Lua: Accessing variables in multidimensional table/array

Question

I'm trying to make a grid, using a table, which stores information such as x & y coordinates, texture information and another table. Here's some of the code used to creating the grid:

for i = i, n do
    local random = math.floor(rainbow.random(1, 13))
    self.grid[i] = {
        sprite = Labyrinth:make_cell_texture(spritebatch, position_x, position_y, i, random),
        y = position_y,
        x = position_x,
        nav = Cell:get_nav(random)
    }
    position_x = position_x + 0.09375
end

It's in the

nav = Cell:get_nav(random)

part that I'm creating the new table:

if random == 1 then
    self.nav = {
        north = true,
        east = false,
        south = false,
        west = false,
    }

The problem is accessing said variables, to check if they are true and false. I return self.nav to grid[i].nav, and I am able to print(self.nav.east) and get a true.

But accessing the table later down the line causes it always to return nil, even though I can get it to print that there is a table there.

I've tried with this loop:

for i = 1, 10 do
    if self.grid[i].nav.north and self.grid[i + 10].nav.south then
        print("Two sides are touching!")
    end
end

Also with self.grid[i][4].north, but to no avail.

Is the way I set it up wrong? Also tried renaming nav to [4].

Am I barking up the wrong tree? Weren't tables designed to be this deep?

I can't see anything obvious that would cause this to break, but debugging problems like this is generally made much easier by the use of a table pretty-printer; Serpent is a standalone one, and Microlight and Penlight both contain decent pretty-printers and are on LuaRocks. — furq
– furq, Commented Aug 1, 2012 at 18:03
Thank you, will check them out. I have currently just been using Sublime Text, and the compiler telling me off as debugging (which is less then optimal!). — kalligula
– kalligula, Commented Aug 2, 2012 at 11:20
I tried them out, and thanks alot. Now I can actually read what my tables contain! Much thanks! — kalligula
– kalligula, Commented Aug 2, 2012 at 12:31

Henry Hollingworth · Accepted Answer · 2012-08-04 12:29:19Z

In your loop where you populate the grid you use this code:

nav = Cell:get_nav(random)

now, from your question it appears it calls a function like this:

function Cell:get_nav(random)
   self.nav = {
    north = true,
    east = false,
    south = false,
    west = false,
}
--more code here
end

If so, what's happening is that you're passing Cell into the identifier self inside the function, so you're effectively saying

Cell.nav = {
    north = true,
    east = false,
    south = false,
    west = false,
}

This is because Dog:jump(height) is the same as Dog.jump(self,height) where Dog is passed into self.

However I believe what you intend to have is where each index in the grid self.grid[i] contains a table which itself contains a key 'nav' and a table containing the directions. If this is so, change the Cell:get_nav function to return the value like so:

if random == 1 then
return {
    north = true,
    east = false,
    south = false,
    west = false,
}

this will assign the table of directions into self.grid[i].nav, where as before it was assigning a nil value to self.grid[i].nav because:

function edit_my_param(f)
f = 3
end 

nav = edit_my_param(4)
print(nav)

sets the value nav to nil because the function doesn't return a value.

regards, henry

Doug Currie · Accepted Answer · 2012-08-01 19:40:44Z

0

In some cases you call it Cell[i] and in other cases grid[i] -- are you sure you are referring to the same table?

What does Cell:get_nav return?; the value it returns is put into self.Cell[i].nav so it may be redundant, and is certainly confusing to also set self.nav in Cell:get_nav

answered Aug 1, 2012 at 19:40

Doug Currie

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1 Comment

kalligula Over a year ago

Thanks for the reply. I realized I had a typo in there. grid[i] refers to the actual grid cells. While Cell is calling a function, which should create a table at grid[i][4]. I return self.nav back to grid[i], but maybe thats where its overkill?

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