William Dunlap Wed, 4 Jan 2012 22:54:41 +0000
> R functions [can] use their enclosing environments to save state.
Aha!
> makeStack <- function () {
> stack <- list()
> list(pop = function() {
> if (length(stack) == 0) { # get from an enclosing env.
> retval <- NULL
> } else {
> retval <- stack[[length(stack)]] # get from an enclosing env.
> stack <<- stack[-length(stack)] # assign in an enclosing env.
> }
> retval
> }, push = function(x) {
> stack[[length(stack) + 1]] <<- x # assign in an enclosing env.
> invisible(x)
> })
> }
Thanks, that's quite clear.
> There are various encapsulations of this method in R. See, e.g.,
> "reference classes" or the "proto" package.
I can't see a reference-class implementation, but I did find
https://stat.ethz.ch/pipermail/r-help/2010-March/230353.html (slightly edited)
> [Subject:] [R] Stack type
> [From:] Gabor Grothendieck ggrothendieck at gmail.com
> [Date:] Tue Mar 2 14:33:43 CET 2010
> library(proto)
> Stack <- proto(new = function(.) proto(Stack,
> stack = NULL,
> push = function(., el) { .$stack <- c(list(el), .$stack) },
> pop = function(.) {
stopifnot(length(.$stack) > 0)
> out <- .$stack[[1]]
> .$stack[[1]] <- NULL
> out
> }))
> mystack <- Stack$new()
> mystack$push( 1 )
> mystack$push( letters )
> mystack$pop()
> mystack$pop()
> mystack$pop() # gives an error
Thanks again! Tom Roche <tom_ro...@pobox.com>
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