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4439 :: storing table value in var changes behaviour

• filip (orginal) [2018-12-06T20:26:26+01:00] view original

  I have two pieces of code that I expect to produce the same result but they don't.

  import tables, times
  
  var t = initTable[int, seq[Duration]]()
  t.add(1, @[])
  t[1].add(Duration())
  echo t
  Run

  Result: {1: @[0 nanoseconds]}

  import tables, times
  
  var t = initTable[int, seq[Duration]]()
  t.add(1, @[])
  var temp = t[1]
  temp.add(Duration())
  echo t
  Run

  Result: {1: @[]}

  Can someone explain why storing a result in a temp variable can change the result like this?

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• Araq (orginal) [2018-12-06T20:40:57+01:00] view original

  Because the value is copied into temp. Nim is not Python, Nim doesn't have reference semantics everywhere.

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• mratsim (orginal) [2018-12-06T21:08:30+01:00] view original

  Translation: use newTable instead of initTable to have a Table with reference semantics.

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• Araq (orginal) [2018-12-06T21:21:54+01:00] view original

  newTable doesn't change anything, var temp = t[1] creates a copy because Duration is an object.

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• filip (orginal) [2018-12-07T07:41:19+01:00] view original

  Coding in Nim gets easier every day but sometime I still stumble on the basics :) Thank you for your help!

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• jyapayne (orginal) [2018-12-10T22:04:48+01:00] view original

  @filip for future reference, you can do something like this:

  import tables, times
  
  type
    Storage = ref object # need this in order to store the reference and not make a copy
      durs: seq[Duration]
  
  proc `$`(st: Storage): string =
    return $st.durs
  
  var t = newTable[int, Storage]()
  t.add(1, Storage(durs: @[]))
  var temp = t[1]
  temp.durs.add(Duration())
  
  echo t
  echo temp
  
  # {1: @[0 nanoseconds]}
  # @[0 nanoseconds]
  Run

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