Rory there are several packages that perform this.
I would use permn() of the combinat library, then, if lexicographical order is important, sort it explicitly. HTH rksh [EMAIL PROTECTED] wrote:
Hi all Here is a rather naive implementation of the SEPA algorithm for generating lexical permutations: lexperm3 <- function(x, n=length(x)) { perms <- list() k <- 1 perms[[k]] <- x k <- k + 1 for (y in 1:(factorial(n)-1)) { i <- n-1 while (x[i] > x[i+1] && i > 0) { i <- i - 1 } # i is largest index st x[i] > x[i+1] j <- n # find min{ x[j], st n>=j>=i+1 and x[j]>x[i] } while (x[j] <= x[i] && j > i) { j <- j - 1 } # swap x[i] and x[j] tmp <- x[i];x[i] <- x[j]; x[j] <- tmp # now sort everything from x[i+1]..x[n] # by reversing the elements within p <- i + 1 q <- n while (p < q) { tmp <- x[p]; x[p] <- x[q]; x[q] <- tmp p <- p + 1 q <- q - 1 } perms[[k]] <- x k <- k + 1 } perms } This, as you can imagine, is severely slow. I would like to speed up this function if possible, which I guess would involve vectorizing the inner loop..does anyone have any ideas about how to improve this code's runtime? One small potential optimization I tried was to shorten the "sort by reverse ordering" near the end of the inner loop : I tried replacing it with rev() and sort(decreasing=TRUE) over a partial subset of the x vector: however rev() was much slower, and I dont think sort() supports lexicographic ordering, so that didnt work. Thanks rory Rory Winston RBS Global Banking & Markets 280 Bishopsgate, London, EC2M 4RB Office: +44 20 7085 4476 ***********************************************************************************The Royal Bank of Scotland plc. Registered in Scotland No 90312. Registered Office: 36 St Andrew Square, Edinburgh EH2 2YB. Authorised and regulated by the Financial Services AuthorityThis e-mail message is confidential and for use by the=2...{{dropped:25}} ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
-- Robin K. S. Hankin Uncertainty Analyst University of Cambridge 19 Silver Street Cambridge CB3 9EP 01223-764877 ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
