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).