Implement an algorithm to shuffle an array of numbers

Question

I wrote codes for the following question but they don't work. I get random numbers but the shuffle method don't shuffle them. Would you please help me?

for( each index i)

   choose a random index j where j>=i.
   swap the elements at index i and j.

My code is:

public static void shuffle(int[] a){
   for( int i = 0; i < a.length-1; i++){
       int range = a.length; 
       int j = (int) (Math.random() * range);
       swap(a, i, j);      
  }
}

public static void swap(int[] array, int i, int j){

        if (i != j) {
            int temp = array[i];
            array[i] = array[j];
            array[j] = temp;
        }
    }

In any case, why shuffle when you can SHUFFLE: see stackoverflow.com/questions/8116872/… — paxdiablo
– paxdiablo, Commented Feb 12, 2013 at 5:15

Srikanth Sridhar · Accepted Answer · 2013-02-12 05:40:33Z

0

Convert your array to Integer Wrapper class and call the Collections.shuffle by converting Integer array to List. The snippet is below

you should have access to the Apache lang library, then you can use the ArrayUtils.toObject(int[]) method like this:

int [] array = {1,2,3,4,5,6};
Integer[] newArray = ArrayUtils.toObject(array);
Collections.shuffle(Arrays.asList(newArray));
for (int i = 0; i < newArray.length; i++) {
    System.out.println(newArray[i]);
}

If you donot have the Apcahe Lang Library, then you can do this way

Integer[] newArray = new Integer[array.length];
int i = 0;
for (int value : array) {
    newArray[i++] = Integer.valueOf(value);
}

answered Feb 12, 2013 at 5:40

Srikanth Sridhar

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Comments

Evgeniy Dorofeev · Accepted Answer · 2013-02-12 05:53:44Z

0

There is java.util.Collections.shuffle which works with List. I suggest to copy-paste the algorithm from src and change it to work with int[]:

public static void shuffle(int[] a) {
    Random rnd = new Random();
    for (int i = a.length; i > 1; i--) {
        swap(a, i - 1, rnd.nextInt(i));
    }
}

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


public static void main(String[] args) {
    int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9};
    shuffle(a);
    System.out.println(Arrays.toString(a));
}

prints

[8, 7, 3, 4, 6, 1, 2, 5, 9]

edited Feb 12, 2013 at 5:53

answered Feb 12, 2013 at 5:24

Evgeniy Dorofeev

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3 Comments

user1995200 Over a year ago

Where I can find java.util.Collections.shuffle ? I can't see it in java API.

Abubakkar Over a year ago

@user1995200 look for Collections class and shuffle is a static method in it

user1995200 Over a year ago

I can see that but there is no code here, it just explanation.

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