R help - Sep 2001 - fastest way to multiply each column of a matrix by a single vlaue

Let A be a m by n matrix and b a length n vector. What is the fastest
vectorized code for doing

for(j in 1:n) A[, j] <- A[, j]/b[j]   ?

solution 1:

t(t(A)/b)

solution 2:

B <- matrix( rep(b, m), byrow=T, nrow=m )
A/B 

anything else?

I have a program that uses this kind of operation million of times and
I appreciate your input.

Thanks.

Jason Liao



====Jason G. Liao
Department of Biometry and Epidemiology
Medical University of South Carolina
135 Rutledge Ave., STE 1148, Charleston, SC 29425
phone (843) 876-1114, fax (843) 876-1126

http://www.geocities.com/jg_liao/index.html

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On Mon, 3 Sep 2001, Jason Liao wrote:
> Let A be a m by n matrix and b a length n vector. What is the fastest
> vectorized code for doing
>
> for(j in 1:n) A[, j] <- A[, j]/b[j]   ?
>
> solution 1:
>
> t(t(A)/b)
>
> solution 2:
>
> B <- matrix( rep(b, m), byrow=T, nrow=m )
> A/B
You don't need the rep because of the re-cycling rule.  Either

B <- matrix(b, m, n, byrow=T)

or, a litle faster

A/rep(b, rep(m, n))


You'll find the S/R system code uses the last most often.

[...]

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Why not make two versions of your program with each alternative and test
their speed with system.time()? Then, you'll have this indication "in
situation".

Best regards,

Philippe Grosjean


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 ) ) ) ) )	 __               	 __
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>Let A be a m by n matrix and b a length n vector. What is the fastest
>vectorized code for doing
>for(j in 1:n) A[, j] <- A[, j]/b[j]   ?
>solution 1:
>t(t(A)/b)
>solution 2:
>B <- matrix( rep(b, m), byrow=T, nrow=m )
>A/B
>anything else?
>I have a program that uses this kind of operation million of times and
>I appreciate your input.
>Thanks.
>Jason Liao


====Jason G. Liao
Department of Biometry and Epidemiology
Medical University of South Carolina
135 Rutledge Ave., STE 1148, Charleston, SC 29425
phone (843) 876-1114, fax (843) 876-1126

http://www.geocities.com/jg_liao/index.html

__________________________________________________
Do You Yahoo!?
Get email alerts & NEW webcam video instant messaging with Yahoo! Messenger
http://im.yahoo.com
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