【Mooc笔记】数组操作（找出100以内的素数）

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本文对比了两种寻找100以内素数的方法：一种是通过遍历排除法，另一种则是利用条件列表排除倍数。两种方法各有优劣，展示了不同的数组操作技巧和优化策略。

数组操作（找出100以内的素数）
Array_operation(Prime_number_1&2)
方法一：

#include <stdio.h>
//find out all the prime numbers smaller than NUMBERS
//solution1: Traverse all the numbers, eliminating composite numbers one by one.

int check_is_prime(int test_number, int prime_list[], int count_of_current_numbers){
    int ret=1;
    int i;
    for(i=0;i<count_of_current_numbers;i++){
        if(test_number%prime_list[i]==0){
            ret=0;
        }
    }
    return ret;
}

int main(void){
    const int NUMBERS = 100;
    int prime_list[NUMBERS]={2};
    int cnt=1; //cnt records the current number of prime numbers
    int i;
    i=3;
    while(i<NUMBERS){
        if(i%2!=0){
            if(check_is_prime(i,prime_list,cnt)){
                prime_list[cnt++]=i;
                printf("successfully added %d\n",i);
                printf("now cnt=%d\n",cnt);
            }
        }
        i++;
    }
    //output all the prime numbers smaller than the given number
    i=0;
    while (i<cnt){
        printf("%d\n",prime_list[i]);
        i++;
    }
    return 0;
}

方法二：

#include <stdio.h>
//find out all the prime numbers smaller than NUMBERS
//solution2:Another method of elimination. Rule out all the multiples of the current prime numbers.

int main(void){
    const int NUMBERS = 100;
    int number_condition_list[NUMBERS]={1,1,0};  //1 means it has not been ruled out, 0 means it may be a prime number
    for(int i=2;i<NUMBERS;i++){
        if(number_condition_list[i]==0){
            for(int j=2;j*i<NUMBERS;j++){
                number_condition_list[j*i]=1;  //Rule out all the multiples of i
            }
        }
    }
    //output all the prime numbers
    for(int i=0;i<NUMBERS;i++){
        if(number_condition_list[i]==0){
            printf("%d\n",i);
        }
    }
    return 0;
}