nimforum mirror - Slices s[1 .. ^1] as proc parameters and missing span iterator

Stefan_Salewski (orginal) [2019-05-04T20:56:09+02:00] view original

I was wondering if slices are still copied when used as proc parameters...

Found again Araqs comment

So my guess was that they are still copied. But maybe overhead is small? So did this test:

# https://github.com/nim-lang/Nim/blob/devel/lib/system/iterators.nim#L1

# https://forum.nim-lang.org/t/4582#28715

iterator span*[T](a: openArray[T]; j, k: Natural): T {.inline.} =
  assert k < a.len
  var i: int = j
  while i <= k:
    yield a[i]
    inc(i)

type
  O = object
    i: int

proc main =
  var
    s = newSeq[O](1000000)
  for i in 0 .. (1000000 - 1):
    s[i] = O(i: i)
  
  var sum = 0
  for x in s[1 .. ^1]:
  #for x in s.span(1, s.high):
    sum += x.i
  
  echo sum

main()

Using span iterator reduces total running time by a factor of two. (And I think use cases are not that rare, I have a few where iteration has to skip first or last element.) Question: Will copying of Slices vanish in the future, or are span iterators useful. (At least they are less ugly than a while loop.)

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4823 :: Slices s[1 .. ^1] as proc parameters and missing span iterator