How can I use math.random but only for certain values?

Question

Here's what I have (it's just an excerpt from my entire code):

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

How can I get it to only come up with a random number of 0, 1, or 2?

in a return statement I have later on, it says that the letter that is acquired here results in a "null"

Here's the entire code if it helps:

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;
}
}

Here's my runner class since someone pointed out I'm not calling any method anywhere:

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

Use the Modulo operator :

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

Math.random() returns a pseudo-random value between 0 and 1.

Multiplying by 100 makes it a number (with decimal values) between 0 and 100.

Casting to (int) drops the decimal value - same as Math.floor() if you have that in your language.

Modulo 3 is the remander of diving by 3: For example, 55 / 3 = 18 remainder 1 (in other words, (18 * 3) + 1 = 55).

Answer 2

Math.random() returns a floating point value between 0 and 1.

If you multiply that by 3, you get a floating point value between 0 and 3.

If you cast the floating point value to an integer, you get 0, 1, or 2.

ie int v = (int) (Math.random() * 3);

If simple multiplication scares you, there is a java.util.Random class. You can do:

/** 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;
}

Using a Random instance allows you to plug-in a more advanced random number generator, which would be necessary if you intended to do stochastic experiments.

Answer 3

Based on the different comments I wanted to try another approach. I just copied your code into my IDE, and ran it.

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's approach is OK, it's just lacking a set of (). If your code is actually the code you gave above and compChoice is always null there's something horribly wrong.

Answer 4

I thought a good way to do this would just be to round the number after multiplying it. I couldn't get it to work the way a few of you answered but how about this?:

var a=Math.random()*3;
var num=Math.round(a);
switch(num)
{
case 0 : compChoice = "R";break;
case 1 : compChoice = "P";break;
case 2 : compChoice = "S";break;
}