[Rd] scoping problem when calling lm(precomputed formula, weights) from function (PR#11540)

[Duncan Murdoch]

On 5/30/2008 11:40 AM, rocket at google.com wrote:
> I've run into a scoping problem in R.

No, in your use of it.

> I'm calling a function that
>    * creates a formula

... incorrectly.

>    * calculates a weight vector
>    * calls lm with that formula and weights
> This fails.
> 
> Here's a simplified reproduce example:
> # f works, g doesn't, h is a workaround
> rm(w)
> data <- data.frame(y=runif(20), x=runif(20), z=runif(20))
> f <- function(k){
>   w <- data$z^k
>   coef(lm(y~x, data = data, weights = w))
> }
> g <- function(k){
>   w <- data$z^k
>   Formula <- parse(text="y~x")[[1]]

If this were simply

Formula <- y ~ x

it would work, because then the formula would get the evaluation 
environment attached.  I assume your real case builds up a formula in 
text and then parses it, in which case you'll have to evaluate the 
formula to get the environment set:

 > g
function (k)
{
     w <- data$z^k
     Formula <- eval(parse(text = "y ~ x"))
     coef(lm(Formula, data = data, weights = w))
}
 > g(2)
(Intercept)           x
   0.7777419  -0.7023185

In more complicated cases, you may need to explicitly set the 
environment of the constructed formula, using e.g.

environment(Formula) <- environment()

Duncan Murdoch

>   coef(lm(Formula, data = data, weights = w))
> }
> h <- function(k){
>   w <- data$z^k
>   Formula <- parse(text="y~x")[[1]]
>   # Following line fails due to scoping bug.  Use workaround.
>   # coef(lm(Formula, data = data, weights = w))
>   Call <- paste("coef(lm(", deparse(Formula), ", data=data, weights=w))")
>   eval(parse(text=Call)[[1]])
> }
> 
> 
>> f(2)
> (Intercept)           x 
>   0.7452512  -0.3413998 
>> g(2)
> Error in eval(expr, envir, enclos) : object "w" not found
>> traceback()
> 9: eval(expr, envir, enclos)
> 8: eval(extras, data, env)
> 7: model.frame.default(formula = Formula, data = data, weights = w, 
>        drop.unused.levels = TRUE)
> 6: model.frame(formula = Formula, data = data, weights = w, drop.unused.levels = TRUE)
> 5: eval(expr, envir, enclos)
> 4: eval(mf, parent.frame())
> 3: lm(Formula, data = data, weights = w)
> 2: coef(lm(Formula, data = data, weights = w))
> 1: g(2)
>> h(2)
> (Intercept)           x 
>   0.7452512  -0.3413998 
> 
> 
> 
> --please do not edit the information below--
> 
> Version:
>  platform = i486-pc-linux-gnu
>  arch = i486
>  os = linux-gnu
>  system = i486, linux-gnu
>  status = 
>  major = 2
>  minor = 7.0
>  year = 2008
>  month = 04
>  day = 22
>  svn rev = 45424
>  language = R
>  version.string = R version 2.7.0 (2008-04-22)
> 
> Locale:
> LC_CTYPE=en_US;LC_NUMERIC=C;LC_TIME=C;LC_COLLATE=C;LC_MONETARY=C;LC_MESSAGES=en_US;LC_PAPER=en_US;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US;LC_IDENTIFICATION=C
> 
> Search Path:
>  .GlobalEnv, package:stats, package:graphics, package:grDevices, package:utils, package:datasets, package:showStructure, package:Rcode, package:splus2R, package:methods, Autoloads, package:base
> 
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