java - 从数组中分离质数

Question

public class HelloWorld {

    public static void main(String []args) {

        int [] arr = {2, 5, 9, 6, 7, 13, 24, 42, 8};
        int [] arr1 = new int[4];
        int [] arr2 = new int[arr.length - arr1.length];

        for(int i = 0; i < arr.length; i++) {
            for(int j = 2; j <= arr[i]/2; j++) {
                if(arr[i] % j == 0) {
                    System.out.println("Number is not prime " + arr[i]);
                    break;
                }
                else {
                    System.out.println("Number is prime " + arr[i]);
                    break;
                }
            }
        }
    }
}

程序应该检查数字数组并打印给定数字是否为质数。有问题，因为前 2 个未标记为素数。那我就不知道为什么把9当质数了。

Answer 1

您的代码中存在多个问题。

1. 为什么不把 2 当作质数。正如其他人所解释的那样，j <= arr[i]/2是罪魁祸首。根据这个条件，j <= 1和 j == 2 .因此，循环不会被执行。

2. 为什么取9为质数。第一次当j == 2和 arr[i] == 9 .作为，9%2 != 0 , 数字打印为素数。

   for(int j = 2; j <= arr[i]/2; j++){
       if(arr[i] % j == 0){
           System.out.println("Number is not prime " + arr[i]);
           break;
       }
       else {
           System.out.println("Number is prime " + arr[i]);
           break;
       }
   }
   

建议: 而不是检查 arr[i]/2 ，您可以使用数字的平方根来检查。

如有需要可引用以下程序:

public class PrimeNumber {
    public static void main(String []args){

        int [] arr = {2,3,4,5,9,6,7,13,24,42,8,400,101};
        int [] arr1 = new int[4];
        int [] arr2 = new int[arr.length - arr1.length];
        boolean flag = true;
        for(int i = 0; i < arr.length; i++){
            if(arr[i] == 2 || arr[i] == 3 )
            {
                System.out.println("Number is prime " + arr[i]);
                continue;
            }
            flag = true;
            for(int j = 2; j <= Math.sqrt( arr[i] ); j++){
                if(arr[i] % j == 0){
                    System.out.println("Number is not prime " + arr[i]);
                    flag = false;
                    break;
                }
            }
            if ( flag )
            {
                System.out.println("Number is prime " + arr[i]);
            }
        }
    }
}