Checking for a Nil value from a table.

[Luska72]

Yesterday I made this post: viewtopic.php?f=3&t=12376
In said post, I asked about Nil vs False, and decided to make air a nil value (in my table) to save memory. I did this without any problems.
Unfortunately, I am not able to check if a certain value from my table is nil or not! I have tried the following bits of code, and can't get them to work.

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 if level[y][x] ~= nil then 

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 if not level[y][x] == nil then 

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 item = level[y][x]
if item ~= nil then
 

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 item = level[y][x]
if not item == nil then
 

You get the picture, I have also tried doing some things I googled such as using Next, but I couldn't find a solution to my problem.

If anyone can give me code or tell my where I might find code that could help me, I would be very grateful.

Thank you very much
--Luska72

PS: I was able to check if a variable is Nil without problems, so I think Lua works like this:

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 If VARIABLE == NIL

Lua directly compares the variable to nil

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 If TABLE[Y][X] == NIL 

Lua first grabs the value from the table, then compares to nil.
However, the program crashes when grabbing the value from the table, so we can't compare to nil...

[norubal]

It works for me.. strange.

[micha]

When doing this,

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if level[y][x] ~= nil then 

or one of the others, it means that level[y] has to exist. If itself is nil as well, then accessing level[y][x] is not possible.
So you need to write

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if level[y] ~=nil and level[y][x]~= nil then

Lua does shortcut evaluation, that means that if the first one is false, then the second one is not checked. So this should work.

Then you can save some typing. The value nil is treated as false so you can simply write

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if a then

instead of

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if a~= nil then

The code then becomes

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if level[y]  and level[y][x] then

[Luska72]

Thank you so much, the "y" not existing was the problem. I was able to fix it by declaring my table like this:

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level = {
{ }, -- # of { }s are equal to the map height
{ },
{ },
{ },
{ },
{ },
{ },
{ },
{ },
{ },
{ },
{ }
}

Thanks again, It's always annoying to make a silly mistake like that

[IndieKid]

Maybe this can help. It looks for variable in array:

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inli.tblchk = function(tbl, chk)
    for key, value in pairs(tbl) do
      if value == chk then
        return true --this is what it returns if it found variable "chk" in array "tbl"
      end
    end
    return false  --returns "false" if it didn't find variable "chk" in array "tbl"
end

You can make it return nil.