Optimized loops

Every one should write code that performs well. Do you use loops in your code? This is a simple trick everyone could apply to his loops.

How you write a loop

Mostly when people are writing loops, it looks like this:

for(var i:int = 0; i < list.length; i++) { var item:MyItem = list[i]; item.doSomething(); item.doAnotherThing(); } [/as] Now this loops works, but there are some small things that could be improved. If you use lots of loops it should be needed to improve your loops, but I hope this is just nice to know anyway.

The optimized version

Take a look at the improved loop:

var item:MyItem;
var total:uint = list.length;
for(var i:uint; i < total; i++) { item = list[i] as MyItem; item.doSomething(); item.doAnotherThing(); } [/as] What is improved in this loop? If you take a close look in both loops you will see I access the list-item once using list[i], and assign it to a variable called item. I don’t what it is, but for me it looks like most people who write javascript always use list[i] instead of creating a var of it, is that right? It is important to know you don’t need to create the var inside the loop, but create it once outside the loop, and (re-)use it while looping. I forget this most of the times, but it is a good practice to check if all the variables you are using inside a loop. Most of the times variables could be defined outside the loop, which could increase performance big time.

The second thing is to define a variable called total, which pre-calculates the length of the loop as a local variable. If you don’t define it, the FlashPlayer checks the length of the Array every time in the loop, now it is like a constant. Note, when you are removing items from the list, you should ajust the total-variable too, since the total does not equal the length of the list anymore ofcourse. 😉

Another thing you probably noticed is that I used the ‘as’ keyword to give the FlashPlayer a direct hint of the type of variable. This another way of casting, and slightly faster than MyItem(list[i]).

Update: Take a look at this great article about the performance of this casting method. http://jacksondunstan.com/articles/1305

Some other small things are to use uints for i and total in loops if you are using positive numbers. Another small thing is that you don’t have to define i = 0, only when you are using nested loops or re-using the i or j in the same function/scope.

Want a more freaky way to define the same optimized loop?

for(var i:uint, item:MyItem, total:uint = list.length; i < total; i++) { item = list[i] as MyItem; item.doSomething(); item.doAnotherThing(); } [/as] Are you using FlashDevelop? You could this for-loop snippet (scroll down).

Most of the loop-optimisation tips also applies to javascript.