[Rd] scoping/non-standard evaluation issue

[John Fox]

Dear Peter,

I played around a bit with your suggestion but wasn't able to get it to
work. 

Thanks for this.

John

--------------------------------
John Fox
Senator William McMaster 
  Professor of Social Statistics
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
web: socserv.mcmaster.ca/jfox


> -----Original Message-----
> From: r-devel-bounces at r-project.org [mailto:r-devel-bounces at r-project.org]
On
> Behalf Of peter dalgaard
> Sent: January-04-11 6:05 PM
> To: John Fox
> Cc: 'Sanford Weisberg'; r-devel at r-project.org
> Subject: Re: [Rd] scoping/non-standard evaluation issue
> 
> 
> On Jan 4, 2011, at 22:35 , John Fox wrote:
> 
> > Dear r-devel list members,
> >
> > On a couple of occasions I've encountered the issue illustrated by the
> > following examples:
> >
> > --------- snip -----------
> >
> >> mod.1 <- lm(Employed ~ GNP.deflator + GNP + Unemployed +
> > +         Armed.Forces + Population + Year, data=longley)
> >
> >> mod.2 <- update(mod.1, . ~ . - Year + Year)
> >
> >> all.equal(mod.1, mod.2)
> > [1] TRUE
> >>
> >> f <- function(mod){
> > +     subs <- 1:10
> > +     update(mod, subset=subs)
> > +     }
> >
> >> f(mod.1)
> >
> > Call:
> > lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
> >    Population + Year, data = longley, subset = subs)
> >
> > Coefficients:
> > (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
> >   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
> >  Population          Year
> >   1.164e+00    -1.911e+00
> >
> >> f(mod.2)
> > Error in eval(expr, envir, enclos) : object 'subs' not found
> >
> > --------- snip -----------
> >
> > I *almost* understand what's going -- that is, clearly mod.1 and mod.2,
or
> > the formulas therein, are associated with different environments, but I
> > don't quite see why.
> >
> > Anyway, here are two "solutions" that work, but neither is in my view
> > desirable:
> >
> > --------- snip -----------
> >
> >> f1 <- function(mod){
> > +     assign(".subs", 1:10, envir=.GlobalEnv)
> > +     on.exit(remove(".subs", envir=.GlobalEnv))
> > +     update(mod, subset=.subs)
> > +     }
> >
> >> f1(mod.1)
> >
> > Call:
> > lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
> >    Population + Year, data = longley, subset = .subs)
> >
> > Coefficients:
> > (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
> >   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
> >  Population          Year
> >   1.164e+00    -1.911e+00
> >
> >> f1(mod.2)
> >
> > Call:
> > lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
> >    Population + Year, data = longley, subset = .subs)
> >
> > Coefficients:
> > (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
> >   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
> >  Population          Year
> >   1.164e+00    -1.911e+00
> >
> >> f2 <- function(mod){
> > +     env <- new.env(parent=.GlobalEnv)
> > +     attach(NULL)
> > +     on.exit(detach())
> > +     assign(".subs", 1:10, pos=2)
> > +     update(mod, subset=.subs)
> > +     }
> >
> >> f2(mod.1)
> >
> > Call:
> > lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
> >    Population + Year, data = longley, subset = .subs)
> >
> > Coefficients:
> > (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
> >   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
> >  Population          Year
> >   1.164e+00    -1.911e+00
> >
> >> f2(mod.2)
> >
> > Call:
> > lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
> >    Population + Year, data = longley, subset = .subs)
> >
> > Coefficients:
> > (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
> >   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
> >  Population          Year
> >   1.164e+00    -1.911e+00
> >
> > --------- snip -----------
> >
> > The problem with f1() is that it will clobber a variable named .subs in
the
> > global environment; the problem with f2() is that .subs can be masked by
a
> > variable in the global environment.
> >
> > Is there a better approach?
> 
> I think the best way would be to modify the environment of the formula.
> Something like the below, except that it doesn't actually work...
> 
> f3 <- function(mod) {
>   f <- formula(mod)
>   environment(f) <- e <-  new.env(parent=environment(f))
>   mod <- update(mod, formula=f)
>   evalq(.subs <- 1:10, e)
>   update(mod, subset=.subs)
> }
> 
> The catch is that it is not quite so easy to update the formula of a
model.
> 
> --
> Peter Dalgaard
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
> 
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