I'm glad I asked. 1) I'd forgotten or never knew about ^:a: 2) I struggled to get the solution for 9, and my solution page faulted to death at 10.
permutations ((i.@:(-&! <:) + !@:<:)@:x: A. >:@:i.)^:(1&<) <@permutations"0 [ 1 3 +-+-----+ |1|2 1 3| | |2 3 1| | |3 1 2| | |3 2 1| +-+-----+ > > Reference: http://rosettacode.org/wiki/Topswops > Date: Mon, 26 Nov 2012 22:19:42 -0500 > From: Dan Bron <j...@bron.us> > To: "programm...@jsoftware.com" <programm...@jsoftware.com> > Subject: Re: [Jprogramming] count iterations > Message-ID: <53154fe6-9368-45b0-8120-ba3e3861e...@bron.us> > Content-Type: text/plain; charset=us-ascii > > 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 ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
