Unity 4.x Game Development by Example Beginner's Guide

The waiting game

The reason why we're declaring these variables at the top of the script, and defining them (giving them their values) further down in the Start function, is partly stylistic, and partly practical. There's a potential problem here: because Unity is superfast at calculating the Screen.width property, it might do so before the game screen has finished resizing and initializing, returning an incorrect value. By moving the variable definition to the Start function, we're giving Unity a few CPU cycles to make itself decent before we go barging in demanding screen properties.

Defining halfButtonW at the top of the class won't break anything, but some programmers would prefer to see the definition separated from the declaration.