Guessing Game Problem

Question

I'm having a problem thinking of a way to think of a solution to this. Basically, what I am trying to do is have a guessing game with the computer. What happens is I think of a number from 1000 to 9999, let's say 3741. Then the computer randomnly guesses from an ArrayList of 1000-9999, and asks if its the right number, or not. Let's say the computer guesses 5312. I would respond by saying how many matches there are 0-4, in this case 4. Then my idea is to update the arrayList, by deleting all the necessary values. I would do this continuously until one number is left, which would be the correct answer. I am trying to think of an algorithm that would do this in 15 guesses or less (aiming for 10ish).
My code so far:
// fill in your information in the line below, and comment it out in order to compile
// Name: ???   Student ID: ???;
import java.util.ArrayList;
import java.util.Random;
import javax.swing.JOptionPane;
public class Assignment2 {
private int totalGuesses;
private ArrayList<Integer> guess;
private int myGuess;
public Assignment2 ( ){
totalGuesses = 0;
myGuess = 0;
guess = new ArrayList<Integer>();
for(int i = 1000; i <= 9999 ; i++) // Adds numbers 1000 to 9999 initially.
   guess.add(i);
public int myGuessIs() {
  return myGuess;
public int totalNumGuesses() {
  return totalGuesses;
public void updateMyGuess(int nmatches) {
  // fill in code here
  // update the guess based on the number of matches claimed by the user
if(guess.size() == 0)
  myGuess = -1;
// you shouldn't need to change the main function
public static void main(String[] args) {
  Assignment2 gamer = new Assignment2( );
  JOptionPane.showMessageDialog(null, "Think of a number between 1000 and 9999.\n Click OK when you are ready...", "Let's play a game", JOptionPane.INFORMATION_MESSAGE);
  int numMatches = 0;
  int myguess = 0;
  do {
   myguess = gamer.myGuessIs();
   if (myguess == -1) {
    JOptionPane.showMessageDialog(null, "I don't think your number exists.\n I could be wrong though...", "Mistake", JOptionPane.INFORMATION_MESSAGE);
    System.exit(0);
   String userInput = JOptionPane.showInputDialog("I guess your number is " + myguess + ". How many digits did I guess correctly?");
   // quit if the user input nothing (such as pressed ESC)
   if (userInput == null)
    System.exit(0);
   // parse user input, pop up a warning message if the input is invalid
   try {
    numMatches = Integer.parseInt(userInput.trim());
   catch(Exception exception) {
    JOptionPane.showMessageDialog(null, "Your input is invalid. Please enter a number between 0 and 4", "Warning", JOptionPane.WARNING_MESSAGE);
    numMatches = 0;
   // the number of matches must be between 0 and 4
   if (numMatches < 0 || numMatches > 4) {
    JOptionPane.showMessageDialog(null, "Your input is invalid. Please enter a number between 0 and 4", "Warning", JOptionPane.WARNING_MESSAGE);
    numMatches = 0;
   if (numMatches == 4)
    break;
   // update based on user input
   gamer.updateMyGuess(numMatches);
  } while (true);
  // the game ends when the user says all 4 digits are correct
  System.out.println("Aha, I got it, your number is " + myguess + ".");
  System.out.println("I did it in " + gamer.totalNumGuesses() + " turns.");
Obviously, I'm having trouble thinking of a solution to updateMyGuess...I don't know why but I'm most likely making this way harder than it actually is.. Can anyone help?

Answer 1

borisan wrote:
What happens is I think of a number from 1000 to 9999, let's say 3741. Then the computer randomnly guesses from an ArrayList of 1000-9999, and asks if its the right number, or not. Let's say the computer guesses 5312. I would respond by saying how many matches there are 0-4, in this case 4. Don't you mean 0?
What if computer's guess is 2795. I assume number of matches then would be 1 (the 7). How exactly is your code going to know that 7 is the lone match and not the 2 or the 9 or the 5?