[R] Persistent state in a function?
Duncan Murdoch
murdoch.duncan at gmail.com
Sat Mar 19 22:57:56 CET 2016
On 19/03/2016 12:45 PM, Boris Steipe wrote:
> Dear all -
>
> I need to have a function maintain a persistent lookup table of results for an expensive calculation, a named vector or hash. I know that I can just keep the table in the global environment. One problem with this approach is that the function should be able to delete/recalculate the table and I don't like side-effects in the global environment. This table really should be private. What I don't know is:
> -A- how can I keep the table in an environment that is private to the function but persistent for the session?
> -B- how can I store and reload such table?
> -C- most importantly: is that the right strategy to initialize and maintain state in a function in the first place?
>
>
> For illustration ...
>
> -----------------------------------
>
> myDist <- function(a, b) {
> # retrieve or calculate distances
> if (!exists("Vals")) {
> Vals <<- numeric() # the lookup table for distance values
> # here, created in the global env.
> }
> key <- sprintf("X%d.%d", a, b)
> thisDist <- Vals[key]
> if (is.na(thisDist)) { # Hasn't been calculated yet ...
> cat("Calculating ... ")
> thisDist <- sqrt(a^2 + b^2) # calculate with some expensive function ...
> Vals[key] <<- thisDist # store in global table
> }
> return(thisDist)
> }
>
>
> # run this
> set.seed(112358)
>
> for (i in 1:10) {
> x <- sample(1:3, 2)
> print(sprintf("d(%d, %d) = %f", x[1], x[2], myDist(x[1], x[2])))
> }
Use local() to create a persistent environment for the function. For
example:
f <- local({
x <- NULL
function(y) {
cat("last x was ", x, "\n")
x <<- y
}
})
Then:
> f(3)
last x was
> f(4)
last x was 3
> f(12)
last x was 4
Duncan Murdoch
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