[R] Idiom for column operations?

[Prof Brian Ripley]

On Tue, 26 Oct 2004, Rob Steele wrote:

> Is there a better way to express operations between matrices and column 
> vectors than transposing the matrix twice?
> 
> This is the kind of thing I'm talking about:
> 
> m = matrix(1:20, 3, 4)

Ouch: that gives a warning as 20 > 3*4.

> v = colSums(m)
> 
> t(t(m) / v)   ## <--  kinda ugly, ain't it?
> 
> I thought of converting the column vector to a matrix:
> 
> m / matrix(v, nrow = nrow(m), ncol = length(v), byrow = TRUE)
> 
> But that seems even worse.

Use the fact that matrix/vector is done as vector/vector and matrices are 
stored down columns:

m/rep(v, each=nrow(m))

You will find that (or using other variants on rep) in lots of S/R code.

-- 
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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595