How to declare a variable without initting it to nil in Lua?

Question

I've found it very useful to do something like:

if not variable then
    variable = value
end

Of course, I'd usually rather that variable was local, but I can't declare it local in my if, or it won't be accessible.

So sometimes I do:

local variable
if not variable then
    variable = value
end

The problem is that when I iterate over this code, the variable declaration sets the variable equal to nil. If I can live with a global value (I can), I can get around it by just not declaring the variable outside of the if block.

But isn't there some way that I can both have my local value and let it keep its value?

Solution

First off, the way or is defined in Lua gives you a nice idiom to avoid the if altogether:

variable = variable or value

If variable is nil, or will evaluate to its second operand. Of course, this will only work, if false is not a valid value for variable (because false and nil are both "false" as far as or is concerned).

However, you still have the problem that you need to declare the variable somewhere. I suppose your problem is that in the case of a global loop you think you have to do either:

while condition do
    variable = variable or value
    process(variable)
end

(which would make variable global) or

while condition do
    local variable
    variable = variable or value
    process(variable)
end

which is pointless because local limits the scope to one iteration and reinitializes variable as `nil.

What you can do though is create another block that limits the scope of local variables but does nothing else:

do
    local variable
    while condition do
        variable = variable or value
        process(variable)
    end
end

Comments

[user2206636]

Thank for you answers. Actually, I was wondering because I wanted to test if "local limits the scope to one iteration" was true or not.

[ChuckCottrill]

Perl and Ruby both have a similar idiom using the '||' or operator, and so you see code such as, val||='default'; this value or default idiom is very useful.

[Martin Ender]

@ChuckCottrill Yup, you can do the same in JavaScript. Probably in most languages that have a concept of "truthy" and "falsy".