[Rd] help with eval()

[Prof Brian Ripley]

On Mon, 18 Apr 2011, Duncan Murdoch wrote:

> On 11-04-18 5:51 PM, Terry Therneau wrote:
>> I've narrowed my scope problems with predict.coxph further.
>> Here is a condensed example:
>> 
>> fcall3<- as.formula("time ~ age")
>> dfun3<- function(dcall) {
>>      fit<- lm(dcall, data=lung, model=FALSE)
>>      model.frame(fit)
>> }
>> dfun3(fcall3)
>> 
>> The final call fails: it can't find 'dcall'.
>> 
>> The relevant code in model.frame.lm is:
>>         env<- environment(formula$terms)
>>         if (is.null(env))
>>              env<- parent.frame()
>> 	eval(fcall, env, parent.frame())
>> 
>> If the environment of the formula is .Globalenv, as it is here, the
>> contents of parent.frame() are ignored.  Adding a
>>             print(ls(parent.frame()))
>> statement just above the  final call shows that it isn't a scope issue:
>> the variables we want are there.
>>
>>    I don't understand the logic behind looking for variables in the place
>> the formula was first typed (this is not a complaint).  The inability to
>> look elsewhere however has stymied my efforts to fix the scoping problem
>> in predict.coxph, unless I drop the env(formula) argument alltogether.
>> But I assume there must be good reasons for it's inclusion and am
>> reluctant to do so.
>
>
> The reason is that when a formula is created, the variables in it are assumed 
> to have meaning in that context.  Where you work with the formula after that 
> should not be relevant:  that's why formulas carry environments with them. 
> When you create the formula before the variables, things go wrong.
>
> There's probably a way to associate the lung dataframe with the formula, or 
> create the formula in such a way that things work, but I can't spot it.

This is why model=FALSE is not the default.  It avoids trying to find 
the data at a later date (and even if you can solve the scoping 
issues, the data may have been changed).

>
> Duncan Murdoch
>
>> Terry Therneau
>> 
>>> sessionInfo()
>> R version 2.13.0 RC (2011-04-12 r55424)
>> Platform: x86_64-unknown-linux-gnu (64-bit)
>> 
>> locale:
>>   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
>>   [3] LC_TIME=en_US.UTF-8        LC_COLLATE=C
>>   [5] LC_MONETARY=C              LC_MESSAGES=en_US.UTF-8
>>   [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
>>   [9] LC_ADDRESS=C               LC_TELEPHONE=C
>> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
>> 
>> attached base packages:
>> [1] stats     graphics  grDevices utils     datasets  methods
>> base
>> 
>> PS. This also fails
>> dfun3<- function(dcall) {
>>      fit<- lm(dcall, data=lung)
>>      model.frame(fit, subset=1:10)
>> }
>> You just need to force model.frame.lm to recreate data.
>> 
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
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>

-- 
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