Re: tables as keys

[Asko Kauppi <askok@...>]

I've faced this in luaSub; the solution being to produce a string  
describing the "fingerprint" of the table. Using sort to make sure  
they are identical, even if string keys are used.

-----
-- str= table_id(tbl)
--
-- Returns a unique ID for the assert template table; subsequent calls  
must
-- give the exact same string.
--
-- [1..N]:  tbl / assert_func / true
-- [str]:   tbl / assert_func
--
local function table_id( t )
    local sort= {}  -- parts collected: "<1..N>=..." "<string>=..."

    -- Collect IDs for each key
    --
    for k,v in pairs(t) do
        local vt= type(v)
        local s
        if vt=="table" then
            s= "{"..table_id(v).."}"

        elseif vt=="function" then
            -- tostring(func) gives "function: 0xNNNNN"; the  
"function: " is
            -- unnecessary for us but it does not really matter  
(except for
            -- the cached memory strings taking up some bytes).  
Remove it anyways.
            --
            s= string.gsub( tostring(v), "function: ", "", 1 )
        else
            assert( v==true )
            s= "..."    -- true means "..." as the last param
        end
        sort[#sort+1]= k.."="..s
    end

    table.sort( sort )

    return table.concat( sort, "," )
end



Matthew Armstrong kirjoitti 11.3.2008 kello 4:48:

> Normally, if I index a table with another table as a key, the keying  
> is done by the table key's instance (essentially, its address).
> What I want is for the keying to be defined by the table key's  
> _contents_ (the structure of the table).
>
> For instance, I want to do something like:
>
> t = makeASpecialTable()
> t[{a=1, b=2}] = 42
> assert(t[{a=1, b=2}] == 42)
>
> One way to solve this problem is to serialize the table key into  
> string.  This works, but probably isn't all that efficient ... or  
> maybe making the generated string smaller would help?
>
> Has anyone dealt with this problem before?