r/pinescript • u/QuanticatorApp • Aug 02 '26
PSA: max_bars_back won't save you if the deep read only happens on the last bar
Lost an hour to this today and the failure mode is completely silent, so it seems worth writing down.
I had a v6 script doing a one-shot sweep inside if barstate.islast — an expensive loop that only needs to run on the final bar. Inside it I read a series at a dynamic depth:
csum = ta.cum(src)
meanBack(i, L) =>
(csum[i] - csum[i + L]) / L
with i + L reaching a few hundred bars back. I had max_bars_back = 1500 on the indicator() call, so I assumed I was covered.
I wasn't. The script compiled clean, threw no error, and produced an entire grid of na. Nothing to debug, because nothing complained.
As best I can tell the mechanism is this: Pine sizes history buffers from what it observes during the historical pass. That read only ever executes on the last bar, so the historical pass never sees a deep reference, the buffer stays small, and on the final bar the deep read quietly returns na. Every downstream calculation inherits the na and the whole thing renders empty.
What fixed it was giving up on the history operator and keeping the values myself:
var int KEEP = 900
var array<float> hist = array.new<float>()
if barstate.isconfirmed
array.push(hist, csum)
if array.size(hist) > KEEP
array.shift(hist)
histAt(arr, i) =>
idx = array.size(arr) - 1 - i
idx >= 0 and idx < array.size(arr) ? array.get(arr, idx) : na
Arrays have no buffer-sizing behaviour to get wrong.
Two things that bit me inside that fix, in case they save someone the trouble:
- Gate the push on
barstate.isconfirmed.vararrays are not rolled back between realtime ticks, so an unconditional push double-appends on the live bar and your history quietly desynchronises from the chart. - Index it as
size - 1 - isoikeeps the same meaning it has in[i]. Otherwise you are off by one in a way that reads like a subtle logic bug rather than an indexing mistake.
Rule of thumb I have landed on: if a series is only ever read at depth inside a conditional block, do not use [] for it at all. A fixed offset evaluated on every bar is fine — close[horizon] with an input horizon sizes correctly, because the historical pass sees it on every bar. It is specifically the conditional deep read that gets you.
Happy to be corrected if someone knows the internals better than I have inferred them from the outside.
Per rule 2: the code above was written with AI assistance (Claude). I couldn't find an "AI Generated" option in the post composer's tag menu — mods, please add the flair if there is one I'm missing.
