[R] making changes to global variables in functions

[William Dunlap]

A third option is to put your state objects in a list
and write a replacement function to modify the state
of each.  E.g.,

  `n<-` <- function(state, value) {
      state[["n"]] <- value
      state
  }
  n <- function(state) state[["n"]]
  states <- list( list(n=100, won=0), list(n=101, won=1) )
  for(i in seq_along(states)) {
      n(states[[i]]) <- i*1000
  }
  invisible(lapply(states, dput))
  # that should show:
  #   structure(list(n = 1000, won = 0), .Names = c("n", "won"))
  #   structure(list(n = 2000, won = 1), .Names = c("n", "won"))

This can make it easier to understand the flow of data.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Janko Thyson
> Sent: Wednesday, December 07, 2011 4:34 AM
> To: R. Michael Weylandt
> Cc: r-help at r-project.org; Yev
> Subject: Re: [R] making changes to global variables in functions
> 
> Basically, I see two options here:
> 
> 1) Using environments
> 
> # Temp environment
> env <- new.env(parent=emptyenv())
> env$state1 <- list(n=100, won=0)
> env$state2 <- list(n=100, won=0)
> 
> fight2 <- function(stateA, stateB, envir){
> #  get(stateA, envir=envir)$n <- 50
>      # The above is what you would want to do, but
>      # 'get<-' is not defined, so:
>      temp <- get(stateA, envir=envir)
>      temp$n <- 50
>      assign(stateA, value=temp, envir=envir)
> 
>      # Same for stateB
>      temp <- get(stateB, envir=envir)
>      temp$n <- 50
>      assign(stateA, value=temp, envir=envir)
> 
>      return(TRUE)
> }
> 
> fight2(stateA="state1", stateB="state2", envir=env)
> 
> # Extract from environment
> state1 <- env$state1
> state1
> state2 <- env$state2
> state2
> 
> 2) Using Reference Classes
> 
> # Class Def
> setRefClass("State",
>      fields=list(n="numeric", won="numeric"),
>      methods=list(
>          fight2=function(...){
>              fight2Ref(.self=.self, ...)
>          }
>      )
> )
> # Set Generic
> setGeneric(name="fight2Ref", signature=".self", def=function(.self, ...)
>      standardGeneric("fight2Ref"))
> # Set Method
> setMethod(f="fight2Ref", signature="State",
>      definition=function(
>          .self,
>          value,
>          ...
>      ){
>          .self$n <- value
>      }
> )
> # Note:
> # You could also put the code inside 'fight2Ref' directly inside the
> class def,
> # but I don't want them to be too crowded, so I go by 'divide and conquer'
> 
> # Instantiate objects
> state1 <- new("State", n=100, won=0)
> state1
> state2 <- new("State", n=100, won=0)
> state2
> 
> # Apply method
> state1$fight2(value=50)
> state1
> state2$fight2(value=50)
> state2
> 
> # Back to list
> stateToList <- function(obj, ...){
>      fields <- names(getRefClass("State")$fields())
>      out <- lapply(fields, function(x.field){
>          obj$field(x.field)
>      })
>      names(out) <- fields
>      return(out)
> }
> state1 <- stateToList(state1)
> state1
> state2 <- stateToList(state2)
> state2
> 
> HTH,
> Janko
> 
> On 06.12.2011 22:06, R. Michael Weylandt wrote:
> > No pointer functionality in R (that I know of), but if you want to
> > return two objects as one the standard way is to put them in a list
> > and to return that list.
> >
> > Michael
> >
> > On Tue, Dec 6, 2011 at 2:35 PM, Yev<kirpich at gmail.com>  wrote:
> >> I'm trying to write a function that takes several objects with many
> >> different attributes and then changes their attributes. So what I wanted to
> >> happen in the simplified example below is for the function to change the
> >> attributes of the objects state1 and state2 that are passed to it. But
> >> because stateA and stateB are local, this isn't working. Are there any easy
> >> solutions?
> >>
> >> e.g., if I could combine the two objects stateA and stateB into a single
> >> object, I could return it and then assign it back to objects state1 and
> >> state2. Or if I could pass a pointer to the original object.. But I cannot
> >> find an easy way of doing either.  Thanks in advance..
> >>
> >> state1<- list(n=100, won=0)
> >> state2<- list(n=100, won=0)
> >>
> >> fight2<- function(stateA, stateB){
> >>    stateA$n<- 50
> >>    stateB$n<-50
> >> }
> >>
> >> fight2(state1,state2)
> >>
> >> state1$n
> >> state2$n
> >>
> >>         [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help at r-project.org  mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guidehttp://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> > ______________________________________________
> > R-help at r-project.org  mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guidehttp://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> >
> 
> 
> --
> ------------------------------------------------------------------------
> 
> *Janko Thyson*
> janko.thyson at ku-eichstaett.de <mailto:janko.thyson at ku-eichstaett.de>
> 
> Catholic University of Eichstätt-Ingolstadt
> Ingolstadt School of Management
> Statistics and Quantitative Methods
> Auf der Schanz 49
> D-85049 Ingolstadt
> 
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