[R] Avoiding a Loop?

[Berton Gunter]

Roland:

Andy Liaw and others have already given perfectly good answers to this (but
note: Using apply() type functions does **not** avoid loops; apply's **are**
loops). However, mostly as an illustration to reinforce Uwe Ligges's
comments (in the "dim vs length" thread) about the usefulness of sometimes
treating arrays as vectors, I offer the following:

nc<-ncol(yourmatrix)
matrix(yourmatrix[,1]*rep(k^seq(0,length=nc),e=nrow(yourmatrix)),ncol=nc)

Alteratively, one could use matrix multiplication:

yourmatrix[,1]%*%matrix(k^seq(0,length=nc),nrow=1)

Both of these will be very fast (although unlikely to make a noticeable
difference without a lot of rows or columns) and I think use less memory
than outer() (again, unlikely to make a noticeable difference).


-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
 

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Rau, Roland
> Sent: Friday, January 21, 2005 4:32 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Avoiding a Loop?
> 
> Dear R-Helpers,
> 
> I have a matrix where the first column is known. The second column is
> the result of multiplying this first column with a constant 
> "const". The
> third column is the result of multiplying the second column with
> "const".....
> So far, I did it like this (as a simplified example):
> 
> nr.of.columns <- 4
> 
> myconstant <- 27.5
> 
> mymatrix <- matrix(numeric(0), nrow=5, ncol=nr.of.columns)
> 
> mymatrix[,1] <- 1:5
> 
> for (i in 2:nr.of.columns) {
> 	mymatrix[,i] <- myconstant * mymatrix[,i-1]
> }
> 
> 
> Can anyone give me some advice whether it is possible to 
> avoid this loop
> (and if yes: how)?
> 
> Any suggestions are welcome!
> 
> Thanks,
> Roland
> 
> 
> 
> 
> +++++
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