Lua for loop reduce i? Weird behavior

Question

Can someone explain me this?

for i = 1, 5 do
  print(i)
  i = i - 1
  print(i)
end

Output is:

1
0
2
1
3
2
and so forth

I exspected i to alter between 1 and 0. But obviously it keeps increasing as if I did not change it at all. What's going on?

I have to delete an i'th table element every now and then. So the next element to process would be i again. In C I would just write --i at the end of my loop content. Any official way in lua? :)

Don't use the loop control variable for it. Use some other condition (even something that is always true with a manual break condition). — Etan Reisner, Apr 21 '15 at 15:56
You could also track a difference: `local d=0; for/do i=i-d; if/then d=d+1 end ... end`. — user3125367, Apr 26 '15 at 02:22

Paul Kulchenko · Accepted Answer · 2015-04-22T15:09:26.067

6

The loop index (i in your case) is a variable local to the body of the loop, so any modifications you do to it have no effect on the loop conditions or the next element being processed (for the for loop).

If you need to have better control over the index to delete elements and keep processing, you should use the while loop form. See For Statement section for details.

edited Apr 22 '15 at 15:09

answered Apr 21 '15 at 16:49

Paul Kulchenko

25,884
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score 2 · Answer 2 · answered Apr 21 '15 at 16:47

2

What about using a while(true) loop instead? Then you'll have to break manually, but that might work.

local i = 5
while(true) do
    print(i)
    i = i - 1
    if (i == 0) then break; end
end

answered Apr 21 '15 at 16:47

Frederik Spang

3,379
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score 1 · Answer 3 · answered Apr 22 '15 at 02:40

Attempting to set the loop control variable causes a failsafe behavior.

I can't find it in the Language Incompatibility section of recent versions of the manual but I recall it being listed somewhere as a change. It was more of a formalization of previous undefined behavior.

If you are really curious, see the listing of your program from luac. The loop control variable is given an internal name so it can't be changed. If the same name is used in an assignment, a local variable is synthesized as a stand-in. It prevents unintentionally causing the for to become infinite.

You need another kind of loop.

Lua for loop reduce i? Weird behavior

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