Re: Why is nil not a valid table key?

[Philippe Lhoste <PhiLho@...>]

Rici Lake wrote:
> I know, it's always been that way. But is there really a compelling 
> reason?
>
> This has been niggling at the back of my mind for a while. From an 
> implementation viewpoint, there would be no problem using a special 
> type for keys for empty Nodes. I don't believe it would slow things 
> down at all or complicate the code (since there is already a special 
> type for dead keys.)
>
> I'm motivated to ask after having glanced at 
> <http://rgrig.blogspot.com/2005/11/writing-readable-code.html>. My 
> little solution to his challenge (which basically involves inverting a 
> table, although that's not the way it was phrased) looks like this:
>
> function firstindex(vec, first, last)
>    first, last = first or 1, last or #vec
>    local index, retval = {}, {}
>    for i = last, first, -1 do index[vec[i]] = i end
>    for i = first, last do retval[i] = index[vec[i]] end
>    return retval
> end

My solution (I didn't looked at your before doing it, to avoid to be 
spoiled...):

local x = { 0, 1, 4, 2, 4, 1, 0, 2 }
local x = { 'a', 'b', 'e', 'c', 'e', 'b', 'a', 'c' }
local y, found = {}, {}

for i, val in ipairs(x) do
 if found[val] == nil then
    y[i], found[val] = i - 1, i
  else
    y[i] = found[val] - 1
  end
end
print(table.concat(y, ", "))

Shows the solution as requested (0-based).

BTW, how can you have a nil element in the list? In Lua 5.0, putting it 
in the init. array will stop the array right there, and setting a value 
to nil removes it. Is it a Lua 5.1 new capability or did I miss something?

--
Philippe Lhoste
--  (near) Paris -- France
--  http://Phi.Lho.free.fr
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