Randomize or shuffle an array

Question

Say I have an array:

myList:Array = new Array();
myList = [1,2,3,4,5,6,7,8,9];

myRandomList:Array = new Array();

for (var i:uint = 0; i < myList; i++) {
            var item:Number = Math.floor(Math.random() * myList.length-1) + 1;
            myRandomList.push(item);
      }

The only thing is, I'd like myRandomList to not have any duplicate numbers...is there a way to select a random number from the first list and then SUBTRACT it so I don't select that number twice?

UPDATE:

I just saw this method of shuffling an array from shadetyler.blogspot.com/2008/12/array-shuffle-as3.html

Array.prototype.shuffle = function(){
for(var i = 0; i < this.length; i++){
var a = this[i];
var b = Math.floor(Math.random() * this.length);
this[i] = this[b];
this[b] = a;
}

However, is there a way to rewrite this as a function? }

I just saw that you can use splice...but there's also a Array.prototype method that's supposed to be faster? shadetyler.blogspot.com/2008/12/array-shuffle-as3.html — redconservatory, Jan 20 '11 at 02:26
Try my implementation for array shuffling, answered here: http://stackoverflow.com/a/25702799/1521021 — Glogo, Sep 06 '14 at 17:24

phwd · Accepted Answer · 2011-01-20T02:47:04.043

3

The title says shuffle an array so if you are looking for an ideal shuffle you may want the Fisher–Yates algorithm that is unbiased.

So if you wanted to use/keep your original, you would initialize myRandomList

var myRandomList: Array = new Array( myList.length );

Then create a random number with the range say a and then swap myRandomList[a] with myRandomList[i] where i is the current element.

// Random number
var a = Math.floor(Math.random() * myList.length);
// A swap
myRandomList[i] = myRandomList[a];
// put whatever is in index a in the ith position
myRandomList[a] = myList[i];
// restore whatever was in the ith position to index a

edited Jan 20 '11 at 02:47

answered Jan 20 '11 at 02:28

phwd

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This is great! But the link doesn't have an Actionscript 3 example...can you elaborate a bit more? – redconservatory Jan 20 '11 at 02:37
@redconservatory you are just switching spots within the array so no need to push elements into it. As for the duplicates if `a==i` then there is no change. – phwd Jan 20 '11 at 02:50

score 1 · Answer 2 · answered Jan 20 '11 at 03:17

I haven't done much actionscript, but if there's a resizable array class, you could randomly transfer the data... Ex:

array from array to

for loop over from with iterator j. Pre-generate this number, because it will change i = get a random index in from to[j]=from[i] remove from[i]

If there's no size mutable array class, you can always do the random swapping

array theArray

rand = a random number for rand idx1, idx2 -> set to random numbers temp = theArray[idx1] theArray[idx1] = theArray[idx2] theArray[idx2] = temp

something like that that's just psudo code.

Randomize or shuffle an array

2 Answers2

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