Bill Dunlap TIBCO Software Inc - Spotfire Division wdunlap tibco.com
> -----Original Message----- > From: r-help-boun...@r-project.org > [mailto:r-help-boun...@r-project.org] On Behalf Of Bryan Keller > Sent: Thursday, September 03, 2009 2:11 PM > To: r-help@r-project.org > Subject: [R] Recursion is slow > > The following recursion is about 120 times faster in C#. I > know R is not known for its speed with recursions but I'm > wondering if anyone has a tip about how to speed things up in R. > > #"T" is a vector and "m" is a number between 1 and sum(T) > > A <- function(T,m) { > lt <- length(T) > > if (lt == 1) { > if (0 <= m & m <= T[1]) { > return(1) > } else { > return(0) > } > } > R <- 0 > for (u in 0:T[lt]) { > R <- (R+(A(T[1:(lt-1)],(m-u)))) > } > return(R) > } > For starters, remove all repeated calculations from the for loop. Then vectorize the length(T)==2 case to avoid a lot of calls to the scalar case. Finally, I noticed that the answer was independent of the order of T and that putting it in reverse sorted order was the faster order, so I did that. I use default values of arguments so you don't have to supply derived values when you start but the recursions don't have to recompute some things. B <- function(T,m,lt=length(T), Tsorted = rev(sort(T))) { if (lt == 1L) { R <- as.integer(m <= Tsorted && 0L <= m) } else if (lt == 2L) { mu <- m - (0:Tsorted[2L]) R <- sum(mu <= Tsorted[1L] & 0L <= mu) } else { R <- 0L lt1 <- lt-1L T1 <- Tsorted[1:lt1] for (mu in m-(0:Tsorted[lt])) { R <- R + B(unused, mu, lt1, T1) } } R } E.g., > system.time(A( c(20,40,35,21,31), 100)) user system elapsed 13.23 0.08 12.93 > system.time(B( c(20,40,35,21,31), 100)) user system elapsed 0.35 0.00 0.33 > A( c(20,40,35,21,31), 100) [1] 193363 > B( c(20,40,35,21,31), 100) [1] 193363 Bill Dunlap TIBCO Software Inc - Spotfire Division wdunlap tibco.com > ------------- > Bryan Keller, Doctoral Student/Project Assistant > Educational Psychology - Quantitative Methods > The University of Wisconsin - Madison > > ______________________________________________ > R-help@r-project.org 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. > ______________________________________________ R-help@r-project.org 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.