How to choose a random element in this array only once across all declared objects in main?

Question

I would like these objects to print the random names in the array only once across all the objects declared in main. My problem is that it would show the names twice or more.

public class Fighters {

    private static String[] names = { "Terminator", "Slicer","Ninja", "cow", "Robot", "littlegirl" };
    private static int id = 0;

    public Fighters(){
        id++;
        name = names[(int) (Math.random() * names.length)];
    }

    public String toString(){
        return String.format(id+" Name:%-5s, name);
    }
}

In main() I have these objects

Fighters a1 = new Fighters();
System.out.println(a1.toString());

Fighters a2 = new Fighters();
System.out.println(a2.toString());

Fighters a3 = new Fighters();
System.out.println(a3.toString());

You could make a parallel array (or a 2d array) of `boolean` to flag whether the name has been shown yet or not. — nhgrif, Oct 01 '13 at 20:44
Or make a copy of the array and use some kind of randomized algorithm to pull out / check an element or shove it all in a stack or queue... I am partial to @nhgrif's suggestion for using a 2d array, though. — Drew, Oct 01 '13 at 20:58

andrewgrz · Accepted Answer · 2013-10-01T21:31:10.280

3

If you want to keep your name assignment in the Fighter class, use a static variable to hold the list of used names. You need to make sure you have not created more Fighters than the names array. Otherwise this will loop indefinitely. I will leave it up to you to figure that part. This should at least get you on the right track.

import java.util.ArrayList;

public class Fighter {

    private String name;

    private static String[] names = { "Terminator", "Slicer","Ninja", 
             "cow", "Robot", "littlegirl" };
    private static int id = 0;
    private static ArrayList<String> usedNames = new ArrayList<String>();


    public void Fighters(){
        id++;
        String checkedName = "";
        do{
            checkedName = names[(int) (Math.random() * names.length)];
            if(!usedNames.contains(checkedName)) break;
        }
        while(true);
        name = checkedName;
        usedNames.add(name);
    }

    public String toString(){
        return String.format(id+" Name:%-5s, name");
    }

}

edited Oct 01 '13 at 21:31

answered Oct 01 '13 at 21:01

andrewgrz

435
3
6

I get this error when compiling with your code `Exception in thread "main" java.lang.NullPointerException at Fighter.(Fighter.java:29)` – answerSeeker Oct 01 '13 at 21:27
java:29 is this line `f(!usedNames.contains(checkedName)) break;` – answerSeeker Oct 01 '13 at 21:28
Forgot to initialize the ArrayList. Code is updated. – andrewgrz Oct 01 '13 at 21:32
I noticed that the loop goes on forever so I modified it with a while loop that makes it stop when id reaches a certain amount – answerSeeker Oct 05 '13 at 00:04

Thomas W · Answer 2 · 2013-10-01T20:55:55.807

Keep the "remaining names" in a List or Set, and when you need to pick one do so randomly & remove it from "remaining".

public class NamePicker {
    protected List<String> names = new ArrayList( Arrays.asList(
         "Terminator", "Slicer","Ninja", "cow", "Robot", "littlegirl" ));
    protected List<String> remaining = names;

    public String pickName() {
        if (remaining.isEmpty()) {
            // could refill 'remaining' here.. but we'll just throw, for now.
            throw new IllegalStateException("names all used");
        }
        int index = (int) (Math.random() * remaining.size());
        String result = remaining.remove( index);
        return result;
    }
}

Putting this behavior in a class also wraps it up nicely, and enables you to better separate "name assignment" for creation of Fighters from eg. loading them from a file or database.

Generally, you shouldn't design just one constructor that assumes that you're "creating" the entity -- you should allow for "load the entity" as well. When you get there, the best way is to have two static methods one for create() the other for load() calling onto internal constructors.

Kaushik Sivakumar · Answer 3 · 2013-10-01T21:21:16.370

-1

its because you ar picking a random element (int) (Math.random() * names.length)..and because of probability, you are picking the same one again by chance

1 option for you problem maybe to randomize the names array and pick in order 2.another option would be to use a list instead of array and remove once used

edited Oct 01 '13 at 21:21

answered Oct 01 '13 at 20:48

Kaushik Sivakumar

205
2
9

It wasn't me to downvote, but your explanation is wrong: it's not because of the "randomize" (which would mean that another kind of randomize is fine), it's the OP's algorithm that is wrong. – Stefano Sanfilippo Oct 01 '13 at 21:15

How to choose a random element in this array only once across all declared objects in main?

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