[R] fill Matrix quicker

[peter dalgaard]

On Aug 1, 2011, at 20:12 , monk wrote:

> dear all,
> 
> i have a quite simple question, i want to fill up a Matrix like done in the
> following function,
> but the performance is very bad for large dimensions
> is there a way to do this like with apply or something similar?
> 
> 
> makeMatrix <- function(a, b,dim) {
> 	X=matrix(0,ncol=dim,nrow=dim)
> 
> 
> 
> 	for (i in c(1:dim)){
> 		for (j in c(1:dim)) {
> 		if (i==j) {X[i,j]<-a}
> 		else { X[i,j]<-  exp(( -1*abs(i-j))/(3*b)) }
> 		}
> 	}
> 	X
> }
> 

I'd go for something like 

X <- outer(1:dim, 1:dim, function(i,j) exp(-abs(i-j)/3/b))
diag(X) <- a


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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
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