One of the misconseptions of programming, is that even though if a piece of code get's something done, it isn't allways the right way. In this series we're taking a look at the efficiency of scripts. The more efficent your scripts are, the faster they are executed in the Flash player. In small projects this can only be a few millisecons faster, but in projects where a lot of calculations need to be calculated per frame, this can really give a boost to the frame rate. For example: a badly scripted 3D engine (a project where LOADS of calculations need to be made) can get a framerate of like 10fps. While a optimised 3D engine, based on the same mathematics can get a framrate of 80fps.

Minimising Calculations

The smaller the amount of calculations that need to be calculated, the faster the script. Sounds logical eh? This is fairly simple to implement. You can save a frequently used value in a variable so it doesn't have to be calculated each time. For example, take a look at both of these preloader scripts:

The second script is the most efficent because it saves the total bytes of the movie ONCE, and not each frame. This value never changes anyway, so why keep updating it? This may not look very useful, but apply this to big scripts, and it can really get the scripts running faster. You can also apply this same technique with loops.

Variable Scopes

Variables have a thing called the scope. The scope is where the variable can be accesed from. You have variables that only can be accesed from the function where they belong to, variables that can be accessed throughout the whole MovieClip and variables that can be accessed throughout the whole movie. Here are the ways of defining different variables with different scopes:

After defining the variables, you don't need to repeat the var or _global. You can just type the name.
Anyway, getting back to efficiency. Using the first type, is allways the fastest, as the variables get deleted when your done with it. They can't be accesed from outside that current level at that time, so if you want to keep it for use in functions/events (like onEnterFrame), you need to use the second type. You can, however, acces this variable from outside the movieclip if you put the path in front of the variable. The slowest is _global. You should also avoid using this, as it increases the chance of accidently using the same variable name (and that allways causes loads of havoc).

Functions & Classes

Using a lot of functions can make the performance decrease. Each time a function is executed, Flash saves all the variables etc, and then executes the function. When the function is done, it takes back all the variables and continues the original script. This can sometimes really take some time, specialy when your using the code in a loop. Sometimes, using function is better because it increases the readibilty and useability of the script. If you want to make scripts accesible for other users, using functions is a must.
It's basicly also the same story for classes. If you want it to be accessible for other users, use functions & classes.Tips

You can go on and on testing your scripts, trying to find the fastest way to do it. But you won't seen any difference in just one execution. So if you want to test a scripts efficiency, put it in a loop of like 10,000 times. Then trace getTimer() minus the timer at the beginning of the test. Here's a little script:

Good luck!