[R] array addition doesn't recycle!

[Raubertas, Richard]

Another alternative is to use the underappreciated function 
'sweep()':

sweep(A, 1:2, a, "+")

Internally this is about the same as your 'A + array(a, c(2,2,2))'.
But it has the advantage that it makes explicit what the 
relationship between the dimensions of 'A' and 'a' is.  I find 
that relying on implicit recycling tends to produce errors that
are hard to trace, and code that is hard to understand six months
later.

Rich Raubertas
Merck & Co.

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Prof 
> Brian Ripley
> Sent: Wednesday, March 31, 2004 10:00 AM
> To: Tamas Papp
> Cc: R-help mailing list
> Subject: Re: [R] array addition doesn't recycle!
> 
> 
> The recycling rules are documented and this is not amongst them.
> Computer packages do have a tendency to follow their rules 
> rather than 
> read your mind.
> 
> I suspect A + as.vector(a) is what you intended.
> 
> On Wed, 31 Mar 2004, Tamas Papp wrote:
> 
> > Hi,
> > 
> > I have noticed the following:
> > 
> > > a <- array(1:4, c(2, 2))
> > > A <- array(1:4, c(2,2,2))
> > > A + a
> > Error in A + a : non-conformable arrays
> > 
> > It works with a matrix + a vector, why doesn't it work with arrays?
> > Am I missing something?
> > 
> > How would you do the above operation efficiently (ie I need to add a
> > matrix to each "plane" of 3-dim array)?  At the moment I am using
> > something like
> > 
> > A + array(a, c(2,2,2))
> > 
> > but it doesn't seem that efficient.
> 
> Why do you think is `not that efficient'?  Does you have a 
> need to save 
> microseconds?
> 
> -- 
> 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
> 
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