#include<stdio.h>
#include<stdbool.h>
int main(void)
{
    unsigned long num;
    unsigned long div;
    bool isprime;
    printf("please enter an integer for analysis;");
        printf("Enter q to quit.\n");
    while (scanf_s("%lu", &num) == 1)
    {
        for (div = 2, isprime = true; (div * div) <= num; div++)
        {
            if (num % div == 0)
            {
                if ((div * div) != num)
                printf("%lu is divisible by %lu and %lu.\n",
                    num, div, num / div);
            else
                printf("%lu is divisible by %lu.\n", num, div);
            isprime = false;
            }
        }
        if (isprime)
            printf("%lu is prime.\n", num);
        printf("please enter another integer for analysis;");
        printf("Enter q to quit.\n");
    }
    printf("Bye.\n");
    return 0;
}
isprime = true，isprime = false;，if (isprime)的作用是什么，请详细解释一下，谢谢。

isprime = true，isprime = false;，if (isprime)的作用是什么，请详细解释一下，谢谢。