[R] Problme with <<- (was Apparent namespace problem)

Prof Brian Ripley ripley at stats.ox.ac.uk
Sat Apr 26 09:26:39 CEST 2003

On Fri, 25 Apr 2003, Ross Boylan wrote:

> I'm seeing some strange behavior while using the snow package for
> networked computers.  I believe it's caused by name space resolution
> issues, and would appreciate any suggestions tracking it down.

`namespace' is a technical term in R (some packages have namespaces), and
not I think involved here. I think you meant `scoping issues', although
the exact issue seems to be an inappropriate use of <<-.

> First, is there a way to find out what frame (as in frames in
> environments, not data frames) a name is being obtained from or put
> into?


> Second, how closely does the evaluation environment in the
> browser/debugger match what you would get in the function at the same
> point?  I ask because if I evaluate a statement in the browser it
> seems to work one way, but if I execute it it works another way.
> The statement is
> clusterEvalQ(cl, crossval.setup(x, y, groups, theta.fit,
> theta.predict))
> This evaluates the crossval.setup function across the cluster cl.
> crossval.setup is a function which puts its arguments in a list g (a
> local variable) and then does gcv <<- g.  The intent is to stuff the
> data into a global variable for use by later function calls.

If that is the intention, please use assign() explicitly.  That is not
what <<- is intended to do in R and it probably should only be used to
change an already existing value somewhere in the environment tree.

assign("gcv", g, envir=NULL), I believe


I have ignored the `snow' aspect, as I don't understand enough of how you 
are using it: it might be relevant.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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