You don't have to do the swaps, you could just calculate them:
nn=. 2 4 1 3
>:i.<./(+:nn i. 1),+:nn i: 1
1 2 3 4
nn=. 2 3 4 5 1 6 7 8
>:i.<./(+:nn i. 1),+:nn i: 1
1 2 3 4 5 6 7 8
On Mon, Nov 26, 2012 at 10:19 PM, Dan Bron <j...@bron.us> wrote:
> I added a solution to the task at RosettaCode. If we let J do the work
> for us, the solution is fairly simple. We use ^:a: ("swop" until you hit
> your fixed point, i.e. until 1={. ) and the count is the number of swops,
> i.e. the length of the result.
>
> The task calls for calculating the "topswop" all possible permutations of
> N integers (i.e. the maximal number of swops for any given permutation of
> rank N). The task only calls for N up to 10, so brute force (calculating
> all !N swops and taking the max) suffices. But for large N, brute force
> would explode, so another method would be required.
>
> Just looking at the series, I suspect a simple arithmetical
> (combinatorical) solution exists. It might even be ready-made in the OEIS.
> But I'll leave that to you smart guys.
>
> -Dan
>
>
> Please excuse typos; composed on a handheld device.
>
> On Nov 26, 2012, at 9:19 PM, David Ward Lambert <b49p23t...@stny.rr.com>
> wrote:
>
> > Reference: http://rosettacode.org/wiki/Topswops
> > What are good ways to count iterations?
> >
> > While=: 2 : 'u^:v^:_'
> >
> > topswop=: (|.@:{. , }.)~ {.
> >
> > topswop While (1 ~: {.) 2 4 1 3
> > 1 2 3 4
> >
> > These ideas seem like structural nightmares:
> > * separate data and counter with boxes;
> > * append a counter to the data;
> > * explicit code with a global noun;
> > * +/@:(0 = (0,~topswop)While(1 ~: {.)) NB. <laughs>.
> >
> > The boxes or explicit code seem most general.
> >
> > Thank you, Dave.
> >
> > Bonus: find a definition topswop using C.
> >
> > ----------------------------------------------------------------------
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> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>
--
Devon McCormick, CFA
^me^ at acm.
org is my
preferred e-mail
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