Looping matrix multiplication

-----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