[Haskell-cafe] split string into n parts

[Clifford Beshers]

Udo Stenzel wrote:
> jim burton wrote:
>   
>> I want to split a string into 5 parts of equal length, with the last fifth
>> padded if necessary, but can't get it right - here's what I've got - 
>>     
>
> fifths s = unwords.take 5.unfoldr (Just . splitAt l) $ s ++ repeat ' '
>   where l = (length s + 4) `div` 5 
>   


Okay, you win.  That's the nicest answer so far, I think.

But here are solutions with a different theme altogether.  They are 
based on groupBy, not unfoldr. I really like the new `on` function.


module Chunk where

import Data.List

(on) f g = \x y -> f (g x) (g y)

groupByIndex test xs =
    map (map snd) $ groupBy (test `on` fst) $ zip [0..] xs

-- chunk : divide the input string into n chunks of equal length (len), 
with padding

-- chunk1 accepts the number of chunks
chunk1 n pad xs =
    unwords $ take n $ groupByIndex ((==) `on` (`div` len)) $ xs ++ 
repeat pad
        where len = (length xs + n - 1) `div` n

-- chunk2 accepts the length of each chunk
chunk2 len pad xs =
    unwords $ take n $ groupByIndex ((==) `on` (`div` len)) $ xs ++ 
repeat pad
        where n = (length xs + len - 1) `div` len

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