Re: Why do we have ipairs?

["Thiago L." <fakedme@...>]

On 10/06/2014 13:14, Daurnimator wrote:

On 10 June 2014 12:03, Thiago L. <fakedme@gmail.com> wrote:

I thought # just used the array size? (you know, with OP_NEWTABLE you have hash size and array size... I thought # just returned that array size...)

Nope, it's a binary search for an item followed by a nil: http://www.lua.org/source/5.2/ltable.c.html#luaH_getn

In the worst case, it's a linear search.

This is because numeric indices are NOT always stored in the array portion of the table.

Also keep in mind that arrays need to grow and shrink, the sizes to NEWTABLE are only suggestions for initial memory allocation.

 

Also wouldn't this be faster and behave the same as ipairs: (in 5.1 at least... 5.2 can screw itself for all I care about... also I didn't test or benchmark it...)
local a,b,c = 1,#t,1
local v = t[a]
while a <= b and v ~= nil do
    -- your stuff here
    -- use this as if it is "for a,v in ipairs(t) do <...> end"
    a = a + c
    v = t[a]
end

No/not for all tables.

#t can be very slow (see the linear search worst case)

How about:
local a = 1
local v = t[a]
while v ~= nil do
    -- your stuff here
    -- use this as if it is "for a,v in ipairs(t) do <...> end"
    a = a + 1
    v = t[a]
end