[Rd] caching frequently used values
Tamas K Papp
tpapp at Princeton.EDU
Wed Dec 13 23:56:32 CET 2006
Hi Robert,
Thanks for your answer. I would create and environment with
new.env(), but how can I assign and retrieve values based on a
numerical index (the derivative)? The example of the help page of
assign explicitly shows that assign("a[1]") does not work for this
purpose.
Thanks,
Tamas
On Wed, Dec 13, 2006 at 01:54:28PM -0800, Robert Gentleman wrote:
> 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
> >
> >______________________________________________
> >R-devel at r-project.org mailing list
> >https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
> --
> 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
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