How do I create an int array with randomly shuffled numbers in a given range

Question

Basically, let's say I have an int array that can hold 10 numbers. Which mean I can store 0-9 in each of the index (each number only once).

If I run the code below:

int[] num = new int[10];
for(int i=0;i<10;i++){
    num[i]=i;
}

my array would look like this:

[0],[1],.....,[8],[9]

But how do I randomize the number assignment each time I run the code? For example, I want the array to look something like:

[8],[1],[0].....[6],[3]

Solution 1

Make it a List<Integer> instead of an array, and use Collections.shuffle() to shuffle it. You can build the int[] from the List after shuffling.

If you really want to do the shuffle directly, search for "Fisher-Yates Shuffle".

Here is an example of using the List technique:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Test {
  public static void main(String args[]) {
    List<Integer> dataList = new ArrayList<Integer>();
    for (int i = 0; i < 10; i++) {
      dataList.add(i);
    }
    Collections.shuffle(dataList);
    int[] num = new int[dataList.size()];
    for (int i = 0; i < dataList.size(); i++) {
      num[i] = dataList.get(i);
    }

    for (int i = 0; i < num.length; i++) {
      System.out.println(num[i]);
    }
  }
}

Solution 2

Collections class has an efficient method for shuffling:

private static Random random;

/**
 * Code from method java.util.Collections.shuffle();
 */
public static void shuffle(int[] array) {
    if (random == null) random = new Random();
    int count = array.length;
    for (int i = count; i > 1; i--) {
        swap(array, i - 1, random.nextInt(i));
    }
}

private static void swap(int[] array, int i, int j) {
    int temp = array[i];
    array[i] = array[j];
    array[j] = temp;
}

Comments

[Patricia Shanahan]

A List can store any reference type, including Integer.

[Admin]

When i typed num.add(1) on eclipse. It says "The method add(String) in the type List is not applicable for the arguments (int)". 'num' is my list variable.

[Admin]

And when I declared it as List<Integer>, it asks me to remove the <Integer> part.

[Patricia Shanahan]

What Java version are you using?

[Patricia Shanahan]

I've added a complete worked example using List<Integer>. However, if you prefer to avoid List until you have learned more about it, see my comment about Fisher-Yates Shuffle.

[Admin]

Your solution look elegant and I can understand it. But When I try to run it I get "The type List is not generic; it cannot be parameterized with arguments <Integer>" as an error. Could it be the java version? (1.7 in my case)

[Admin]

I changed List to ArrayList and it worked. Thank you so much for the code.

[Patricia Shanahan]

Be careful which List class you are importing. It must be java.util.List.