[R] xyplot does not find variable in data

[Romain Francois]

On 10/12/2009 08:30 PM, Deepayan Sarkar wrote:
> On Mon, Oct 12, 2009 at 9:32 AM, Jacob Wegelin<jacob.wegelin at gmail.com>  wrote:
>> When we call a lattice function such as xyplot, to what extent does
>> the "data" designation cause the function to look inside the "data"
>> for variables?
>>
>> In the examples below, the "subset" argument understands that
>> "Variety" is a variable in the data.
>>
>> But the "scales" argument does not understand that "nitro" is a
>> variable in the data.
>>
>> What principle is at work?
>
> A strange one called "standard non-standard evaluation"; see
>
> http://developer.r-project.org/nonstandard-eval.pdf
>
> for a nice overview by Thomas Lumley.
>
>
> ?xyplot says:
>
>      data: For the ‘formula’ method, a data frame containing values (or
>            more precisely, anything that is a valid ‘envir’ argument in
>            ‘eval’, e.g. a list or an environment) for any variables in
>            the formula, as well as ‘groups’ and ‘subset’ if applicable.
>            If not found in ‘data’, or if ‘data’ is unspecified, the
>            variables are looked for in the environment of the formula.
>            For other methods (where ‘x’ is not a formula), ‘data’ is
>            usually ignored, often with a warning.
>
> so the non-standard evaluation only applies to 'groups' and 'subset'.
> The list may be different for other functions, e.g., densityplot()
> also evaluates 'weights' in 'data'.
>
> -Deepayan

In addition, you might want to use with, as in:

with( Oats, xyplot(
	yield ~ nitro,scales=list(x=list(at=unique(nitro))), 
subset=Variety=="Victory"
) )

Romain

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