How to randomly pick element from array without repeating?

Question

I am working on an quiz app. I managed to randomized the questions(from an array) with

int position = new Random().nextInt(questions.length);

but the questions keep on repeating. How do i make it stop getting when it reaches lets say 10 questions without repeating? Here is my code if it helps:

gcfQuiz.setText(questions[position]);

    gcfButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {

            EditText Answer = (EditText) findViewById(R.id.txtAns1);
            String finAns = Answer.getText().toString();

            if (finAns==answers[position]){

                correct++;
            }
            position++;
            if (position<questions.length)
            {
                gcfQuiz.setText(questions[position]);

            }else {
                Intent in = new Intent(getApplicationContext(), gcfResult.class);
                startActivity(in);
            }
        }
    });

Use ArrayList and remove the question after you randomly picked. — additionster, Jan 05 '17 at 09:30

score 2 · Answer 1 · edited May 23 '17 at 12:01

You could use a List instead, and then remove the element if it is chosen:

List<Question> questions = new ArrayList<Question>();
// For Question class, see below.

// Get some random valid index from the list.
int index = new Random().nextInt(questions.size());
// Remove the question from the list and store it into a variable.
Question currQuestion = questions.remove(i);

For your information, I assumed the Question class looks like this. This approach is generally better than two separate arrays with questions and answers respectively:

class Question {

    private String text;
    private String answer;

    public Question(String text, String answer) {
        this.text = text;
        this.answer = answer;
    }

    public boolean isCorrectAnswer(String inputAnswer) {
        return Objects.equals(answer, inputAnswer);
    }
}

Some tips

I see you are using == to compare Strings. Never ever do that. Use equals() instead. See How do I compare strings in Java?.
I see you have a variable named Answer. According to the Java Naming Conventions, variables should start with a lowercase character. See Code Conventions at the Oracle website.
See above; you could transfer the responsibility of checking whether an answer is correct to the Question class, because it holds the answer string. See An Object Must be Responsible for Itself.

My questions and answers arrays are all in the same class as the code. Will it be better if it was in a seperate class? Thanks for answering — desperateStudent, Jan 05 '17 at 10:10
Yes, that's the whole [idea of object-oriented programming](https://www.cs.drexel.edu/~introcs/Fa15/notes/06.1_OOP/Advantages.html?CurrentSlide=3), especially encapsulation: you put things that logically belongs to eachother in a class. Each question has a correct answer, and you transfer the responsibility of checking whether the answer is correct or not to the question. — MC Emperor, Jan 05 '17 at 10:16

score 1 · Answer 2 · answered Jan 05 '17 at 09:31

    int n=questions.length;
//if n=100 means it give random numb with no duplicate values with in the range 100.
    Random r = new Random();
    Set<Integer> positionvalue = new HashSet<>();
    for(int i = 0; i <n; i++){
            while(true) {
            int number = r.nextInt(n) + 1;
            if (positionvalue.contains(number) == false) {
                positionvalue.add(number);                   
                break;
            }
        }
    }

positionvalue set have non-repeating random number between your range(n)

Thanks but in which part do i insert the question? sorry im new to this — desperateStudent, Jan 05 '17 at 10:07

score 1 · Answer 3 · answered Jan 05 '17 at 10:16

1

Create a list of questions and shuffle it (via Collections.shuffle()) one time to get a random order of questions:

List<String> randomQuestionList = new ArrayList<>();
randomQuestionList.addAll(Arrays.asList(questions));
Collections.shuffle(randomQuestionList);

Now you have a randomly ordered list of your questions.

Note I saw in your code, that you have your answers in a separate array. To make this shuffle solution work for you, you will need to hold question and answers together. The best way to achieve that, would probably be to create a Question class that holds the question and the answers. Then you can shuffle the questions and still have the answers to every question right at hand.

class Question {
    String question;
    String[] answers;
}

List<Question> randomQuestionList ...

answered Jan 05 '17 at 10:16

Ridcully

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how do i know if the answer is correct tho? Thanks for answering – desperateStudent Jan 05 '17 at 10:29
You should know, which of your answers is the correct one :-) You could add a field to Question class with the index of the correct answer or create an Answer class with a text and a right/wrong flag (this would also handle cases where you can have more than 1 correct answer). – Ridcully Jan 05 '17 at 10:32
Note, that you should probably display the answers in a random order as well -- you can again use shuffle() for that :-) – Ridcully Jan 05 '17 at 10:33

score 0 · Answer 4 · answered Jan 05 '17 at 09:40

Take two global variables

int nextQuestion;
List<Integer> posList;

Then initialise it:

private void initRandomLogic() {
    nextQuestion = 0;
    posList  = new ArrayList<>();
    for(int i=0;i<questions.length;i++)
        posList.add(i);
    Collections.shuffle(posList);
}

private int getRandomPosition() {
    return posList.get(nextQuestion++);
}

Hope will help you!

How to randomly pick element from array without repeating?

4 Answers4

Some tips

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