c - 使用 C 查找素数不起作用

Question

我目前正在学习 C 并试图解决一个问题。我需要使用数组和循环找到从 2 到 100 的所有素数。

我已经知道这个问题的解决方案，但是我无法找到代码中的错误。这是我第一次使用 StackOverflow，所以希望我正确地评论了所有内容:)

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

int main(){

  int prime_numbers[50] = {2,3};                //initializes the array which will be printed at the end
  int counter = 1;                              //initializes the index of the last prime element in array
  int checker = 0;                              //initializes a checker used to determine if the number is prime

  for(int i = 5; i <= 100; i++) {               //goes through numbers 5 to 100 as the first two primes are hard coded
    for(int j = 0; j <= counter; j++){          //goes through array untill it reaches last prime using the before initialized counter
        if(i % prime_numbers[j] != 0) {         //check to see if a number that is being checked is not divisible by j'th element in array
            checker++;                          //if so, checker is incremented
        }
        if(checker == counter + 1) {            //check to see if number was not divisible by any prime in our array
            checker = 0;                        //if so checker is reset to 0 for the next iteration
            ++counter;                          //counter is incremented as there is one more prime in our array
            prime_numbers[counter] = i;         //add inside array the found prime number
            break;                              //break should not be necessary, however for some reason, it yields a different result when I don't put it in
        }                                       //most likely the error in the code. Need to find out why loop does not stop after second if is done
    }
  }

  for(int g = 0; g <= 50; g++) {                //prints out all the prime numbers in array
    if(prime_numbers[g] != 0) {
        printf("%d ", prime_numbers[g]);
    }
  }

  return 0;
}

我希望程序打印从 0 到 100 的所有质数，中间有空格。

Answer 1

该程序还可以查找非素数。它的逻辑是一个相当奇怪的颠倒。候选者只能被数组中的一个素数整除。该报告还打破了数组界限。

更正后的代码:

#include <stdio.h>

int main(void) {                            // correct function signature

    int prime_numbers[50] = {2,3};
    int counter = 1;
    int checker;                            // initialise within each loop

    for(int i = 5; i <= 100; i++){
        checker = 0;                        // inintialise here
        for(int j = 0; j <= counter; j++) {
            if(i % prime_numbers[j] == 0) { // opposite test to yours
                checker++;                  // flag a non-prime
                break;
            }
        }

        if(checker == 0) {                  // moved outside the loop
            ++counter;
            prime_numbers[counter] = i;
        }
    }

    for(int g = 0; g < 50; g++) {           // corrected the bounds error
        if(prime_numbers[g] != 0){
            printf("%d ", prime_numbers[g]);
        }
    }
    printf("\n");                           // flush the output buffer
    return 0;
}

程序输出:

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97

The reporting loop relies on the array being initialised to 0s and a better way would be

for(int j = 0; j <= counter; j++) {
    printf("%d ", prime_numbers[j]);
}
printf("\n");