[Haskell-cafe] Mutable arrays

[Rodrigo Queiro]

Sorry, I was lazy. New maximum':
maximum' = foldl1' max

On 02/02/2008, Rodrigo Queiro <overdrigzed at gmail.com> wrote:
>
> This is my attempt at some nicer code:
>
> maximum' (x:xs) = foldl' max x xs
> maximum' _ = undefined
>
> modifyArray :: (MArray a e m, Ix i) => (e -> e) -> a i e -> m ()
> modifyArray fn arr = do
>     bounds <- getBounds arr
>     forM_ (range bounds) (modifyElement fn arr)
>
> modifyElement :: (MArray a e m, Ix i) => (e -> e) -> a i e -> i -> m ()
> modifyElement fn arr i = do
>     x <- readArray arr i
>     writeArray arr i (fn x)
>
> normalizeArray :: (MArray a e m, Ix i, Fractional e, Ord e) => a i e -> m
> ()
> normalizeArray arr = do
>     arr_elems <- getElems arr
>     let max_elem = maximum' arr_elems
>     modifyArray (/max_elem) arr
>
> On 02/02/2008, Jeff φ <jeff1.61803 at gmail.com> wrote:
> > Hello,
> >
> > I'm trying to write code that will take a mutable 2D array and normalize
> it
> > by dividing all elements by the largest element.
> >
> > I managed to write code to do this, but it seems overly complex.  I
> could
> > write something much simpler in Clean or C++.  Most likely, my code is
> > complex because I don't have any experience with mutable arrays in
> Haskell.
> > I couldn't find any tutorials on the Internet.  I'd be grateful for
> > suggestions on simplifying the following code.   Thanks.
> >
> >
> > {-# OPTIONS_GHC -fglasgow-exts -fbreak-on-exception #-}
> >
> > -- normalize_ary This takes a mutable array.  Determines the largest
> >  -- element in the array (max_elem) and then divides every element by
> > -- max_elem.
> >  normalize_ary :: (Num t1,
> >                   Num t,
> >                    Ix t,
> >                   Ix t1,
> >                    MArray a e t2,
> >                   Ord e,
> >                    Fractional e,
> >                   Enum t,
> >                    Enum t1) =>
> >                  a (t, t1) e -> t2 ()
> >  normalize_ary ary =
> >     do
> >          -- The following two commented out lines of code show my first
> >         -- attempt at determining a value for max_elem.  However, this
> >          -- produces a stack overflow.
> >
> >         -- elem_ary <- getElems ary
> >       -- let max_elem = foldl1 max elem_ary
> >
> >     max_elem <- calc_max_2d_elem ary
> >      max_elem `seq` map_in_place_2d_arr (\x -> x / max_elem) ary
> >
> >
> >  map_in_place_2d_arr :: (MArray a e t, Enum t2, Enum t1, Ix t1, Ix t2)
> =>
> >                        (e -> e) -> a (t1, t2) e -> t ()
> >  map_in_place_2d_arr fn arr = ret
> >     where
> >        ret = do ((i1,j1),(i2,j2)) <- getBounds arr
> >                ( mapM_ (\i ->  do v <- readArray arr i
> >                                    writeArray arr i (fn v)
> > )
> >                        [(i,j) | i <- [i1..i2], j <- [j1..j2]])
> >
> >
> > calc_max_2d_elem :: (Ord t, MArray a t t1, Ix t2, Ix t3, Num t3, Num t2)
> =>
> >                      a (t3, t2) t -> t1 t
> > calc_max_2d_elem arr =
> >      do m <- readArray arr (0,0)
> >        (_,(i_max, j_max)) <- getBounds arr
> >         let calc_max_loop arr m (i,j)
> >          | j == j_max     = return m
> >           | otherwise      = do e <- readArray arr (i,j)
> >                    let m2 = max e m
> >                                     m2 `seq` calc_max_loop
> > arr m2 nxt_idx
> >          where nxt_idx | i == i_max - 1 = (0,j+1)
> >                             | otherwise      = (i+1,j)
> >        calc_max_loop arr m (0,0)
> >
> >
> > _______________________________________________
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> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
>
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