[R] fastest way to multiply each column of a matrix by a single vlaue

[Jason Liao]

Thanks, Prof. Ripley! That is really helpful.

Jason Liao

--- Prof Brian D Ripley <ripley at stats.ox.ac.uk> wrote:
> 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
> 


=====
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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