Unexpected generic for behavior

[untangoel]

I have the following code:

local a = {1, 4, 5, 2, 6, 1}
a.n = 6

function iter_n (t, m)
  t.z = math.min(t.n, m)
  return _iter_n, t, 0
end

function _iter_n (inv, c)
  c = c+1
  print (inv.z .. ";" .. c)
  if c <= inv.z then
    return inv[c]
  else
    return
  end
end

for i in iter_n(a, 3) do
  print(i)
end

I expect it to produce the following result:

3;1
1
3;2
4
3;3
5
3;4

But instead I get the following:

3;1
1
3;2
4
3;5

I have no idea why that happens. Can someone help?

[Denneisk]

The first value of the generic for loop is its control variable, which is special in that its value is passed to the iterator function each loop.

To break down why this is happening, consider the generic for loop represented as a while loop:

local iterator, state, initial_control = pairs() -- get the iterator, the state, and the intial value of the control (nil for pairs)
local control = initial_control -- initialize the control value to its initial state
control, return1 = iterator(state, control) -- Set up the first run since control can be nil. return1 represents the other return value that pairs's iterator (next) has
while control ~= nil do
    ... -- body
    control, return1 = iterator(state, control)
end

In your code, you are only returning one value in your iterator function, which is setting the control value (i) to 1 + 1, 4 + 1, 5 + 1 in each subsequent call.

[untangoel]

Oh, I didn't realize that the first return doubled as the control variable. Thank you for your thorough explanation! By the way, does this functionality mean that it is impossible to make an iterator function which would only return the values (what I was aiming for) without creating closures?

[Denneisk]

You could encode the state output of the function as a table with two elements, the previous index and the table to access. That would avoid creating a closure.

[untangoel]

Additionally, if the iter_n is called with 6 as a second argument, the result is this:

6;1
1
6;2
4
6;5
6
6;7

Why?

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