java - `++arr[1+r.nextInt(6)];` 这行代码是做什么的

Question

我想知道，当我从教程中学习Java时，有一个程序可以掷骰子1000次并打印其频率。

import java.util.Random;
public class RollDicewitharray {

    public static void main(String[] args) {
        Random r=new Random();
        int arr[]= new int[7];

        System.out.println("diceNo.\tFrequency");
        for (int roll = 1; roll < 1000; roll++) {

            ++arr[1+r.nextInt(6)];                  /* this line */

        }
        for (int i = 1; i < arr.length; i++) {
            System.out.println(i+"\t"+arr[i]);
        }

    }

Answer 1

总而言之，该程序模拟滚动六面骰子 1000 次，并记录每个滚动数字的出现。

public static void main(String[] args) {
    Random r=new Random();
    int arr[]= new int[7];                    //Craete an array with 7 int elements
    System.out.println("diceNo.\tFrequency");

    for(int roll=1;roll<1000;roll++){         //Loop 1000 times
        ++arr[1+r.nextInt(6)];                //Randomly pick arr[1] to 
    }                                         //arr[6] and plus one to it        

    for(int i=1;i<arr.length;i++){
        System.out.println(i+"\t"+arr[i]);    //Print occurrence of 1-6
    }    
}

破坏以下代码:

++arr[1+r.nextInt(6)];     //r.nextInt(6) will be evaluated first:

<小时/>

r.nextInt(6) 返回 (0-5) 的随机值，因此您有:

++arr[1+(random 0 to 5)];  //+1 will be evaluated next:

因此，您将生成一个随机值 1-6。接下来将 1 添加到数组中:

++arr[random 1 to 6];      //+1 to arr[1] or arr[2] or arr[3] or arr[4] or arr[5] or arr[6]

现在可以解释为:

arr[1] +=1;  //or
arr[2] +=1;  //or
arr[3] +=1;  //or
arr[4] +=1;  //or
arr[5] +=1;  //or
arr[6] +=1;

<小时/>

因此，运行程序后，如果您的数组如下所示:

 [0] [1] [2] [3] [4] [5] [6]   Array index
+---+---+---+---+---+---+---+
| 0 |175|170|165|170|165|175| <-- arr 
+---+---+---+---+---+---+---+

It means 1 was rolled 175 times, 
2 was rolled 170 times, 
3 was rolled 165 times,
and so on..