! − 99! , both have exceededunsigned long long representation range (≥2⁶⁴), resulting in modulo 2⁶⁴ wraparound; coincidentally obtaining the same remainder, thus the difference is0. This is not a true mathematical equality, but a pseudo-zero result caused by finite word length overflow.

请输入数字 x, y（负数将按正数处理）: 10099两个数阶乘之差是: 0 （错误答案）