scoping/non-standard evaluation issue

Dear Gabor,

I used str() to look at the two objects but missed the difference that you
found. What I didn't quite understand was why one model worked but not the
other when both were defined at the command prompt in the global
environment.

Thanks,
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 Gabor Grothendieck
Sent: January-04-11 6:56 PM
To: John Fox
Cc: Sanford Weisberg; r-devel at r-project.org
Subject: Re: [Rd] scoping/non-standard evaluation issue

On Tue, Jan 4, 2011 at 4:35 PM, John Fox <jfox at mcmaster.ca> 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 there is something wrong with R here since the formula in the
call component of mod.1 has a "call" class whereas the corresponding
call component of mod.2 has "formula" class:

class(mod.1$call[[2]])

[1] "call"

class(mod.2$call[[2]])

[1] "formula"

If we reset call[[2]] to have "call" class then it works:

mod.2a <- mod.2
mod.2a$call[[2]] <- as.call(as.list(mod.2a$call[[2]]))
f(mod.2a)

Call:
lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
   Population + Year, data = longley, subset = subs)

Coefficients:
(Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
Population          Year
  3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
 1.164e+00    -1.911e+00


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