setting the chances of occurrence of elements in an array - actionscript-3

Question

For example I have 3 elements in an array:
public function randomTile():Number
{
var tiles:Array = new Array(fire,ice,water);
var index:Number=Math.floor(Math.random()*tiles.length);
return tiles[index];
}
How to set the chances of occurrence of fire(70%), ice(10%), and water(20%)?

Answer 1

This should work for any number of elements and you can specify any chance value.
var tiles:Array = [
{"item":"fire", "chance":70 },
{"item":"ice", "chance":10 },
{"item":"water","chance":20}
];
var picked:Object = pickRandomByChance(tiles);
trace(picked.item);
public function pickRandomByChance(options:Array):Object
{
var copy:Array = [];
var range:Number = 0;
for (var i:int = 0; i < options.length; i++)
{
copy.push( { "item":options[i].item, "chance":options[i].chance } );
range += copy[i].chance;
if (i > 0)
copy[i].chance += copy[i - 1].chance;
}
var pick:Number = Math.floor(Math.random() * range);
for (i = 0; i < copy.length; i++)
{
if (pick <= copy[i].chance)
return copy[i];
}
return null;
}

Answer 2

There quite a few ways you could do this, and it largely depends of the scope of your project. If you just have the three elements, using a switch statement would be easy:
var rand:Number = Math.random();
switch(true){
case rand >= .3:
//use fire
break;
case rand >= .1
//use water
break;
default:
//use ice
}
Someone else may have a better way though