[R] Use of expand.model.frame()

[John Fox]

Dear Gabor,

That solves the problem -- thank you very  much for the quick response.

(I did look at the source for expand.model.frame but didn't isolate the
problem, which seems clear now that you've pointed it out.)

Regards,
 John

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Gabor 
> Grothendieck
> Sent: Saturday, January 01, 2005 10:26 AM
> To: r-help at stat.math.ethz.ch
> Subject: Re: [R] Use of expand.model.frame()
> 
> John Fox <jfox <at> mcmaster.ca> writes:
> 
> : 
> : Dear list members,
> : 
> : I've encountered a problem using expand.model.frame() with 
> a model specified
> : without an explicit data argument. To illustrate (R 2.0.1 
> under Windows XP):
> : 
> :     > x <- rnorm(10)
> :     > y <- x + rnorm(10)
> :     > mod <- lm(y ~ x)
> :     > z <- 1:10
> :     > expand.model.frame(mod, "z")
> :     Error in eval(expr, envir, enclos) : Object "y" not found
> :     > environment(formula(mod))
> :     <environment: R_GlobalEnv>
> :     >
> : 
> : But when data is specified in the call to lm(), I get the expected
> : behaviour:
> : 
> :     > DF <- data.frame(x, y, z)
> :     > remove(x, y, z)
> :     > mod <- lm(y ~ x, data=DF)
> :     > expand.model.frame(mod, "z")
> :                 y          x  z
> :     1   1.0321783 -0.8145171  1
> :     2  -1.7639244 -0.5965655  2
> :     3  -0.8150426 -1.4833768  3
> :     4   0.6047758  0.2030853  4
> :     5  -1.9870830 -1.5559511  5
> :     6  -0.4807146 -1.0751953  6
> :     7   1.3683736  0.1610050  7
> :     8   0.9323799  0.3537983  8
> :     9  -1.0797075 -0.4218457  9
> :     10 -3.0528776 -1.9750668 10
> :     > environment(formula(mod))
> :     <environment: R_GlobalEnv>
> : 
> : >From ?expand.model.frame:
> : 
> :     "Usage
> :     expand.model.frame(model, extras,
> :                        envir = environment(formula(model)),
> :                        na.expand = FALSE)
> : 
> :     Arguments
> :     model a fitted model
> :     extras one-sided formula or vector of character strings 
> describing new
> : variables to be added
> :     envir an environment to evaluate things in
> :     na.expand logical; see below"
> : 
> : So, if "things" are evaluated by default in 
> environment(formula(model)), why
> : does the first example fail and the second one work? 
> Obviously, I'm not
> : understanding something properly here.
> : 
> : Any help would be appreciated -- either to get 
> expand.model.frame() to work
> : or to suggest an alternative approach.
> : 
> : Happy new year to all.
> : 
> : John
> 
> Look at the source of expand.model.frame.
> 
> The second line of expand.model.frame uses the data component 
> of mod$call and if its not there expand.model.frame chokes.  
> As a workaround check if its missing and supply it yourself:
> 
> if (is.null(mod$call$data)) mod$call$data <- 
> environment(formula(mod)) expand.model.frame(mod, "z")
> 
> expand.model.frame should probably be doing that itself.
> 
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