[Haskell-cafe] Rewrite this imperative in FP way

[Haisheng Wu]

*d = [sum $ map (a !!) [i | i <- b, j <- c, i + j < 3, i + j == dIndex] |
dIndex <- [0..3]] *

This is cool.

-Simon


On Sun, Feb 5, 2012 at 5:07 PM, L Corbijn <aspergesoepje at gmail.com> wrote:

> On Sun, Feb 5, 2012 at 7:28 AM, Haisheng Wu <freizl at gmail.com> wrote:
> > a = [1,1,1,1]
> > b = [0,1,2,3]
> > d = [0,0,0,0]
> >
> > for i in b:
> >   for j in c:
> >     if (i+j)<3:
> >       d[i+j] += a[i]
> >
> > My just work implementation in Haskell
> > http://hpaste.org/57452
> >
> > Another people implementation in Haskell with Monad and it turns out
> complex
> > and very imperatively.
> > http://hpaste.org/57358
> >
> > Do you have any cool solution in FP way?
> >
> > Thanks.
> > -Simon
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
>
> There are several ways to make it nicer.
> Without assumption on what lists a, b and c contain it can be written as
> d = [sum $ map (a !!) [i | i <- b, j <- c, i + j < 3, i + j == dIndex]
> | dIndex <- [0..3]]
>
> With the assumption that b and c are both [0..3] this can be 'improved' to
> d = (take 3 . map sum . tail $ inits a) ++ replicate (4 - 3) 0
> This generates the first three values by the complicated expression
> and then adds the extra zero's by using the replicate expression. This
> works as the value of the i-th element of d is the sum over the first
> i elements in a if i < 3 and 0 otherwise. A list of lists with the
> first i elements is generated with 'tail $ inits a' which is then
> summed and restricted to length 3.
> An alternative for this is
> d = (take 3 . snd $ mapAccumL (\acc ai -> (acc + ai, acc + ai)) 0 a)
> ++ replicate (4 - 3) 0
> Where the summation and tail generating is done in the mapAccumL function.
>
> Greetings,
> Lars
>
> P.S. Yes the replicate (4-3) 0 can be replaced by [0], but I wanted to
> explicitly include the length (4) of the expected list.
>
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