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

[Jón Fairbairn]

I wrote:
> jim burton <jim at sdf-eu.org> 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 at cl.cam.ac.uk