Small LUA question

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zd
Prole
Posts: 5
Joined: Thu Mar 08, 2012 2:27 am

Small LUA question

Post by zd » Thu Mar 08, 2012 2:31 am

This isn't specific to LOVE, but I'm having trouble finding an answer elsewhere so I figured I'd give it a shot here.

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function countWalls(tiles)

	local walls = 0

	for x = self.x - 1, self.x + 1 do
		for y = self.y - 1, self.y + 1 do
			if tiles[x][y] == nil then -- doesn't work, but needs to
				walls = walls + 1
			elseif tiles[x][y].blocked == true then
				walls = walls + 1
			end
		end
	end

	return walls

end
My question is how exactly should

Code: Select all

if tiles[x][y] == nil then -- doesn't work, but needs to
Is there a handy and quick way to check whether or not a given variable exists?

I just keep getting

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line 28: attempt to index field '?' ( a nil value )
Now, I've actually been able to rectify this pretty easily by adding code to the function to ignore border tiles. That being said, I would really like to know of an elegant way to check whether or not a variable might exist without it throwing an error when it doesn't. I'll post up a sample of my game once I get this refactor done.

User avatar
veethree
Inner party member
Posts: 820
Joined: Sat Dec 10, 2011 7:18 pm

Re: Small LUA question

Post by veethree » Thu Mar 08, 2012 3:36 am

What's on line 28?

It'd help if you'd post the whole .love, That makes it easier for us to troubleshoot.

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Kadoba
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Posts: 399
Joined: Mon Jan 10, 2011 8:25 am
Location: Oklahoma

Re: Small LUA question

Post by Kadoba » Thu Mar 08, 2012 3:42 am

You've got the right idea. The problem is that something in the first dimension of your 2d array is nil. To prevent this from causing an error you need to change your code here:

Code: Select all

if tiles[x][y] == nil then 
to this:

Code: Select all

if tiles[x] == nil or tiles[x][y] == nil then
Another trick is that nil is the only thing besides false that does not resolve to true in logical expressions, so you can do this as a shortcut:

Code: Select all

if not tiles[x] or not tiles[x][y] then
I created a grid class to make working with 2d arrays easier. Here it is if you're interested.

zd
Prole
Posts: 5
Joined: Thu Mar 08, 2012 2:27 am

Re: Small LUA question

Post by zd » Fri Mar 09, 2012 12:26 am

Kadoba wrote:You've got the right idea. The problem is that something in the first dimension of your 2d array is nil. To prevent this from causing an error you need to change your code here:

Code: Select all

if tiles[x][y] == nil then 
to this:

Code: Select all

if tiles[x] == nil or tiles[x][y] == nil then
Another trick is that nil is the only thing besides false that does not resolve to true in logical expressions, so you can do this as a shortcut:

Code: Select all

if not tiles[x] or not tiles[x][y] then
I created a grid class to make working with 2d arrays easier. Here it is if you're interested.
Thanks man, this worked perfectly.

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