Lua local question

[slime]

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function my_func(someArg)
	someArg = someArg == nil and someDefaultValue or someArg
end

Unless someDefaultValue is false, in which case the OP's solution is the only way to do it.

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function my_func(someArg)
	someArg = someArg ~= nil and someArg or someDefaultValue
end

[Robin]

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function my_func(someArg)
	someArg = someArg ~= nil and someArg or someDefaultValue
end

If someArg is false:

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someArg = false ~= nil and false or someDefaultValue

so it is exactly the same as

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someArg = someArg or someDefaultValue

So to deal with the argument like this, being false and nil in different ways, you need an if-statement.

[slime]

Dammit! You've won the battle, but not the war!

[benloran]

Heh, Robin beat me to it. Anyway, the point is that and/or assignments can lead to hard-to-track bugs if you're using them with boolean values.

[pancakepalace]

I tested this profusely. The if/then structure seems like the only one which works in this case and it's easy to read. My default value can be false, or some string.

If only Lua supported ternary conditionals. If only...

[benloran]

Well it does. You just can't use them if you need nil and false to be separate values.

In Lua,

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result = C and A or B

is the same as

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result = C ? A : B

in other languages, except for the single situation in which you want to assign a value of false to the result if C is true (in which case you have to test for nil instead).

[Robin]

If A is nil and B is true, you could do:

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result = not C and B or A

If A is false, you can also do:

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result = not C and B

But if both A and B are or might be false, none of that works.

[Hydra]

Hi guys,

I read all the docs I could find on the Lua language, but the use of local keyword when creating default values for function parameters is still unclear to my mind.

This is actually a pretty important subject with lua, i'll do my best to explain this for you.

Lua script uses two environments, local and global.

Variables, functions etc. in the local environment can only be accessed within the file in which they are created. For example:

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local msg = "Hello World!"

This message cannot be called outside of its file with "msg", whereas this:

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msg = "Hello World!"

Is global, and CAN be accessed anywhere within your program.

As well local variables that are created within functions cannot be used outside of that function, another example:

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local doSomething = function()
	local x = 1
end

print(x)

this would return nil, because "x" is local within that function, even if you were to call doSomething() before trying to print "x", it's not going to return any value.

If a variable/function/table doesn't have the word "local" in front of it, it's automatically going to be in the global environment.

It's worth noting that lua script runs faster in the local environment (the code will be executed faster and with less processing time), so using local variables and functions wherever possible is a good way to improve your code.

I'm pretty tired right now so hopefully that clears things up and I didn't just make an ass of myself.

[TechnoCat]

You guys aren't far from just talking about boolean algebra anymore.
http://en.wikipedia.org/wiki/De_Morgan's_laws

[T-Bone]

One thing I've always wondered. If you create a local variable inside a .lua-file, that variable will only be available there. If you create a local variable inside a function, it will only be available inside the function. Is there a way to create a "local" variable (local as in local for that .lua-file) from withing a function?

Like this

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a="global"
local a = "local level 1"

function lol()
   local b = "local level 2"
   semilocal c = "local level 1"
end

print(b) --nil
print(c) --"local level 1"