[R] A more efficient code using whole object approach?

[Greg Snow]

A couple of thoughts (hopefully others will have even better
suggestions):

1. Could you do a some of the computations on the log scale?  This would
turn exponents into multiplications and multiplications into additions
and may turn a lot of the work into matrix multiplications.

2. It does not appear that the order matters (i=1, 2, ...), so this
could be easily parallelized, the nws package allows a fairly straight
forward way to do this if you have access to multiple computers (I for
one don't have a problem stealling cycles from my coworkers computers
when they are not doing computationally intesive things).

Hope this helps, 


-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at intermountainmail.org
(801) 408-8111
 

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Muzaffer Musal
Sent: Wednesday, October 04, 2006 11:36 AM
To: r-help at stat.math.ethz.ch
Subject: [R] A more efficient code using whole object approach?

Dear All,
  I have the following code which works fine but takes 16+ hours to run
and I was hoping someone could give me a hint as to how I could make it
more efficient.
  Let me explain what the code is supposed to do. Each single row of the
m.o.f matrix (n*p dim) has to go through the following operation
Dm*m.o.f[i]^alpha1m where both Dm and alpha1m have n*1 dimensions.  Each
row of the m.o.f is to be evaluated n times for the different values of
alpha1m and Dm. My code changes the Dm and alpha1m (n*1) matrices into
(n*p) matrices (not shown below, done only 1 time) and also change the
(1*p) m.o.f row into a m.o (n*p) matrix (shown below, done n times). For
each of the n rows of the m.o.f matrix we repeat this operation which we
record in the temp matrix.  The third line of the loop multiplies for
each row, the columns of the matrix and takes the resultant mean of the
vector. 
  I know I should use the whole object approach somehow but after many
tries I gaave up for the long approach. I know the explanation above is
rather long and I apologize for that, any hint/help will be greatly
appreciated.
     
  for (i in 1:numrows)
  {
  mo.o<-t(array(m.o.f[i,],dim=c(numcols,numrows)))
  t<-Dm*m.o^alpha1m
  l1[i]<-mean(apply(t,1,"prod"))
  }
  Best Regards,

 		
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