R help - Jun 2007 - Another loop avoidance question.

Hi

I want to sum over one of the dimensions of a n x k1 x k2 array in
which each column is the product of the corresponding columns from two
matrices with dimensions n x k1 and n x k2. I can see two approaches:
a loop on k1 and a loop on k2. But I cannot figure a solution that
avoids the loop? Is it possible? (I don't refer to apply or lapply etc
either as they are just hidden loops so, correct me if I'm wrong, I
can't see they hold any real advantage here). The following does
exactly what I want (except for the loops). My aim is to find the
solution which minimises processing time.

cheers
Dave

## Basic paramters
k1=3
k2=2
n=10
m1 = matrix (1:(n*k1), nrow=n, ncol=k1)
m2 = matrix (1:(n*k2), nrow=n, ncol=k2)
## Approach 1: loop on k1
output1 = matrix(0,nrow=n,ncol=k2)
pt1 = proc.time(for (i in 1:k1) output1 = output1 + m1[,i]*m2)
## Approach 2: loop on k2
output2 = matrix(0,nrow=n,ncol=k2)
pt2 = proc.time(for (i in 1:k2) output2[,i] =  rowSums( m1*m2[,i] ))
## Same result
sum(output1-output2)

-- 
David Pleydell

Laboratoire de Biologie Environnementale
USC INRA-EA 3184
Universit? de Franche-Comt?
Place Leclerc F
25030
Besan?on
France

(0033) 0381665763
dpleydel at univ-fcomte.fr

Sorry folks I posted the code with a bug in approach 2. aarrrhh. it should have
been...

## Approach 2: loop on k2
output2 = matrix(0,nrow=n,ncol=k2)
pt2 = proc.time(for (i in 1:k2) output2[,i] = rowSums( m1*(m2[,rep(i,3)]) ))
 
> I haven't seen any other response yet (after 8 hours!) but doesn't:
> rowSums(m1)*m2
> do what you want?
> 
> HTH
> Ray Brownrigg
... which is a very long winded way to do exactly what Ray did very
easily. Thanks for the pointer Ray.

Dave