[Rd] caching frequently used values

Robert Gentleman rgentlem at fhcrc.org
Wed Dec 13 22:54:28 CET 2006

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:

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
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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
rgentlem at fhcrc.org

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