程序说每个数字都是素数

Question

我已经创建了这个C程序，它应该告诉我1-25之间有多少素数，但是打印的每个数字都是素数。请帮忙

#include<stdio.h>

int main(void) {

int n = 1, counter = 0;
int i, flag = 0;

while ( n <= 25 ) 
{
    for ( i = 2; i <= (n/2); i++ ) 
    {
        if (n%i == 0) 
        {
            flag = 0;
        }
    }
    if (flag == 0) 
    {
        counter++;
        printf("%d is Prime Number.\n", n);
    } 
    else 
    {
        printf("%d is not Prime Number.\n", n);
    }
    n++;
}
return 0;
}

Answer 1

flag = 0;

您的旗帜始终为0。进入1循环

时，您必须将其设置为while

while ( n <= 25 ) {
    flag = 1; //<-- here
    for ( i = 2; i <= (n/2); i++ ) {

您也可以在细分中break，仅用于效率

if (n%i == 0) 
{
    flag = 0;
    break;
}

Answer 2

您可以将代码用作

for ( i = 2; i <= sqrt(n); i++ ) 
    {
        if (n%i == 0) 
        {
            flag = 0;
        }
    }
    if (flag == 0) 
    {
        counter++;
        printf("%d is Prime Number.\n", n);
        flag=1;
    } 
    else 
    {
        printf("%d is not Prime Number.\n", n);
    }
    n++;

使用sqrt()代替n/2程序会更有效率。