How about this: import Array a = [1,1,1,1] b = [0,1,2,3] c = [0,2] d = elems $ accumArray (+) 0 (0,3) [(i+j,a!!i) | i<-b, j<-c, i+j<3] -- Thomas H On 2012-02-06 12:01 , Haisheng Wu wrote:

*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 mailto:aspergesoepje@gmail.com> wrote:

On Sun, Feb 5, 2012 at 7:28 AM, Haisheng Wu mailto:freizl@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@haskell.org mailto:Haskell-Cafe@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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