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5722 :: understanding risk of shadowing

• koin (orginal) [2019-12-22T13:55:05+01:00] view original

  I try to understand

  Warning: Special variable 'result' is shadowed. [ResultShadowed]

  in following code:

  import math
  
  proc toNumArray(n: int): array[6, int] =
    var n = n
    var result = [0, 0, 0, 0, 0, 0]
    for i in 0..<6:
      result[5-i] = trunc(float(n mod 10)).int
      n = n div 10
    result
  
  echo toNumArray(123456)
  Run

  I know shadowing is used, because parameter are immutable. So I got some questions about shadowing

  1. I shadow n are there some drawbacks of doing so?
  2. Why is result shadowed? And also, what are the drawbacks?
  3. Is there a better way to accomplish this is Nim?

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• juancarlospaco (orginal) [2019-12-22T15:05:23+01:00] view original

  Nothing wrong with n.

  All proc and func have an automatic result variable created from the type on the signature of the function.

  Change var result = to result =

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• doofenstein (orginal) [2019-12-22T15:17:33+01:00] view original

  As @juancarlospaco said, Nim has an implicitly declared result variable. It is zero like all variables zero initialised and returned by default which is great to remove some boilerplate code:

  import math
  
  proc toNumArray(n: int): array[6, int] =
    var n = n
    for i in 0..<6:
      result[5-i] = trunc(float(n mod 10)).int
      n = n div 10
  
  echo toNumArray(123456)
  Run

  See also: https://nim-lang.org/docs/manual.html#statements-and-expressions-return-statement

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• koin (orginal) [2019-12-22T16:13:35+01:00] view original

  thanks @juancarlospaco and @doofenstein! I've red the manual but totally forgot about the implicit result variable.

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• Stefan_Salewski (orginal) [2019-12-22T16:48:56+01:00] view original

  Is trunc(float(n mod 10)).int not identical to plain n mod 10? And you may use for i in countdown(5, 0) if you like.

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• koin (orginal) [2019-12-22T17:38:31+01:00] view original

  yes It should be identical, thank you! was a c&p leftover, where I used trunc(log10(i) + 1) :D

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