Java select a random item from a multi array & not over use it again?

Question

    private String[][] questions = 
    {
        {"Sky's color is black", "false"},
        {"The earth is round.", "true"}, 
        {"Google is written in PHP", "false"},
        {"This program is written in JAVA", "true"}, 
        {"Daniel is white", "true"}
    };

    public void selectRandomQuestion() {

    }

I want the system to select a random question from the array, and after it's selected, I want to print it + make the system remember that this question already was printed, so the system won't select that question again.

Example:

I got the question "Google is written in PHP", answered it, now I can not get this question anymore. After I run out of questions, system will turn boolean game to false, so the game will end.

How can I do this?

Oh, cause two way arrays, I always forget that, thanks. But all the methods above, I want the system to know the correct answer, without duplicating the array. — Jony Kale, Jul 03 '13 at 19:19

Richard Tingle · Accepted Answer · 2013-07-03T19:58:43.827

By far the simplest way to achieve this (but not what I recommend) is to use an Arraylist of Strings and then remove the used questions as you go

public class Test { 
    public static void main(String[] args) { 

        Random rnd=new Random();
        ArrayList<String> questions = new ArrayList<String>();
        ArrayList<Boolean> answers = new ArrayList<Boolean>();

        questions.add("Question 1");
        answers.add(true);

        questions.add("Question 2");
        answers.add(false);

        while (array.isEmpty()==false){
            int index=rnd.nextInt(questions.size());

            String question=questions.get(index);
            boolean answer=answers.get(index);

            questions.remove(index);
            answer.remove(index);

            //do whatever with the question
        }


    }

}

Object orientated alternative
A nicer object orientated way to do this, however, would be to create an object to hold the question and answer together

public class QAndA {
    public final String question; 
    public final boolean answer;

    public QAndA(String question, boolean answer) {
        this.question = question;
        this.answer = answer;
    }


}

And then hold those objects in an Arraylist

public class Test { 
    public static void main(String[] args) { 

        Random rnd=new Random();

        ArrayList<QAndA> array = new ArrayList<QAndA>();

        array.add(new QAndA("Question 1",true));
        array.add(new QAndA("Question 2",true));


        while (array.isEmpty()==false){
            int index=rnd.nextInt(array.size());

            QAndA question=array.get(index);
            array.remove(index);

            //do whatever with the question
        }


    }

}

Removing objects from an Arraylist is not a very quick option but given that the Arraylist is probably short this is unlikely to be an important factor. If it is consider some of the other collections. The fields in QAndA are declared public as the QAndA class is a glorified struct, again consider whether this is appropriate dependant on your usage.

How will the system know the correct answer for that question? considering I need a quesiton + it's answer. — Jony Kale, Jul 03 '13 at 19:15
The object orientated way would actually be to create a QAndA class that held both question and answer strings together and then actually hold an arraylist of QAndA objects (thats what I'd do), but alternatively you could hold two arraylists; one with questions one with answers — Richard Tingle, Jul 03 '13 at 19:17

score 1 · Answer 2 · answered Jul 03 '13 at 21:41

Removing previously selected items from ArrayLists, as suggested by others, could work. An alternate solution is to shuffle your Array, and then just iterate through it. Once you get to the end, you'll have touched each question once and in a random order.

score 0 · Answer 3 · answered Jul 03 '13 at 19:02

0

See Random class javadoc and checkout Set<Integer>

answered Jul 03 '13 at 19:02

jmj

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401
438

Probably a `Map` would be better. – Luiggi Mendoza Jul 03 '13 at 19:06
@Luiggi why to store unnecessary boolean – jmj Jul 03 '13 at 19:51
I thought the `boolean` was part of the data to display, notice OP's example. – Luiggi Mendoza Jul 03 '13 at 19:53

Marco Corona · Answer 4 · 2013-07-03T19:46:59.140

You could create an arrayList that indexes the question and has a boolean value, that points to if it has been asked or not. And to see when the game ends, just create a variable that is equal to questions.length and iterate that many times.

If you are against using an arrayList you could just add a 3rd row to your array:

private String[][] questions = 
{
    {"Sky's color is black", "false", "false"},
    {"The earth is round.", "true", "false"}, 
    {"Google is written in PHP", "false", "false"},
    {"This program is written in JAVA", "true", "false"}, 
    {"Daniel is white", "true", "false"}
};

The last row can point to if the question was accessed. Just to clear things up for the first question "Sky's color is black":

questionAsked = questions[0][0]
answer = questions[0][1]
ifAsked = questions[0][2]

score 0 · Answer 5 · edited Nov 03 '21 at 10:59

You can insert Objects to a Set, it can be HashSet. A Set will maintain list of unique items. HashSet will maintain unique set of question objects. I also override equals method since it is called by HashSet fo uniqueness.

      public class Test { 
        public static void main(String[] args)
      { 

     HashSet<Question> setOfItems = new HashSet<String>();
        setOfItems.add(new Question("Sky's color is black",true));
        setOfItems.add(new Question("The earth is round.",false));
        setOfItems.add(new Question("Google is written in PHP",false));
        setOfItems.add(new Question("This program is written in JAVA",true));
        setOfItems.add(new Question("Daniel is white",false));
        
       while(!setOfItems.isEmpty())
       {
            Question selectedQuestion = selectRandomQuestion(setOfItems);
            //DO whatever needed.
       }
     public Question selectRandomQuestion(HashSet<Question> setOfItems) {
        String result = null;
        if(setOfItems.size() >0)
        {
            Random generator = new Random( );
            int index = generator.nextInt( setOfItems.size() );
            Iterator<Question> i = setOfItems.iterator(); 
            
            while(index<0)
            {
                 result = i.next();
                 index--;
            }
            setOfItems.remove(result); //Remove from Set
            
         }
         return current;
     }
   }
}


public class Question
        {
              private String question;
              private boolean answer;
              public Question(String question, boolean answer)
              {
                   this.question = question;
                   this.answer = answer;
              }
              @Override
              public boolean equals(Question q)
              {
                   return this.question.equals(q.getQuestion());
              }
              //Getter Setters
        }

Its worth noting that if you want to override equals (which isnt strictly nessissary) you should also override the hashcode function to match, a good IDE will generate a warning about this and will generate the hashcode function for you. http://stackoverflow.com/questions/27581/overriding-equals-and-hashcode-in-java — Richard Tingle, Jul 04 '13 at 07:01

score 0 · Answer 6 · answered Jul 03 '13 at 19:34

You could make it somewhat object oriented by making a class representing each question and a class to draw random questions.

The class representing the question public class Question{

private boolean notShown = true;
private String question;

public Question(String s){

    this.question = s;
}
public boolean isNotShown(){

    return notShown;
}
public String GetQuestion(){


    notShown = false;
    return question;
}

public void setQuestionNotSeen(){

    notShown = true;
}
}

The class representing the draw

public class QuestionLister {


private ArrayList<Question> listOfQuestions;

public QuestionLister() {

    listOfQuestions = new ArrayList<>(0);

}
public QuestionLister(String s) {

    listOfQuestions = new ArrayList<>(0);
    listOfQuestions.add(new Question(s));

}


public void addQuestion(String s){

    listOfQuestions.add(new Question(s));
}

public String getRandomQuestion(){
    int r;
    boolean allSeen = true;

    for (Question question : listOfQuestions) {

        if(question.isNotShown()){

            allSeen = false;
        }
    }

    if(!allSeen){
        do{
            r = (int) Math.random() * listOfQuestions.size();

        } while(!listOfQuestions.get(r).isNotShown());

        return(listOfQuestions.get(r).GetQuestion());
    }

    return "All questions shown";
}

public void setAllQuestionsNotSeen(){

    for (Question question : listOfQuestions) {
        question.setQuestionNotSeen();
    }
}
}

Java select a random item from a multi array & not over use it again?

6 Answers6