I found the following slightly surprising and thought that it might (eventually) be useful to someone to document it:
Problem: I wanted to reorder the columns of the matrix produced by combn(n,p) so that adjacent columns only differed by one element. I assumed that this was a problem known to the Assyrians and resisted the temptation to write to r-help, and through assiduous googling finally discovered that this was still a topic of current research in computer science. Solution: Limin Xiang and Kazuo Ushijima (2001) "On O(1) Time Algorithms for Combinatorial Generation," Computer Journal, 44(4), 292-302. provides a nice algorithm (in Pascal) which I then translated into ratfor and plan to incorporate eventually into quantreg. It manages to do n=500 p = 3 in about 3 secs on my old G5. It probably isn't of interest to do problems too much bigger than this since we are already at 20 x 10^6 columns. url: www.econ.uiuc.edu/~roger Roger Koenker email [EMAIL PROTECTED] Department of Economics vox: 217-333-4558 University of Illinois fax: 217-244-6678 Champaign, IL 61820 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
