In java, how to replace a duplicate number in an ArrayList?

Question

Possible Duplicate:
How do I remove repeated elements from ArrayList?

I want to replace a duplicate number (if any) that i store in an ArrayList. I obtianed the numbers for a Random variable. Here is the class.

import java.util.*;

public class RandomNumbers {
// instance variables
private ArrayList<Integer> randomNumberList = new ArrayList<Integer>(); 
private Random randomNums = new Random();

public RandomNumbers(){
    // generating and adding random numbers to the list
    for (int i=0; i<MemoryGame.totalAnswers; i++)
        randomNumberList.add(randomNums.nextInt(32));

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

public ArrayList<Integer> getRandomNumbers(){
    return randomNumberList;
}
}

My school book tells me how to add,remove and retrieve a number, but not how to replace one that is duplicated.

thanks.

What do you mean by "replace"? Is it that you want to keep generating random numbers until you get one you haven't already got? — Bohemian, Dec 29 '12 at 02:58
duplication in duplication question: http://stackoverflow.com/questions/203984/how-do-i-remove-repeated-elements-from-arraylist — İsmet Alkan, Dec 29 '12 at 03:04
Just a comment, BTW, is `MemoryGame.totalAnswers` always smaller than 32? If not, you may run into problems.... — Yanick Rochon, Dec 29 '12 at 03:05
For a reasonably sized set of distinct numbers to randomize, I would ***use a [Fisher-Yates] shuffle*** and then pick the first N (could be up to the entire input size) results. I find this approach cleaner in general. (The Fisher-Yates shuffle can also be implemented in such a way that numbers are generated as a stream without requiring full computation.) — , Dec 29 '12 at 03:43
I need 8 random numbers from 0-31 that are not duplicated. The Value in MemoryGame.totalAnswers is 8. I'm going to resource HashSet tonight. I didn't know about it since I'm still I'm my first semester at school. — chr.solr, Dec 29 '12 at 03:46

score 2 · Accepted Answer · edited May 23 '17 at 11:43

If you are not limited to ArrayList, use a HashSet<Integer> instead (or better yet, a LinkedHashSet). HashSet<Integer> will garantee you not to have duplicated values in the collection, LinkedHashSet<Integer> will do the same and also preserve the ordering the the items.

If you insist on having an ArrayList, then do this :

public RandomNumbers(){
    HashSet<Integer> set = new HashSet<Integer>();

    // generating and adding random numbers to the list
    //for (int i=0; i<MemoryGame.totalAnswers; i++)
    while (set.size()<MemoryGame.totalAnswers)
        set.add(randomNums.nextInt(32));

    randomNumberList.addAll(set);  // dump the set in your ArrayList

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

Also, if MemoryGame.totalAnswers == 32, then you can speed up this process with a random list instead :

public RandomNumbers(){
    // generating and adding random numbers to the list
    for (int i=0; i<MemoryGame.totalAnswers; i++)
        randomNumberList.add(i);

    Collections.shuffle(randomNumberList);

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

** Update **

Since MemoryGame.totalAnswers == 8, forget the last snippet. I'm leaving it there if anyone would happend to need it. You can skip the use of a Set entirely by folllowing pst's suggestion :

public RandomNumbers(){
    // generating and adding random numbers to the list
    for (int i=0; i<32; i++)
        randomNumberList.add(i);

    Collections.shuffle(randomNumberList);

    // keep only the first ones we need
    randomNumberList.removeRange(MemoryGame.totalAnswers + 1, 32);

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

With the updated comments, I added yet another workable solution, without using a `Set`. Cheers! — Yanick Rochon, Dec 29 '12 at 05:41

score 1 · Answer 2 · answered Dec 29 '12 at 02:58

For replacing an element in a position use the set() method:

randomNumberList.set(index, element);

Of course, the logic to find out which elements are duplicated in the first place is up to you; if it doesn't make sense to have duplicate numbers for your problem, better use a Set data structure (for instance: HashSet, or LinkedHashSet if preserving insertion order when iterating is important for you) and keep adding random elements to the set until it has the desired size.

score 1 · Answer 3 · answered Dec 29 '12 at 03:13

1

If the ultimate objective is a random list of distinct numbers, and the range of possible numbers is not much larger than the number of results required, consider a shuffle rather than replacing duplicates.

answered Dec 29 '12 at 03:13

Patricia Shanahan

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score 0 · Answer 4 · answered Dec 29 '12 at 05:18

0

If all you want is as per your last comment

8 random, non-repeated numbers from 0-31

You don't need any instance variables, random number generating code, or instance methods.
All you need is just this:

public static List<Integer> getRandomNumbers() {
    List<Integer> list = new ArrayList<Integer>();
    for (int i = 0; i < 32; i++)
        list.add(i);
    Collections.shuffle(list);
    return list.subList(0, 8);
}

answered Dec 29 '12 at 05:18

Bohemian

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I tried this method, but somehow i get duplicate number with this method. – chr.solr Dec 29 '12 at 10:04
It is totally impossible to get duplicate numbers with this code, because each number 0-31 is added only once. What did you do to "try" it, exactly? – Bohemian Dec 29 '12 at 11:24
I replace my getRandomNumbers method with yours and delete what i had in the constructor method. But if I use it in the constructor and return the value through getRandomNumbers method it works just fine. is there any way to remove the non-use elements after shuffling. ei delete index from 9-31 after shuffling. I dont if it make since to delete them but i would think so, because efficiency. Thanks for the help. – chr.solr Dec 29 '12 at 19:00
to "delete" the used ones, do this: `myList = list.subList(8, list.size())` – Bohemian Dec 29 '12 at 19:15

In java, how to replace a duplicate number in an ArrayList?

4 Answers4