[Haskell-cafe] Re: split string into n parts

I wrote:

jim burton wrote:

...

Thankyou! It's http://www.rubyquiz.com - They are mostly well suited to haskell, lot of mazes etc. I've done 5 or 6 with varying degrees of success but have learned a lot. This thing about strings in fifths is from #1, the solitaire cipher.

At a quick glance I can't see which bit needs it. The only mention of five is where it asks to split the string into groups of five characters (not into five equal parts), padded with Xs.

You can do that like this:

splitAtMb n l = let p = splitAt n l in if null $ fst p then Nothing else Just p

Gah! Brain AWOL. I'm surprised no-one picked me up on that. Why didn't I use: splitAtMb n [] = Nothing splitAtMb n l = Just $ splitAt n l ?

in_fives l = unfoldr (splitAtMb 5) (l ++ replicate (length l `mod` 5) 'X')

And using length makes this over-strict. maybe something like groups_of n = unfoldr (splitPad 5) where splitPad [] = Nothing splitPad l = Just $ mapFst (padwith 'X') (splitAt n l) padwith c l = take n $ l ++ replicate n c mapFst f (a,b) = (f a, b) -- in Data.Graph.Inductive.Query.Monad which is a little bit inefficient, but less clunky than checking for the end of list in order to apply padwith just once. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk