Re: filter an array in place

[Miles Bader <miles@...>]

Emmanuel Oga <emmanueloga@gmail.com> writes:
> Two methods I came up with:
>
> 1) popping from the array and pushing to a new one if the condition is met.
> 2) Iterating first, storing indexes to remove, then removing one by
> one (compensating index offset as a result of each removed elements)

You don't actually have to store the indices to remove -- during the
first iteration, just set "deleted" entries to nil, and keep track of
the highest index.  Then add a second pass that compacts the array
(using the stored index for the iteration bound, to avoid problems with
ipairs on non-proper arrays).

E.g. (untested):

   local max_index = #ARRAY
   local do_removals = false

   -- first loop, set removed elements to nil
   for i = 1, max_index do
     if ARRAY[i] == "a" then
       ARRAY[i] = nil
       do_removals = true
     end
   end

   -- second loop, compact array to remove holes
   if do_removals then
     local offset = 0
     for i = 1, max_index do
       if ARRAY[i] == nil then
         offset = offset + 1
       elseif offset ~= 0 then
         ARRAY[i - offset] = ARRAY[i]
       end
     end
   end


-miles

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