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3389 :: Array indexed by a subrange type

• adrien79 (orginal) [2017-12-01T21:41:23+01:00] view original

  I want to define an array indexed by another type. I can make it work like this:

  type Point = range[0 .. 360]
  ...
  type MyArray = array[Point(0) .. Point(360), int]
  Run

  but not like this:

  type Point = range[0 .. 360]
  const AllPoints = Point(0) .. Point(360)
  ...
  type MyArray = array[AllPoints, int]
  
  >>> Error: ordinal type expected
  Run

  I don't understand the difference between the two, and why the former is allowed and the latter is not.

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• jlp765 (orginal) [2017-12-02T00:43:26+01:00] view original

  try

  
  type
    Point = range[0 .. 360]
    AllPoints = Point(0) .. Point(360)
    MyArray = array[AllPoints, int]
  Run

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• adrien79 (orginal) [2017-12-02T07:52:59+01:00] view original

  Thanks, this works to define the array, but then it breaks this in other places:

  for pt in AllPoints:
    do_sth(pt)
  Run

  No matter, I've just realized that I can use low and high with a type, I'll use this:

  type MyArray = array[Point.high .. Point.low, int]
  Run

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• Udiknedormin (orginal) [2017-12-02T08:38:59+01:00] view original

  @adrien79 Actually, if you tried to iterate over Points as you illustrated, it would break too:

  for pt in Points:
    do_sth(pt)
  Run

  You should use explicit low and high:

  for pt in Points.low .. Points.high:
    do_sth(pt)
  Run

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• adrien79 (orginal) [2017-12-02T09:31:26+01:00] view original

  No, I'm iterating like this at several places and it works fine:

  type Point = range[0 .. 360]
  const AllPoints = Point(0) .. Point(360)
  for pt in AllPoints:
    do_sth(pt)
  Run

  What I can't do is insert the same AllPoints in the array type declaration:

  type MyArray = array[AllPoints, int]
  Run

  I guess this is because the array declaration expects a type whereas my const AllPoints is a slice, but I don't fully understand the details.

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• leledumbo (orginal) [2017-12-02T17:35:56+01:00] view original

  I guess this is because the array declaration expects a type whereas my const AllPoints is a slice, but I don't fully understand the details.

  Correct, when you use type you define a subrange, but with const it's a slice. As you can read in the docs, array declaration expects an ordinal type for its index, which subrange fulfills but slice doesn't. OTOH, for .. in can loop over a slice, but not a subrange. Using x.low .. x.high you created a slice, so for .. in works with it.

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• Udiknedormin (orginal) [2017-12-02T19:44:43+01:00] view original

  @adrien79 Yes, that's how it works. There is no items for types, as far as I know.

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