Deeper Modes of Expression, Part 7: Making Decisions by Chris Meyer
Learning how to craft if/then/else statements plus do/while loops will open the door to a wide range of advanced techniques.
As your expressions become more complex, you will want to start making decisions in the middle of them – for example, if the current value is less than a certain number, do one thing; if it is greater than that number, do something different. Other decisions include "wait until the current time is past the next marker – then do this animation" or even "don't freak out and give me an error message if you can’t find the marker, keyframe, or other layer you’re looking for."
A variation on this theme is a do/while loop, which is an essential tool for overcoming expressions' inability to remember what happened beforehand: You can use these loops to walk through every frame of the animation – accumulating information – until you get to the current frame.
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