Re: Basic speed optimization question

[Rici Lake <lua@...>]

On 30-Dec-06, at 9:07 PM, Aladdin Lampé wrote:

> Hi all,
>
> In other programming languages, I used to declare objects before loops 
> in order not to re-create one at each loop, for instance in Java :
> MyObject hello = MyObject.new();
> for(...)
> {
> // use and reuse 'hello' here, without the overhead of recreating the 
> object at each loop
> }
>
> In Lua, I often do the same:
> local a = ""
> for ...
>  a = ... -- use variable a, change its value, etc. (here variable 'a' 
> is 'non-local' thus with no indexed search)
> end
>
> Would it be better Lua programming (in terms of *speed* optimization) 
> to do directly:
> for
>  local a = ... -- because here the variable 'a' is local and not 
> 'non-local', thus very quickly resolved (indexed) even if recreated at 
> each loop
> end
>
> Thank you for your thoughts and comments about it!
> Genio

You'll find it quicker (and, imho, more readable) to use the second 
idiom, particularly if the variable is being assigned to the result of 
a function call.

In any event, no "object" is created by declaring a variable local. In
the usage:

local a = ""
for i, w in ipairs(t) do
  a = "(" .. w .. ")"
  -- do something with a
end

The initial string assigned to a is simply thrown away when a is 
reassigned. So you're not saving anything; all you're doing is
needlessly assigning a to the empty string (which doesn't use up
much time, since the string already exists).

On the other hand, if the usage were:

local a, b
for i, w in ipairs(t) do
  a, b = w:match"(%a*)(%d*)"
  -- do something with a and b
end

you'll end up incurring two VM move operations in each loop, which
could be avoided by the more natural:

for i, w in ipairs(t) do
  local letters, digits = w:match"(%a*)(%d*)"
  -- do something, etc.
end