Greg Fitzgerald garious at gmail.com
Tue Jun 5 14:04:09 EDT 2007

```Kevin,

Below is my attempt, which hopefully is bad enough to get this thread
rolling for you. :)

It rotates the 'i'th element 'n' times by swapping the 'i'th element with
the element to its right 'n' times.  It looks horribly inefficient to me,
but is fairly simple and only depends on the prelude.  I think it ought to
be possible to reduce this to a very simple function with no recursion and
one strategic splitAt, but I've run out of time to work more on it. :(

rotate :: [a] -> Int -> Int -> [a]
rotate xs i n = snd (iterate swap (i,xs) !! n)

swap :: (Int, [a]) -> (Int, [a])
swap (i,xs)
| i == length xs - 1 = (0, last xs : init (tail xs) ++ [head xs])
| otherwise          = (i + 1, start ++ [right,left] ++ end)
where
(start, left:right:end) = splitAt i xs

-Greg

On 6/4/07, kevin birch <kbirch at pobox.com> wrote:
>
> On 火, 2007-6月-05, at 02:54, Greg Fitzgerald wrote:
>
> > rotating the fourth element 2 positions would result in: [1, 2, 4, 3, 5]
>
> Seems odd.  Should that be [4,1,2,3,5]?
>
> Yes, I meant to use the 5 element in my second example.  Sorry for the
> confusion.
>
>  > Is there an idomatic way to handle both of these cases in a function?
> Generally people like to see your attempt at a solution before giving the
> idomatic one so that they are sure it's not a homework question.  What do
> you have so far?
>
> Yeah, I only wish I had gone to a school that would be forward thinking
> enough to each FP.  ;-)
>
> Here is my version:
>
> rotate :: Array Integer Card -> Integer -> Integer -> Array Integer Card
> rotate a i n
>     | i <= u - n = a // [(i, a ! (i + 1)), (i + 1, a ! (i + 2)), (i + 2, a
> ! i)]
>     | otherwise = a // zip [l..u] (h ++ [a ! i] ++ filter (not . (== (a !
> i))) t)
>     where (l, u) = bounds a
>           (h, t) = splitAt (fromInteger ((i - u) + n)) \$ elems a
>
> This function is part of my implementation of the Solitaire encryption
> algorithm, so that is why I have the reference to a Card data type.  This
> does what I want, and seems basically idiomatic, but perhaps it could be
> better.
>
> Thanks,
> Kevin
>
>
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