Thread: shuffle array items?

Hi guys, so I have the following piece of code:

 
public class Test{
 
     public static void main (String args[]){
         int[] myList = {1,2,3,4,5};
         for (int i = 0; i < myList.length; i++) {
 
	      System.out.println(myList[i] +" before");
 
              int index = (int) (Math.random()* myList.length);
 
              int temp = myList[i]; 
              myList[i] = myList[index];
              myList[index] = temp;
 
              System.out.println(myList[i] +"after swap");
            }
     }
}

I have seen this from a Java book which says this is one way to shuffle the items in a given array. I don't believe it works though as I have tried it for myself and the Math.random() method is basically changing the values of the array cells and at the end I get a new array, say: {2,3,4,5,3}

Can somebody tell me some good way of shuffling array items? I am still quite new to programming so would appreciate some simple answer! (if possible )

Few days ago I added explanation of simple shuffling algorithm to my site and dedicated task on it:

Cards Shuffling - CodeAbbey

Here it is proposed to be used on the deck of cards (which could be useful for some game etc.)

Hope this explanation is simple enough...

Thanks Junky, used another loop outside the first one and it worked.
@Norm, I looked at the doc material but I was a bit confused on how to actually use the shuffle method so I googled it and most of the examples looked like this:

import java.util.ArrayList;
import java.util.Collections;
 
public class ShuffleElementsOfArrayListExample {
 
  public static void main(String[] args) {
 
    //create an ArrayList object
    ArrayList arrayList = new ArrayList();
 
    //Add elements to Arraylist
    arrayList.add("A");
    arrayList.add("B");
    arrayList.add("C");
    arrayList.add("D");
    arrayList.add("E");
 
    System.out.println("Before shuffling, ArrayList contains : " + arrayList);
 
    /*
      To shuffle elements of Java ArrayList use,
      static void shuffle(List list) method of Collections class.
    */
 
    Collections.shuffle(arrayList);
 
    System.out.println("After shuffling, ArrayList contains : " + arrayList);
 
  }
}

So I tried compiling and running it but I got the following error:

Note: Test.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Not entirely sure what this means, any ideas?

Also I was wondering if you could (please) hint me another way of shuffling array items because the one above I just posted is good but not that efficient if I had, say 100 array elements and I wanted them to be initialised by the user at run-time, using a loop for example. I am quite happy to use this one if I manage to figure out why I'm getting that error but I just want to stay on the safe side, in case I had a lot of array elements!

Oh I see, okay. Any idea why I'm getting the error in the above example?

Yep I tried this but I haven't really encountered warnings of this type. To be frank never really encountered warnings when compiling a program at all... I am quite new to java so I guess that's why. If it's not too much to ask, can I ask you to have a look at them and see if you know what they mean?

Nevermind, I didn't know it would still run if I tried it so I don't really care about the warnings anymore, thanks.

Not sure what you mean by using the -Xlint option. I compiled it with -Xlint and I looked at the warnings already which didn't really make sense to me because as I mentioned before I'm quite new to programming. I contacted my lecturer who was really helpful and explained that I am getting these warnings due to ArrayList now being a generic class and apparently the preferred way to do it would be using ArrayList<String> instead of ArrayList. This topic has not been introduced yet so of course I was not expected to know that. So yeah, this pretty much saved me some time. I suppose I will return to the warnings one day when I actually understand what they are trying to say.