Lua loop knowing when loops ends - loops

Question

Is there a statement in Lua that would allow me to determinate whether it's last loop cycle?
When I'm not able to determinate how many time loop will loop?
Example:
for _, v in pairs(t) do
if I_know_this_is_your_last_cycle then
-- do something
end

Answer 1

This is simplified version of missingno's answer: :-)
for _, v in pairs(t) do
if next(t,_) == nil then
-- do something in last cycle
end
end

Answer 2

You could probably try to write something like this:
--count how many elements you have in the table
local element_cnt = 0
for k,v in pairs(t) do
element_cnt = element_cnt + 1
end
local current_item = 1
for k,v in pairs(t)
if current_item == element_count then
print "this is the last element"
end
current_item = current_item + 1
end
or this:
local last_key = nil
for k,v in pairs(t) do
last_key = k
end
for k,v in pairs(t) do
if k == last_key then
--this is the last element
end
end

Answer 3

In general, no. As you can see from the Lua docs, the for loop is syntax sugar for a while loop on top of an iterator so it only knows if the loop is over at the start of the loop.
If you really want to check if you are entering the last iteration then I would simply code things explicitly with an while loop.
local curr_val = initial_value
while curr_val ~= nil then
local next_val = get_next(initial_value)
if next_val ~= nil then
--last iteration
else
--not last iteration
end
curr_val = next_val
end
If you want to translate the example with the pairs function, you can use the next function as the iterator.
As an aside, I would suggest thinking twice before coding a loop like this one. The way its coded means its very easy to write code that doesn't work right when you are iterating over 0 or 1 elements or to write code that doesn't treat the last element properly. Most of the time writing a plain loop and putting the "at the end" code after the loop is more reasonable.

Answer 4

There are a couple of ways to do this. The easiest is to just use the standard for loop and check it yourself, like this:
local t = {5, nil, 'asdf', {'a'}, nil, 99}
for i=1, #t do
if i == #t then
-- this is the last item.
end
end
Alternatively, you could roll your own iterator function for tables that tells you when you're on the last item, something like this:
function notifyPairs(t)
local lastItem = false
local i = 0
return
function()
i = i + 1
if i == #t then lastItem = true end;
if (i <= #t) then
return lastItem, t[i]
end
end
end
for IsLastItem, value in notifyPairs(t) do
if IsLastItem then
-- do something with the last item
end
end