On Thu, Nov 8, 2012 at 6:16 PM, Nordlund, Dan (DSHS/RDA) <nord...@dshs.wa.gov> wrote: > >> -----Original Message----- >> From: r-help-boun...@r-project.org [mailto:r-help-bounces@r- >> project.org] On Behalf Of wilkesma >> Sent: Thursday, November 08, 2012 5:49 AM >> To: r-help@r-project.org >> Subject: [R] Looping matrix multiplication >> >> I am dealing with a number of time-series consisting of 2250 samples of >> flow >> velocity in 2D (Vx, Vy). >> >> For each time-series I have determined the resultant vector angle (d) >> and >> magnitude (Vres) based on the ensemble means of Vx and Vy and now want >> to >> rotate each point in the time-series by d so that mean(Vx)=Vres and >> mean(Vy)=0. >> >> Here is the code I have written to perform the rotation for a single >> time-step: >> >> n <- 2250 #number of samples in time series >> R <- matrix ((c ((cos(d)), (-sin(d)), (sin(d)), (cos(d)))), 2) >> #rotation >> matrix, where d is the resultant vector angle in degrees >> V <- t (matrix ((c (Vx, Vy)), n)) #velocity matrix >> Vadj <- R %*% (V) [, 1] #e.g. for the first Vx, Vy pair in the time- >> series >> >> Now what I want to do is loop this to give me the solution at each >> time-step >> and then append the adjusted Vx and Vy time-series to a new matrix. >> Can >> someone please suggest how I should about this? >> >> Thanks, >> Martin >> >> > > Perhaps the following will get what you want > > Vadj <- V %*% t(R) >
? crossprod suggests that tcrossprod(V, R) is to be slightly preferred. Cheers, Michael ______________________________________________ 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.
