Forwarding to -cafe
-------- Original Message --------
Subject: Re: [Haskell-cafe] Period of a sequence
Date: Mon, 27 Jun 2011 04:46:10 -0700 (PDT)
From: michael rice
To: Steffen Schuldenzucker
Hi Steffen,
Repeating decimals.
5/7 == 0.714285 714285 7142857 ... Period = 6
It does seem like a difficult problem.
This one is eventually repeating, with Period = 3
3227/555 = 5.8144 144 144…
Michael
--- On *Mon, 6/27/11, Steffen Schuldenzucker
//*wrote:
From: Steffen Schuldenzucker
Subject: Re: [Haskell-cafe] Period of a sequence
To: "michael rice"
Cc: haskell-cafe@haskell.org
Date: Monday, June 27, 2011, 4:32 AM
On 06/26/2011 04:16 PM, michael rice wrote:
> MathWorks has the function seqperiod(x) to return the period of
sequence
> x. Is there an equivalent function in Haskell?
Could you specify what exactly the function is supposed to do? I am
pretty sure that a function like
seqPeriod :: (Eq a) => [a] -> Maybe Integer -- Nothing iff non-periodic
cannot be written. If "sequences" are represented by the terms that
define them (or this information is at least accessible), chances
might be better, but I would still be interested how such a function
works. The problem seems undecidable to me in general.
On finite lists (which may be produced from infinite ones via
'take'), a naive implementation could be this:
>
> import Data.List (inits, cycle, isPrefixOf)
> import Debug.Trace
>
> -- Given a finite list, calculate its period.
> -- The first parameter controls what is accepted as a generator.
See below.
> -- Set it to False when looking at chunks from an infinite sequence.
> listPeriod :: (Eq a) => Bool -> [a] -> Int
> listPeriod precisely xs = case filter (generates precisely xs)
(inits xs) of
> -- as (last $ init xs) == xs, this will always suffice.
> (g:_) -> length g -- length of the *shortest* generator
>
> -- @generates prec xs g@ iff @g@ generates @xs@ by repitition. If
@prec@, the
> -- lengths have to match, too. Consider
> --
> -- >>> generates True [1,2,3,1,2,1,2] [1,2,3,1,2]
> -- False
> --
> -- >>> generates False [1,2,3,1,2,1,2] [1,2,3,1,2]
> -- True
> generates :: (Eq a) => Bool -> [a] -> [a] -> Bool
> generates precisely xs g = if null g
> then null xs
> else (not precisely || length xs `mod` length g == 0)
> && xs `isPrefixOf` cycle g
>
-- Steffen
