How to pull elements from an enum without getting repetitions

Question

I've been trying to pull values from an enum and store them as a String in an array. However, depending on a variable, the values have to be unique, i.e. the same value can't be used twice.

I've used the following code to pull values:

public enum Colour
{
    ROOD, GEEL, GROEN, BLAUW, PAARS;

    public Colour getRandomColour(Random rn)
    {
        return values()[rn.nextInt(values().length)];
    }
}

However, this can give duplicate values.

It seems that the values of my enum refuse to be put in code blocks. Sorry!

EDIT:

for (int i = 0; i < code.length; i++) 
            code[i] = kleur.getRandomColour(rn).toString();

It fills up the array 'code'. The array-length depends on several factors but it will always be smaller than or equal to the amount of values in the enum.

it would be interesting to see the part where you call `getRandomColour` and push the value in your array — jhamon, Mar 21 '18 at 11:08
Do you want to get random [permutation](https://en.wikipedia.org/wiki/Permutation) of all your enum values? Because if you call `getRandomColour` more than there is values in your enum you must get duplicate value at least once. — MatheM, Mar 21 '18 at 11:10
[Java generating non-repeating random numbers](https://stackoverflow.com/q/16000196) — Bernhard Barker, Mar 21 '18 at 11:16
Possible duplicate of [Generating a random enum value continuously without getting the same value twice](https://stackoverflow.com/questions/18145766/generating-a-random-enum-value-continuously-without-getting-the-same-value-twice) — Bernhard Barker, Mar 21 '18 at 11:17
The difference is that due to the nature of my project, I have to use an array since it has to be 2-dimensional. The post Dukeling linked used an ArrayList. — Nick Lersberghe, Mar 21 '18 at 11:26

Tim Biegeleisen · Accepted Answer · 2018-03-21T11:18:49.943

1

You could populate a list with all values from your Colour enum, shuffle, and then just access values sequentially:

List<Colour> colourList = Arrays.asList(Colour.values());
Collections.shuffle(colourList);

Then, just iterate the list and access the colors in order, which of course should be random since the collection was shuffled:

for (Colour c : colourList) {
    // do something with c
}

You could also go with your current approach, and store the colors into a set, rather than an ArrayList. But the problem there is that you may draw a duplicate value any number of times, and the chances of that happening as the set increases in size will go up.

edited Mar 21 '18 at 11:18

answered Mar 21 '18 at 11:11

Tim Biegeleisen

502,043
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You have to use `List colours = Arrays.asList(Colour.values()); Collections.shuffle(colours);` instead of `Collections.shuffle(Arrays.asList(Colour.values()));` since Collections.shuffle doesn't return any value. – statut Mar 21 '18 at 11:18
Is there a way to apply this to an Array rather than a list? The 2-dimensions are quite important in my project (or since I'm a java novice, i don't know a better way to do it yet). – Nick Lersberghe Mar 21 '18 at 11:27
I don't recall seeing two dimensions in your question, but you can just run what I suggested N times to generate N dimensions of random sets of enum values. You could go with what you have, but using a set, but then you have to deal with needing to draw multiple times to get the next unique value. – Tim Biegeleisen Mar 21 '18 at 11:29
I just realised, this specific array is 1-D rather than 2-D. My project is a game of Mastermind and I'm using a 2-D array for the board while this was about the solution. I just need the values. ArrayList it is then. Thanks! – Nick Lersberghe Mar 21 '18 at 11:41

score 0 · Answer 2 · answered Mar 21 '18 at 11:41

0

Use a temporary list to check if the new value already exist and continue to generate a new value until a unique is found.

for (int i = 0; i < code.length; i++) {
   String next = kleur.getRandomColour(rn).toString();
   List<String> tempList = Arrays.arrayAsList(code);
   while (tempList.contains(next);
     next = kleur.getRandomColour(rn).toString();
   }
   code[i] = next;
}

answered Mar 21 '18 at 11:41

Joakim Danielson

43,251
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I found a slightly better solution to my problem. Store the values as String in an array and use the indexes to reference that String. – Nick Lersberghe Mar 21 '18 at 14:51

score 0 · Answer 3 · answered Mar 23 '18 at 15:53

0

For those still wondering, I might have found a good solution. Store the values of the array in an ArrayList. Using a for-loop to pull a value from the ArrayList then remove that index, essentially eliminating duplicates.

answered Mar 23 '18 at 15:53

Nick Lersberghe

166
8

How to pull elements from an enum without getting repetitions

3 Answers3