[R] Looping matrix multiplication

[Nordlund, Dan (DSHS/RDA)]

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of wilkesma
> Sent: Thursday, November 08, 2012 5:49 AM
> To: r-help at 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)


Hope this is helpful,

Dan

Daniel Nordlund
Olympia, WA 98504-5204