Re: [Haskell-beginners] list splitting - nice implementation?

Lists are good if they are short; otherwise, lists are good if you are only traversing them from head to tail or decapitating them. You want a more complex data structure. On Sat, Nov 17, 2012 at 11:51 PM, Emmanuel Touzery wrote:

well for isSorted, better use the implementation from Data.List.Ordered. That part was poor in performance for sure, but it wasn't my main focus, I was more interested in the rest.

On Sun, Nov 18, 2012 at 8:45 AM, Emmanuel Touzery wrote:

...

Hello,

i wonder what would be the idiomatic way to achieve that algorithm in haskell:

[1,4,56,450,23,46,52] => [1,4,56,450] [1,4,56,450,23,46,52] => [23,46,52]

in other words split the list when one element gets smaller than the previous one. Tge rest of the time the list is sorted. There would be only two lists, not N. I always need the first or second sublist, I don't need both at once. But of course a more complete algorithm handling the N case and/or returning both sublists would be good.

i could code this by hand, but i'm trying to use as much as possible builtin higher-order functions. However in this case so far I've only come up with this:

import Data.List

isSorted :: Ord a => [a] -> Bool isSorted l = (sort l) == l

secondPart :: Ord a => [a] -> [a] secondPart l = head $ filter isSorted (tails l)

firstPart :: Ord a => [a] -> [a] firstPart l = last $ filter isSorted (inits l)

It is concise alright, but it seems contrived and also in terms of performance I don't think it's OK (for small lists sure but for big lists?).

Anyway, somehow I think something as simple as this must be doable very concisely and with optimal performance using only builtin higher-order functions. Any idea?

Thanks!

Emmanuel

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-- -- Regards, KC