Question

我知道我可以使用一个额外的变量计算dvisors但我的问题是为什么这条线不行？ prime = num / 1 == num＆amp;＆amp; num / num == 1; 另请查看以下有问题代码中的注释。感谢。

#include <stdio.h>
int main()
{
    long num = 0;
    long req = 0;
    int control = 0;
    int div = 1;             
    int prime;

    printf("Give starting number: ");
    scanf("%d", &num );
    printf("\nGive required numbers: ");
    fflush(stdin);
    scanf("%d", &req);;
    while(req > control)
    {       
        printf("\nNumber is: %d ", num);
        prime = num / 1 == num && num / num == 1; /*This lines here why dosnt thi work?*/

        if(prime)   /*Also here lets say the above line work with what do i avaluate the if with so it only prints tha asterisk for the prime numbers?*/
            printf("* ");
        printf("Equal Divisors are: ");

            while(div <= num)
            {
                if(num % div ==0)
                    printf("%d ", div);
                div++;
            }

        if(req == control)
            break;

        num++;
        control++;
        div=1;                      


        printf("\n");       
    }
    return 0;
}

Answer 1

@Leeor在他/她的评论中指出了这一点：那不是你如何检查素数。每个号码都符合您的标准。

Primes是仅由它们自己和1均分的数字。所以你必须遍历所有其他除数，最多为num / 2，并确保它们不给出整数结果。（从技术上讲，你只需循环遍历所有PRIME数字除数 - 2,3,5,7,11等 - 但这样做更难。）

编辑：正如@thb在评论中指出的那样，你必须遍历所有其他除数，最多为sqrt（num），而不是num / 2.

这是一些伪代码：

Set a Boolean variable to "True" (meaning "yes, it's a prime").
Loop from 2 to sqrt(num).
    If num divided by the loop counter is an integer, then...
        It's NOT a prime number.
        Set the Boolean variable to "False".
        If you can exit the loop there, great. But no big deal if you can't.
After the loop, if the Boolean variable = "True" then it's a prime number.  If it's "False" then it's not a prime number.

Answer 2

一个问题是表达式的排序。这条线

prime = num / 1 == num && num / num == 1;

相当于

{
    const bool a = ((num / 1) == num);
    const bool b = ((num / num) == 1);
    prime = a && b;
}

这是一个容易犯的错误。无论如何，有人怀疑prime = a && b;是你的意思。

找不到素数

2 个答案: