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Fix inefficient Seq.item usage in sumSeq example causing O(n²) complexity #49135

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Fix inefficient Seq.item usage in sumSeq example causing O(n²) complexity #49135

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Copilot Oct 14, 2025
56c2d95
Fix inefficient sumSeq implementation using Seq.item
Copilot Oct 14, 2025
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13 changes: 5 additions & 8 deletions samples/snippets/fsharp/fssequences/snippet15.fs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,14 +23,11 @@ let squaresSeries = generateInfiniteSequence (fun index -> float (index * index)

// This function sums a sequence, up to the specified number of terms.
let sumSeq length sequence =
(0, 0.0)
|>
Seq.unfold (fun state ->
let subtotal = snd state + Seq.item (fst state + 1) sequence
if (fst state >= length) then
None
else
Some(subtotal, (fst state + 1, subtotal)))
sequence
|> Seq.skip 1 // skip first item (matching the original behavior)
|> Seq.truncate length // don't take more than length items
|> Seq.scan (+) 0.0 // generate running sums
|> Seq.skip 1 // skip the initial 0.0 from sequence of running sums

// This function sums an infinite sequence up to a given value
// for the difference (epsilon) between subsequent terms,
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