looking for a better idiom

["J.Jørgen von Bargen" <jjvb.primus@...>]

Hi, folks
I'm looking for a better idiom for

    -- initial
    local lookup={}
    --

    for item in item_source() do
        -- vv this is the uggly code
        local data=lookup[item] or {}
        lookup[item]=data
        -- ^^ this is the uggly code

        -- do something more with data, e.g.
        data.set=true
        data.count=(data.count or 0)+1
        --
    end

(Where item_source will return about 20 different items about 1000 times 
in random order)

I have replaced it with

local function ta(t,a)
    local n={}
    t[a]=n
    return n
end

       -- vv better code
       local data=lookup[item] or ta(lookup,item)
       -- ^^ better code

eliminating multiple assigns to lookup[item], which already gave me a 
speedup of about 11%.
Any suggestions to make it even faster or prettyer (and without a 
function call?)

Regards Jørgen

PS: This is not the real code, but I come across this quite often and 
like to do it the best way.