tcl/tk return problem

Prof Brian Ripley wrote:

On Wed, 31 Aug 2005, Philippe Grosjean wrote:

deggle wrote:

Hello,

I'm very new in working with tcl/tk in R and have a problem which will
probably
sound silly to most of you.
Here is the code I have problems with:

readcelfiles <- function()
{
  require(tcltk)
  tt <- tktoplevel()
  tkgrid(tklabel(tt,text="Choose a directory!"))

OnOK <- function()
{
   fileDir<-tclvalue(tkchooseDirectory())
   data.raw <- ReadAffy(celfile.path=fileDir)
   #return(data.raw)
}

  OK.but <- tkbutton(tt,text="OK",command=OnOK)
  tkgrid(OK.but)
  tkfocus(tt)
}

So after clicking on my "OK" button, I choose my directory and read the
cel-files.
But now I want to return the object to my workspace ... "return" doesn't
work here.

I suppose you mean in the User Workspace. Your OnOK function should look
like that:

OnOK <- function() {
     fileDir<-tclvalue(tkchooseDirectory())
     data.raw <<- ReadAffy(celfile.path=fileDir)
}

Note that the function overwrites any existing 'data.raw', so this could
be dangerous. Writting directly in the User Workspace is not advised
from inside a function, but here, it is the simplest way to return a
result from a tk widget action.

Maybe simplest, but not a very good way.  See
R_SOURCES/src/library/tcltk/R/utils.R for ideas on how to write a modal
dialog box that returns the value selected.

One problem with <<- is that it does not necessarily write in the
workspace.  You need

   assign("data.raw", ReadAffy(celfile.path=fileDir), envir=.GlobalEnv)

to be sure of that.  (The example code I quote does use <<- but in a
controlled way.)

This works, and you weren't suggesting it as a good style, but I'd like
to say it's really a bad style to write to .GlobalEnv.

The controlled use of <<- as in tk_select.list from the file you
quoted is really the best way to solve this problem.  As a general
rule, you shouldn't stomp on something you don't own, and functions
don't own variables in .GlobalEnv.

What tk_select.list does is define OnOK locally, and use <<- to write to
the tk_select.list environment.  Then the result can be manipulated and
returned politely, without stomping on anything anywhere.