初学素数筛选打表法

创建一个数组 从2开始找是素数数组当前下标为true 再把当前下标的倍数标为false 因为质数的倍数一定不是质数。打好表就可以用了

import java.util.Arrays;import java.util.Scanner;public class Main {@SuppressWarnings("resource")public static void main(String[] args) {Scanner sc = new Scanner(System.in);while (sc.hasNext()) {int n = sc.nextInt();boolean arr[] = new boolean[n + 1];int arr1[] = new int[n + 1];Arrays.fill(arr, true);for (int i = 0; i < arr1.length; i++) {arr1[i]=i;}int sum = 0;for (int i = 2; i < arr1.length; i++) {if (arr[i] == true) {sum++;int flag = 0;for (int j = 2; j <= Math.sqrt(i); j++) {if (i % j == 0) {flag++;break;}}if (flag == 0) {for (int j = 2; j * i < arr1.length; j++) {arr[i * j] = false;}}}}System.out.println(sum);}}}