Re: absence of a __hash metamethod

[Asko Kauppi <askok@...>]

Programming in Lua as in any language has a certain mindset, which  
makes one do things "the Lua way". So maybe the reason you need a  
'signature' for table contents is because of doing something unlike  
Lua in higher levels.

Having said that, I actually had the same need yesterday (first time  
ever) and made the following code. This is for caching products based  
on table contents, so earlier products can be reused. Please take use  
of the code, if you like. Disclaimer: not tested yet!

Note that even if Lua had a hash metamethod for tables, it wouldn't  
give same hash for different tables of equal contents.

-----
-- str= table_id(tbl)
--
-- Returns a unique ID for the assert template table; subsequent  
calls must
-- give the exact same string.
--
-- { [tbl/assert_func [, ...] [, true]], [name= tbl/assert_func [,  
name2= tbl/assert_func]] }
--
local function table_id( t )
    local sort= {}  -- parts collected: "num=..." "name=..."

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

        elseif tv=="function"
            -- 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)
            --
            s= string.gsub( tostring(v), "function: ", "", 1 )
        else
            assert( v==true )
            s= true
        end
        sort[#sort]= k.."="..s
    end

    table.sort( sort )

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





Peter Cawley kirjoitti 14.11.2007 kello 23:13:

> The 0x12345678 value produces by tostring is the pointer to the
> object. As the table is being regenerated each time, this value will
> vary:
>
> > a = {1,2,3}
> > b = {1,2,3}
> > = tostring(a)
> table: 003297C8
> > = tostring(b)
> table: 003299F0
> > a = {1,2,3}
> > = tostring(a)
> table: 0032B138
>
> On 14/11/2007, Patrick Donnelly <batrick.donnelly@gmail.com> wrote:
> > I'm not saying this is the most efficient way, but here's  
> > something you can do:
> >
> > > hold = {}
> > > somenewtable = {}
> > > hold[somenewtable] = true
> > > hold[tostring(somenewtable)] = somenewtable
> > > print(tostring(somenewtable))
> >
> > > print(tostring(somenewtable))
> > table: 0x807b6a0
> >
> > Notice tostring on a table produces a unique string identifier for
> > that table... You can use it as your "hash".
> >
> > --
> > -Patrick Donnelly
> >
> > "One of the lessons of history is that nothing is often a good thing
> > to do and always a clever thing to say."
> >
> > -Will Durant