[BioElectro]

Hi,

I realize this is an old thread, but I do not understand how this works. Specifically, I don't understand how an element avoids being selected more than once when the same index value reoccurs. This seems like magic to me. Clearly, I am missing something important here. How does

int pos = order[index]

know to select the

order[--orderLen]

value rather than the value from the random calculation?

baavgai, on 13 March 2009 - 07:23 AM, said:

Awww, that's cheating!

Here's how you'd do it just another array, if you're interested:

import java.util.*; 

public class Shuffle3 { 
	public static void main(String[] arguments) { 
		String names[] = { "Peter", "Patricia", "Hunter", "Sarah", 
			"Gabe", "Gina", "Rob", "John", "Zoey", "Tammy", "Robert", 
			"Sean", "Paschal", "Kathy", "Neleh", "Vecepia" };
		
		int [] order = new int[names.length];
			
		System.out.println("The original order:"); 
		for (int i = 0; i < names.length; i++){
			order[i] = i;
			System.out.println(i + ": " + names[i]); 
		}
		
		System.out.println("The new order:"); 
		int orderLen = names.length;
		for (int i = 0; i < names.length; i++){
			int index = (int) (Math.random() * orderLen);
			int pos = order[index];
			System.out.println(pos + ": " + names[pos]); 
			order[index] = order[--orderLen];
		}
	}
} 

[m-e-g-a-z]

Create a new thread and post your question on there. Please don't bring up threads that are nearly 2 years old.

[BioElectro]

m-e-g-a-z, on 19 December 2010 - 03:24 PM, said:

Create a new thread and post your question on there. Please don't bring up threads that are nearly 2 years old.

Umm, no. The thread is short and has important context. Is there anyone else who would actually like to be helpful who would like to respond to this? Thanks.

[cfoley]

So, the array order is a list of indexes. One index is chosen at random and this line stores the index in pos:

int pos = order[index]

To ensure the same index isn't chosen again, we need to delete it from the array... except in Java you can't delete from arrays. What baavgai did was to overwrite the chosen value with the last value in the array, and at the same time decrement the variable storing the length of the array.

order[index] = order[--orderLen];

Does this help?

This post has been edited by cfoley: 19 December 2010 - 04:56 PM

[BioElectro]

cfoley, on 19 December 2010 - 03:55 PM, said:

So, the array order is a list of indexes. One index is chosen at random and this line stores the index in pos:

int pos = order[index]

To ensure the same index isn't chosen again, we need to delete it from the array... except in Java you can't delete from arrays. What baavgai did was to overwrite the chosen value with the last value in the array, and at the same time decrement the variable storing the length of the array.

order[index] = order[--orderLen];

Does this help?

Yes, that is awesome, thanks!

[cfoley]

No problem.

[macosxnerd101]

Topic split. Please avoid necroposting. You are more than welcome to copy the code into a new thread.