How to refer to a table inside another table?

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szmol96
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How to refer to a table inside another table?

Post by szmol96 » Mon May 12, 2014 6:43 pm

Hi. I have this table, where I store my map segments and their data:

Code: Select all

map = {
	a1 = {
		x = 0,
		y = 0,
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapa1.png"),
		data = love.image.newImageData("img/mapa1.png")
	},
	a2 = {
		x = a1.x + a1.width, --this is line 11
		y = a1.y,
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapa2.png"),
		data = love.image.newImageData("img/mapa2.png")
	},
	b1 = {
		x = a1.x,
		y = a1.y + a1.height,
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapb1.png"),
		data = love.image.newImageData("img/mapb1.png")
	},
	b2 = {
		x = a1.x + a1.width,
		y = a1.y + a1.height,
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapb2.png"),
		data = love.image.newImageData("img/mapb2.png")
	}
}
But i get this error: "map.lua:11:attempt to index global 'a1' (a nil value)".
If I put "map" there, like this:

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x = map.a1.x + a1.width,
I get another error, which says: "map.lua:11:attempt to index global 'map' (a nil value)"
So, how do I refer to 'a1'?
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Robin
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Re: How to refer to a table inside another table?

Post by Robin » Mon May 12, 2014 7:02 pm

Well, you can't refer to a table when it doesn't exist yet, so the solution is this:

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map = {
	a1 = {
		x = 0,
		y = 0,
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapa1.png"),
		data = love.image.newImageData("img/mapa1.png")
	},
	a2 = {
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapa2.png"),
		data = love.image.newImageData("img/mapa2.png")
	},
	b1 = {
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapb1.png"),
		data = love.image.newImageData("img/mapb1.png")
	},
	b2 = {
		width = 640,
		height = 480,
		image = love.graphics.newImage("img/mapb2.png"),
		data = love.image.newImageData("img/mapb2.png")
	}
}
map.a2.x = map.a1.x + map.a1.width
map.a2.y = map.a1.y
map.b1.x = map.a1.x
map.b1.y = map.a1.y + map.a1.height	
map.b2.x = map.a1.x + map.a1.width
map.b2.y = map.a1.y + map.a1.height
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szmol96
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Location: Horvátkút, Hungary

Re: How to refer to a table inside another table?

Post by szmol96 » Mon May 12, 2014 7:15 pm

Oh, thank you Sir!

EDIT: Oh, and also I need some help with this function. The game says "attempt to index local 'image' (a function value)"

Code: Select all

function maskedHLine(x, y, length, image)
	for i = 1, length do
		local r, g, b, a = image:getPixel( x + i, y )
		if a > 0 then
			return true
		end
	end
	return false
end
It is in connection with this piece of code:

Code: Select all

if maskedHLine(player.x - 11, player.y + 16 + player.ySpeed * dt, 22, currMapSegment) == false then
		if player.ySpeed < player.maxFallSpeed then
			player.ySpeed = player.ySpeed + (1000 * dt)
		end
		player.onGround = false
	else
		player.ySpeed = 0
		player.onGround = true
	end
And I get currMapSegment from this function:

Code: Select all

function currMapSegment()
	local currMapSegment
	local i, segment
	for i, segment in ipairs(map) do
		if player.x >= segment.x and player.x <= segment.x + segment.width and player.y >= segment.y and player.y <= segment.y + segment.height then
			currMapSegment = segment
		end
	end
	return currMapSegment
end
How to solve this one?
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Robin
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Re: How to refer to a table inside another table?

Post by Robin » Mon May 12, 2014 8:15 pm

Like the error says, currMapSegment is a function.

So:

Code: Select all

if not maskedHLine(player.x - 11, player.y + 16 + player.ySpeed * dt, 22, currMapSegment()) then
(Generally, you'd want to use not something instead of something == false. Boolean logic FTW!)
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ejmr
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Re: How to refer to a table inside another table?

Post by ejmr » Tue May 13, 2014 2:32 am

Robin wrote:(Generally, you'd want to use not something instead of something == false. Boolean logic FTW!)
I disagree. Every one of the following results in boolean false:

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print(1 == false)
print(0 == false)
print(true == false)
print(" " == false)
print(not 1)
print(not 0)
print(not true)
print(not " ")
So I do not believe there is an inherent readability difference between ‘==’ and ‘not’.
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Robin
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Re: How to refer to a table inside another table?

Post by Robin » Tue May 13, 2014 8:22 am

ejmr wrote:So I do not believe there is an inherent readability difference between ‘==’ and ‘not’.
Wat. Your conclusion does not follow from your argument, or you missed a step in between.

People using == true and == false are the bane of my existence.
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foo0
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Re: How to refer to a table inside another table?

Post by foo0 » Tue May 13, 2014 9:08 am

Code: Select all

if stuff == not false then
	print("blood boiling")
end

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Ref
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Re: How to refer to a table inside another table?

Post by Ref » Tue May 13, 2014 3:05 pm

Code: Select all

happy = true
if happy then print( 'Smile ') end

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Exasperation
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Re: How to refer to a table inside another table?

Post by Exasperation » Tue May 13, 2014 3:22 pm

Wait a second... this:

Code: Select all

not nil
returns true while this:

Code: Select all

nil == false
returns false.

So, since the two options behave differently for the same input, shouldn't we use the one that provides the desired behavior in whatever particular situation we're using it?

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szmol96
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Re: How to refer to a table inside another table?

Post by szmol96 » Tue May 13, 2014 5:24 pm

I have difficulties again... :oops: The same function gives me a nil value error. It says: "attempt to index local 'image' (a nil value)".
What do I do now?
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