2023/08/16

练习题:生成一个顺序数组,将这个数组的元素打乱顺序后输出

package 练习;

import java.util.Arrays;
import java.util.Random;

public class ShuffleArray {
    public static void main(String[] args) {
        int[] a = f();
        for(int i=0; i<a.length; i++) {
            System.out.println(a[i]);
        }
        System.out.println("\n\n----------------");

        shuffle(a);
        System.out.println(Arrays.toString(a));
    }

    public static int[] f() {
        int[] a = new int[5];
        for(int i=0; i<a.length; i++) {
            a[i] = i+1;
        }

        return a;
    }

    public static void shuffle(int[] a) {

        for (int i = 0; i < a.length; i++) {
            int j = new Random().nextInt(a.length);
            int t = a[i];
            a[i] = a[j];
            a[j] = t;
        }
    }
}

练习题:接收用户输入的数字,判断在此范围内质数的个数

package cn.cxy.exec;

import java.util.Scanner;

public class CountPrimeNumber {
    public static void main(String[] args) {
        System.out.println("输入整数n，求n内质数的数量");
        int n = new Scanner(System.in).nextInt();

        count(n);
    }

    public static void count(int n) {
        if(n<2) {
            System.out.println("没有质数");
            return;
        }
        if(n==2) {
            System.out.println("有1个质数");
            return;
        }
        int count = 1;//已知有一个质数
        outer:   //从3到n寻找质数
        for(int i=3; i<=n ;i++) {
            double max = 1+ Math.sqrt(i);
            for(int j=2; j<max; j++) {//在2到<max，找能把i整除的数
                if(i%j == 0) {
                    continue outer;
                }
            }
            count++;
        }
        System.out.println(n+"内质数的数量："+count);
    }
}