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1205 :: cannot update object / tuple in a for loop.

• kuba (orginal) [2015-05-07T19:52:22+02:00] view original

  I'm having a hard time with updating a sequence of objects in a for loop. It seems (at least for the objects) the type has to be declared as a "ref object" in order to be able to mutate in a loop. Otherwise loop throws an Error: 'i.x' cannot be assigned to.

  How about sequence of tuples? It does not seem I can use 'ref' in this case.

  type AAA = tuple[x:int]
  type BBB = object
      x:int
  type CCC = ref object
      x:int
  
  var a : AAA = (x:1)
  var b : BBB = BBB(x:2)
  var c : CCC = CCC(x:2)
  
  
  var aaa : seq[AAA] = @[a,a,a]
  var bbb : seq[BBB] = @[b,b,b]
  var ccc : seq[CCC] = @[c,c,c]
  
  
  # for i in aaa:
  #     i.x=3          # does not work
  
  # for i in bbb:      # does not work - 'i.x' cannot be assigned to
  #     i.x=3
  
  for i in ccc:
      i.x=3         # works
      echo i[]
  Run

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• jdm (orginal) [2015-05-07T20:07:51+02:00] view original

  This works:

   for i in 0..aaa.high:
    aaa[i].x=3
  
  echo aaa
  # @[(x: 3), (x: 3), (x: 3)]
  Run

  It seems that the for ... in ... syntax works a bit like using let. This does not compile:

   for i in 0..aaa.high:
    let elem = aaa[i]
    elem.x=3          # does not compile
  Run

  Whereas this (with var) compiles, but does not work:

   for i in 0..aaa.high:
    var elem = aaa[i]
    elem.x=3          # does not compile
  
  echo aaa
  # @[(x: 1), (x: 1), (x: 1)]
  Run

  To get a kind of "mutable reference" to a sequence item, you'd have to use a pointer (at least I know no more elegant way):

   for i in 0..aaa.high:
    var elem = addr(aaa[i])
    elem.x=3          # shortcut for elem[].x=3
  
  echo aaa
  # @[(x: 3), (x: 3), (x: 3)]
  Run

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• Araq (orginal) [2015-05-07T20:27:44+02:00] view original

  Use the system.mitems iterator instead of the implicit items iterator.

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• Sixte (orginal) [2015-05-07T20:45:52+02:00] view original

  Quote: "Use the system.mitems iterator instead of the implicit items iterator." (Araq)

  How to bring the systems.mitems iterator into the scope of the for loop?

  By the way, I did'nt know that I can assign something to an extracted value. I thougt that the

  for i in 0..aaa.high:
  Run

  i is an immutable (value).

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• Jehan (orginal) [2015-05-07T21:48:35+02:00] view original

  jdm: Whereas this (with var) compiles, but does not work:

  Tuples have copy semantics and you're modifying the copy. Try one of the following:

  for i in 0..aaa.high:
    template t: expr = aaa[i]
    t.x = 3
  
  echo aaa
  Run

  Or, as suggested by Araq:

  for item in aaa.mitems:
    item.x = 3
  
  echo aaa
  Run

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