There's a very useful shortcut in if...else tests when you test whether a variable equals 0 or not. But you have to know how to use the ! operator. This operator is used to invert a boolean.
What does that mean? It means that if you have:

myBool = true;
trace (!myBool);
// returns false

It is also used to test the inequality between two numbers, objects... Remember that in Flash, 0 is equivalent to false, and any other number is considered as true.

myValue = 5;
if (myValue != 0)
{
     trace ("myValue is not 0");
}
// returns myValue us not 0

Now to test whether a number is or isn't equal to 0, instead of doing:

if (myValue == 0) //...
if (myValue != 0) //...

You can do respectively:

if (!myValue) //...
if (myValue) //...

Imagine that myValue = 0. !myValue will be evaluated as true. The same way if myValue = 5, !myValue will be evaluated as false.