Get random element from array without repeats in Java

Question

I'm creating a java GUI based program, and I have an array of questions. I need to create a method that gets the next question after I click a JButton. The next question should be chosen randomly and shouldn't repeat a previously-asked question. See the code below -- how would getNextQuestion be implemented?

public class QuestionBank {

    String [] questions;
    int currQuestionIndex;

    public QuestionBank() {
        questions = new String [10]; //increase array size if you will add more questions
        questions[0]= "Which three words describe you best?";
        questions[1]= "Which is your best feature?";
        questions[2]= "Which common saying or phrase describes you?";
        questions[3]= "What’s the best thing that’s happened to you this week?";
        questions[4]= "Who was your role model when you were a child?";
        questions[5]= "Who was your favorite teacher and why?";
        questions[6]= "What was your favorite subject at school?";
        questions[7]= "What did you want to be when you grew up?";
        questions[8]= "If you could have one wish come true what would it be?";
        questions[9]= "Which would you prefer — three wishes over five years or one wish right now?";
        //add more questions
    }

    public String getNextQuestion() {
        //get the next question, randomly from the array and without repeats
    }
}

Increment `currQuestionIndex` and verify it's not out bounds... — MadProgrammer, May 14 '18 at 00:08
@zlakad And the difference being?! Sorry finding it hard to see the issue — MadProgrammer, May 14 '18 at 00:10
@MadProgrammer He want to randomize the array. Is it not the difference? — zlakad, May 14 '18 at 00:12
Possible duplicate [How to randomly pick an element from an array](https://stackoverflow.com/questions/8065532/how-to-randomly-pick-an-element-from-an-array) should answer the first part of the question — MadProgrammer, May 14 '18 at 00:17
The second problem (uniqueness) would require a second array (of the same size), into which you would place the "used" indices of the questions you've already asked. Each time you "pick" a random question, you would need to check this second array to see if contains the index, if it does, you try and pick again, if it doesn't, you can use the index and it would be added to the second (used) array — MadProgrammer, May 14 '18 at 00:20

k_ssb · Accepted Answer · 2018-05-14T00:32:25.513

If you want to get items from a collection in a random order without repeats (as clarified in the comments), you can first shuffle the collection and then iterate through the collection normally.

In Java it's easier to shuffle an ArrayList compared to an array, so consider the following:

public class QuestionBank {

    List<String> questions;
    int currQuestionIndex;

    public QuestionBank() {
        questions = new ArrayList<>(); // no size needed
        questions.add("Which three words describe you best?");
        questions.add("Which is your best feature?");
        // add more questions
        currQuestionIndex = 0; // it's 0 by default but this makes it explicit
        Collections.shuffle(questions); // shuffle all questions
    }

    public String getNextQuestion() {
        // get the next question, randomly from the list and without repeats
        if (currQuestionIndex >= questions.size()) {
            return null; // no more questions!
        }
        String nextQuestion = questions.get(currQuestionIndex);
        currQuestionIndex++; // move forward in the shuffled list
        return nextQuestion;
    }
}

If you have to use an array instead of a List, see this question on shuffling an array. The rest of the strategy stays the same.

The likely hood is the OP isn't allowed to use `ArrayList` or `Set` or all the really nice stuff that would make this simple — MadProgrammer, May 14 '18 at 00:16

Rann Lifshitz · Answer 2 · 2018-05-14T05:23:29.173

How about using a Queue ?

import java.util.*;

public class QuestionBank {

    static Queue<String> questions;

    public QuestionBank() {
        List<String> tmpQuestions = new ArrayList<>(); // no size needed
        tmpQuestions.add("Which three words describe you best?");
        tmpQuestions.add("Which is your best feature?");
        tmpQuestions.add("Which common saying or phrase describes you?");
        tmpQuestions.add( "What’s the best thing that’s happened to you this week?");
        tmpQuestions.add("Who was your role model when you were a child?");
        tmpQuestions.add("Who was your favorite teacher and why?");
        tmpQuestions.add("What was your favorite subject at school?");
        tmpQuestions.add("What did you want to be when you grew up?");
        tmpQuestions.add("If you could have one wish come true what would it be?");
        tmpQuestions.add( "Which would you prefer — three wishes over five years or one wish right now?");
        Collections.shuffle(tmpQuestions, new Random()); // shuffle all questions

Using ArrayDeque as suggested by pkpnd with the following reason

        questions = new ArrayDeque<>(tmpQuestions); 
    }
    public String getNextQuestion() {
        // get the next question, from the randomly populated queue
        if (questions.isEmpty()) {
            return null; // no more questions!
        }
        return questions.remove();
    }

    public static void main(String[] args) {
        QuestionBank qb = new QuestionBank();
        String question;
        while((question = qb.getNextQuestion()) != null){
            System.out.println(question);
        }
    }
}

And the outputs are:

    // 1st execution
Who was your role model when you were a child?
What?s the best thing that?s happened to you this week?
Which common saying or phrase describes you?
Which is your best feature?
Which three words describe you best?
What was your favorite subject at school?
Which would you prefer ? three wishes over five years or one wish right now?
If you could have one wish come true what would it be?
Who was your favorite teacher and why?
What did you want to be when you grew up?

//2nd execution
Which common saying or phrase describes you?
What did you want to be when you grew up?
What?s the best thing that?s happened to you this week?
Which three words describe you best?
What was your favorite subject at school?
Which would you prefer ? three wishes over five years or one wish right now?
Who was your role model when you were a child?
Which is your best feature?
Who was your favorite teacher and why?
If you could have one wish come true what would it be?

//3rd execution and so on ....
What?s the best thing that?s happened to you this week?
Which common saying or phrase describes you?
Which is your best feature?
What was your favorite subject at school?
Which would you prefer ? three wishes over five years or one wish right now?
If you could have one wish come true what would it be?
What did you want to be when you grew up?
Which three words describe you best?
Who was your role model when you were a child?
Who was your favorite teacher and why?

score 0 · Answer 3 · answered May 14 '18 at 00:26

If you want to use the int currQuestionIndex to access the next question in the array, make sure you initialise it in the constructor!

i.e.

public QuestionBank() {
    // Initialise and define 'questions' String array here ...
    this.currQuestionIndex = 0;
}

Assuming you just want to access the current String in the questions array, and then advance to the next question for the next time the method is called:

public String getNextQuestion() {
    // Wrap around when we overshoot the array
    if (currQuestionIndex > questions.length)
        currQuestionIndex = 0;

    currQuestionIndex++;
    return questions[currQuestionIndex-1];
}

Also can select a random question, make sure you import java.util.Random:

public String getRandomQuestion() {
    Random rand = new Random();
    return questions[rand.nextInt(questions.length)];
}

Get random element from array without repeats in Java

3 Answers3