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On 02/11/13 14:32, Pasi Mankinen wrote:

  Hisham wrote:
After a number of times when I had to iterate arrays, mark some
elements for removal, and then remove them in a second loop, I wished
I'd be able to do it in one go. Doing it "by hand" with a while loop
managing the index variable explicitly sounded cumbersome, so it
occured to me I could write a custom iterator instead.

Hello Hisham and all, this is my first post to this great list.

Removing elements from array is classic problem that can be solved easily with backwards loop:

print("** Remove backwards: **")
local t = { "a", "b", "c", "d", "e" }
for i=#t,1,-1 do
   local v = t[i]
   print(i, v)
   if v == "c" then
     table.remove(t, i)

Looping is extremely fast, moving items in memory is very slow. By looping backwards we have least amount of items to move. Think of situation where you want to remove { "a", "c", "d", "e" } from t, in this case there will be only one memory move: "b" from index 2 to index 1.

Basically when you optimize memory allocations and memory moves you have optimized your code well - in any language.


Pasi Mankinen

This is a best case senario for the situation and in your example code it does not seem to matter what position elements are in. I would therefore recommend not to use such as method which has a very poor worst case instead I would personally use the same trick which is used in C++ swap and pop.

local t = { "c", "a", "b", "c", "d", "e", "c", "f" }
local index = 1
local size = #t
while index <= size do
	if t[index] == 'c' then
		t[index] = t[size]
		t[size] = nil
		size = size - 1
	else index = index + 1