我如何才能只对某些值使用math.random？

Question

下面是我所拥有的(这只是我的整个代码的摘录)：

int num = (int) Math.random()*100;
    switch(num)
    {
    case 0 : compChoice = "R";break;
    case 1 : compChoice = "P";break;
    case 2 : compChoice = "S";break;
    }

我如何才能让它只得到一个0、1或2的随机数？

在我后面的一个return语句中，它说这里获取的字母结果是一个"null“

如果有帮助，下面是完整的代码：

import java.util.Scanner;
import static java.lang.System.*;

public class RockPaperScissors
{
private String playChoice;
private String compChoice;

public RockPaperScissors()
{



}

public RockPaperScissors(String player)
{
    playChoice = player;
}

public void setPlayers(String player)
{
    playChoice = player;
    int num = (int) Math.random()*100 %3;
    switch(num)
    {
    case 0 : compChoice = "R";break;
    case 1 : compChoice = "P";break;
    case 2 : compChoice = "S";break;
    }
    out.print(compChoice);
}

public String determineWinner()
{
    String winner="";

    if(playChoice == "R")
    {
        switch(compChoice)
        {
        case "R" : winner = "!Draw Game!";break;
        case "P" : winner = "!Computer wins <<Paper Covers Rock>>!";break;
        case "S" : winner = "!Player wins <<Rock Breaks Scissors>>!";break;
        }
    }
    else if(playChoice == "P")
    {
        switch(compChoice)
        {
        case "R" : winner = "!Player wins <<Paper Covers Rock>>!";break;
        case "P" : winner = "!Draw Game!";break;
        case "S" : winner = "!Computer wins <<Scissors Cuts Paper>>!";break;
        }
    }
    else if(playChoice == "S")
    {
        switch(compChoice)
        {
        case "R" : winner = "!Computer wins <<Rock Breaks Scissors>>!";break;
        case "P" : winner = "!Player wins <<Scissors Cuts Paper>>!";break;
        case "S" : winner = "!Draw Game!";break;
        }
    }
    return winner;
}

public String toString()
{
    String output="";

    output = "player had " + playChoice + "\n computer had " + compChoice + "\n " + determineWinner();

    return output;
}
}

下面是我的runner类，因为有人指出我没有在任何地方调用任何方法：

import java.util.Scanner;
import static java.lang.System.*;

public class Lab10d
{
public static void main(String args[])
{
    Scanner keyboard = new Scanner(System.in);
    char response;

    //add in a do while loop after you get the basics up and running

        String player = "";

        out.print("Rock-Paper-Scissors - pick your weapon [R,P,S] :: ");

        //read in the player value
        player = keyboard.next();

        RockPaperScissors game = new RockPaperScissors(player);
        game.determineWinner();
        out.println(game);
            while(response == response)
    {
        out.print("Rock-Paper-Scissors - pick your weapon [R,P,S] :: ");
        player = keyboard.next();
        game.setPlayers(player);
        game.determineWinner();
        out.println(game + "\n");
        out.println("would you like to play again? (y/n):: ");
        response = (char) keyboard.next();




    }

}
}

Answer 1

使用Modulo operator

int num = (int) Math.random()*100 % 3;

Math.random()返回一个介于0和1之间的pseudo-random value。

乘以100会得到一个介于0和100之间的数字(带有十进制值)。

强制转换为(int)会丢弃十进制值-与Math.floor()相同，如果您的语言中有该值的话。

模3是跳水的余数为3:例如，55 /3= 18余数1(换句话说，(18 * 3) +1= 55)。

Answer 2

Math.random()返回一个介于0和1之间的浮点值。

如果将其乘以3，则得到一个介于0和3之间的浮点值。

如果将浮点值转换为整数，则会得到0、1或2。

即int v = (int) (Math.random() * 3);

如果你害怕简单的乘法运算，这里有一个java.util.Random类。您可以执行以下操作：

/** Re-usable source of random numbers */
private Random rand = new Random();

/** Choose a value of 0, 1, or 2 */
public int myChoice() {
    int v = rand.nextInt(3);
    return v;
}

使用Random实例允许您插入更高级的随机数生成器，如果您打算进行随机实验，这将是必要的。

Answer 3

基于不同的评论，我想尝试另一种方法。我只是将您的代码复制到我的IDE中，并运行它。

public class NewMain {
    public static void main(String[] args) {
        int num;
        String compChoice = "";
        for (int ii = 0; ii < 10; ii++) {
            num = (int) (Math.random() * 100) % 3;

            switch (num) {
                case 0:
                    compChoice = "R";
                    break;
                case 1:
                    compChoice = "P";
                    break;
                case 2:
                    compChoice = "S";
                    break;
            }

            System.out.println(num + " " + compChoice);
        }
    }
}

Gregmac的方法还可以，只是缺少一组()。如果您的代码实际上就是上面给出的代码，并且compChoice总是为空，那么一定是出了什么大问题。