有没有办法在过滤非素数后打印素数本身和总计数？

Question

我有下面的代码并且有注释。它本质上是基于 'Sieve of Eratosthenes.' 我正在修改它以在代码的最后一个 for 循环中打印和计算剩余的质数。但是，输出是“1There are 1 primes.”

#include <stdio.h>

#define SIZE 50000

int main(void) {
    int i, mult, count, count2;
    int flag[SIZE + 1];

    count = 0;
    count2 = 0;

    //sets up all numbers
    for (i = 1; i <= SIZE; i++) {
        flag[i] = 1;
        //printf("%d",flag[i]);
    }

    //step 1: selects the prime number
    for (i = 2; i <= SIZE; ++i) {
        if (flag[i] == 1)
            ++count;
        mult = i;

        //step 2: filters out numbers multiple of that prime number in step 1
        while (mult <= SIZE) {
            flag[mult] = 0;
            mult = mult + i;
        }
    }

    //Now that the non-prime numbers have been filtered, this then prints the number and counts the prime numbers
    for (i = 1; i <= SIZE; i++) {
        if (flag[i] == 1) {
            ++count2;
            printf("%d", i);
        }
    }

    printf("There are %d primes. \n", count2);

    return 0;
}

Answer 1

在你的第二个 for 循环中，你开始于：

mult = i;

然后在 while 中，就在您设置之后：

flag[mult] = 0;

本质上你说这个数不是素数。

如果将行 mult = i 替换为：

mult = i + i ;

那么你的程序运行正常了。

有没有办法在过滤非素数后打印素数本身和总计数？

Is there a way to print the prime number itself and the overall count after having filtered non-prime?

c

arrays

sieve-of-eratosthenes