How do I detect the first empty space in an array ?

Hello,

I am a beginner with arrays, so I apologize if my question seems obvious. I think it is a very a simple one:

I have a one-dimension array, it looks like this

0 : "boat"

1: 0

2: "tank"

3: "plane"

4 : "infantry"

And I just want to create a condition that checks if this array contains an empty slot (in this exemple, this is the slot 1), to create some event accordingly.

How would you do that ?

Thanks

Here's an example for you.

[attachment=0:2bluz9g7][/attachment:2bluz9g7]

Thank you very much