C 求一个数的阶乘问题

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问题

Write a program that reads a nonnegative integer, and computes and prints its factorial

(For example, 5! = 5x4x3x2x1 = 120)

#include<stdio.h>
#include<stdlib.h>

int main(void) {

	int num;
	int product=1;
	int j;
	printf("Enter a positive integer:");
	scanf_s("%d", &num);

	if (num <= 0) {

		while (num < 0) {
			printf("Enter a positive integer:");
			scanf_s("%d", &num);
		}
		
		while (num = 0) {
			printf("0! is 1");
			break;
		}
	}

	else {
		j = num;   //实现储存num的原有值，不然会被下面的num=num-1改变

		while (num > 1) {
			product = product * num;
			num = num - 1;   //这一步最后的num=1,会导致最后的printf里面的num输出永远是1!
		}
	}


	printf("%d! is %d", j, product);
	system("pause");
	return 0;
}