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\name{cache}
\alias{cache}
\docType{methods}
\title{Evaluate an expression if its value is not already cached.}
\description{
Cache the evaluation of an expression in the file system.
}
\usage{
cache(expr, dir=".", prefix="tmp_R_cache_")
}
\arguments{
\item{expr}{An expression of the form \code{LHS <- RHS}, Where
\code{LHS} is a variable name, \code{RHS} is any valid expression,
and \code{<-} must be used (\code{=} will not work).}
\item{dir}{A string specifying the directory into which cache files
should be written (also where to go searching for an appropriate
cache file).}
\item{prefix}{A string giving the prefix to use when naming and
searching for cache files. The default is \code{"tmp_R_cache_"}}
}
\details{
This function can be useful during the development of computationally
intensive workflows, for example in vignettes or scripts. The
function uses a cache file in \code{dir} which defaults to the current
working directory whose name is obtained by \code{paste(prefix, name,
".RData", sep="")}.
When \code{cache} is called and the cache file exists, it is loaded
and the object whose name is given on the left of \code{<-} in
\code{expr} is returned. In this case, \code{expr} is \emph{not}
evaluted.
When \code{cache} is called and the cache file does not exist,
\code{expr} is evaluted, its value is saved into a cache file, and
then its value is returned.
The \code{expr} argument must be of the form of \code{someVar <-
{expressions}}. That is, the left hand side must be a single symbol
name and the next syntactic token must be \code{<-}.
To flush the cache and force recomputation, simply remove the cache
files. You can use \code{\link{file.remove}} to do this.
}
\value{
The (cached) value of \code{expr}.
}
\author{
Wolfgang Huber, \email{huber@ebi.ac.uk}
Seth Falcon, \email{sfalcon@fhcrc.org}
}
\section{Note}{
The first version of this function had a slightly different interface
which is no longer functional. The old version has arguments
\code{name} and \code{expr} and the intended usage is: \code{foo <-
cache("foo", expr)}.
}
\examples{
bigCalc <- function() runif(10)
cache(myComplicatedObject <- bigCalc())
aCopy <- myComplicatedObject
remove(myComplicatedObject)
cache(myComplicatedObject <- bigCalc())
stopifnot(all.equal(myComplicatedObject, aCopy))
allCacheFiles <-
list.files(".", pattern="^tmp_R_cache_.*\\\\.RData$", full.name=TRUE)
file.remove(allCacheFiles)
}
\keyword{manip}
\keyword{array}
|
