[Haskell-beginners] Howto reverse a Data.Array

[David McBride]

The problem isn't with array, but rather your index.  Ix instances are
always sorted in ascending order, as you might imagine.  You can however,
use your own index in arrays and they can be indexed in whatever order you
like.


{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Data.Array
import Data.Ix

newtype MyIx = MyIx Int deriving (Eq, Num, Show)

instance Ord MyIx where
  compare (MyIx a) (MyIx b) =
    case compare a b of
      LT -> GT
      GT -> LT
      EQ -> EQ

instance Ix MyIx where
  range (MyIx a, MyIx b) = map MyIx $ reverse [b..a]
  index (MyIx a, MyIx b) (MyIx c) = a - c
  inRange (MyIx a, MyIx b) (MyIx c) = c <= a && c >= b

blah :: Array MyIx Char
blah = array (3,0) [(0,'a'),(1,'b'),(2,'c'),(3,'d')]

Warning:  I only very lightly tested the above code.

You can mix your index and normal indexes to get the row / col ordering you
are hoping for.

blah2 :: [((MyIx, Int), Char)] -> Array (MyIx, Int) Char
blah2 = array ((3,0),(0,3))

Finally, if you are really looking for something that is designed to be a
matrix, you might try one of several libraries that are out there, like
hmatrix.

Hopefully this helps.

On Thu, Jun 23, 2016 at 3:58 AM, Timothy Washington <twashing at gmail.com>
wrote:

> I'm still trying to get an intuitive understanding of Haskell's Data.Array
> <http://hackage.haskell.org/package/array-0.5.1.1/docs/Data-Array.html#g:5>,
> in contrast to Data.List or Data.Vector.
>
> I very much want a nested array (a matrix), where the parent list (or
> rows) are reversed. But neither *A.array* nor *A.istArray* allow indicies
> to be reversed in their constructors, nor the list comprehensions that
> generate the elements
>
> The only reason I'm using an array, is for the *A.//* function (operating
> on a matrix). Otherwise, I'd use Data.Vector
> <https://hackage.haskell.org/package/vector-0.11.0.0/candidate/docs/Data-Vector.html>
> which does have a reverse function, but a less powerful *V.//* , that
> doesn't accept coordinates in a matrix.
>
> Can I reverse a Data.Array? If not, then why.
>
>
> Thanks
> Tim
>
>
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