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-Vect... 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
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
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-Vect... 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
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
Hmm, this is very helpful - didn't know it existed. Thanks very much, I'll
check it out!
Tim
On Thu, Jun 23, 2016 at 7:41 AM, 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.
participants (2)
-
David McBride -
Timothy Washington