Is it better to declare a local inside or outside a loop? [duplicate]

Question

I am used to do this:

do
    local a
    for i=1,1000000 do
        a = <some expression>
        <...> --do something with a
    end
end

instead of

for i=1,1000000 do
    local a = <some expression>
    <...> --do something with a
end

My reasoning is that creating a local variable 1000000 times is less efficient than creating it just once and reuse it on each iteration.

My question is: is this true or there is another technical detail I am missing? I am asking because I don't see anyone doing this but not sure if the reason is because the advantage is too small or because it is in fact worse. By better I mean using less memory and running faster.

Answer 1

Like any performance question, measure first. In a unix system you can use time:

time lua -e 'local a; for i=1,100000000 do a = i * 3 end'
time lua -e 'for i=1,100000000 do local a = i * 3 end'

the output:

 real   0m2.320s
 user   0m2.315s
 sys    0m0.004s

 real   0m2.247s
 user   0m2.246s
 sys    0m0.000s

The more local version appears to be a small percentage faster in Lua, since it does not initialize a to nil. However, that is no reason to use it, use the most local scope because it it is more readable (this is good style in all languages: see this question asked for C, Java, and C#)

If you are reusing a table instead of creating it in the loop then there is likely a more significant performance difference. In any case, measure and favour readability whenever you can.