[R] Multiple operations on list

[Berton Gunter]

Stephane:

1. apply() functions ARE (implicit) loops.
2. ?outer might be useful, but I suspect would be even more inefficient

... so I think the answer is, no, you must loop either explicitly or
implicitly.

However, my guess is that whatever you're trying to do is either built into
R or can be done much more efficiently. Explicit matrix multiplication is
almost always avoidable in statistics.

-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
 
"The business of the statistician is to catalyze the scientific learning
process."  - George E. P. Box
 
 

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Stephane DRAY
> Sent: Monday, September 20, 2004 10:25 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Multiple operations on list
> 
> Hello,
> 
> suppose I have a list with matrices:
> 
> a=list(x1=matrix(rnorm(10),5,2),x2=matrix(rnorm(10),5,2),x3=ma
> trix(rnorm(10),5,2))
> 
> I want to compute for all combination of xi and xj (x1,x2 
> x1,x3 and x2,x3) 
> a value.
> This value is given for the pair x1,x2 by 
> trace(x1%*%t(x1)%*%x2%*%t(x2)) / 
> trace(x1%*%t(x1))*trace(x2%*%t(x2))
> 
> I know that product matrices t(xi)%*%xi can be obtained by:
> 
> aa=lapply(a,crossprod)
> 
> but I do not know how to "mix" the values in aa to obtain the 
> desired values.
> 
> Is there a way to do it without for loop ?
> 
> Thanks in advances,
> 
> sincerely
> 
> Stéphane DRAY
> --------------------------------------------------------------
> ------------------------------------ 
> 
> Département des Sciences Biologiques
> Université de Montréal, C.P. 6128, succursale centre-ville
> Montréal, Québec H3C 3J7, Canada
> 
> Tel : (514) 343-6111 poste 1233         Fax : (514) 343-2293
> E-mail : stephane.dray at umontreal.ca
> --------------------------------------------------------------
> ------------------------------------ 
> 
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