caching frequently used values

the idea you are considering is also, at times, referred to as 
memoizing. I would not use a list, but rather an environment, and 
basically you implement something that first looks to see if there is a 
value, and if not, compute and store. It can speed things up a lot in 
some examples (and slow them down a lot in others).

Wikipedia amongst other sources:
 http://en.wikipedia.org/wiki/Memoization

Environments have advantages over lists here (if there are lots of 
matrices the lookup can be faster - make sure you use hash=TRUE), and 
reference semantics, which you probably want.

Tamas K Papp wrote:

Hi,

I am trying to find an elegant way to compute and store some
frequently used matrices "on demand".  The Matrix package already uses
something like this for storing decompositions, but I don't know how
to do it.

The actual context is the following:

A list has information about a basis of a B-spline space (nodes,
order) and gridpoints at which the basis functions would be evaluated
(not necessarily the nodes).  Something like this:

bsplinegrid <- list(nodes=1:8,order=4,grid=seq(2,5,by=.2))

I need the design matrix (computed by splineDesign) for various
derivatives (not necessarily known in advance), to be calculated by
the function

bsplinematrix <- function(bsplinegrid, deriv=0) {
 x <- bsplinegrid$grid
 Matrix(splineDesign(bslinegrid$knots, x, ord=basis$order,
                     derivs = rep(deriv,length(x))))
}

However, I don't want to call splineDesign all the time.  A smart way
would be storing the calculated matrices in a list inside bsplinegrid.
Pseudocode would look like this:

bsplinematrix <- function(bsplinegrid, deriv=0) {
 if (is.null(bsplinegrid$matrices[[deriv+1]])) {
   ## compute the matrix and put it in the list bsplinegrid$matrices,
   ## but not of the local copy
 }
 bsplinegrid$matrices[[deriv+1]]
}

My problem is that I don't know how to modify bsplinegrid$matrices
outside the function -- assignment inside would only modify the local
copy.

Any help would be appreciated -- I wanted to learn how Matrix does it,
but don't know how to display the source with s3 methods (getAnywhere
doesn't work).

Tamas

-- 
Robert Gentleman, PhD
Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
1100 Fairview Ave. N, M2-B876
PO Box 19024
Seattle, Washington 98109-1024
206-667-7700
rgentlem at fhcrc.org