[R] Persistent state in a function?

[Boris Steipe]

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