[R] Persistent state in a function?

[Greg Snow]

Boris,

You may want to look into the R6 package.  This package has tools that
help create objects (environments) with methods that can use and
change the object.  You can have your persistent table stored as part
of your object and then create methods that will use and modify the
table within the object.

On Sat, Mar 19, 2016 at 10:45 AM, Boris Steipe <boris.steipe at utoronto.ca> 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])))
> }
>
>
> Thanks!
> Boris
>
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-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com