> On Jan 6, 2017, at 11:03 AM, Jacob Wegelin <jacobwege...@fastmail.fm> wrote:
>
> Given any regression model, created for instance by lm, lme, lmer, or rqs,
> such as
>
> z1<-lm(weight~poly(Time,2), data=ChickWeight)
>
> I would like a general way to obtain only those variables used for the model.
> In the current example, this "minimal data frame" would consist of the
> "weight" and "Time" variables and none of the other columns of ChickWeight.
>
> (Motivation: Sometimes the data frame contains thousands of variables which
> are not used in the current regression, and I do not want to keep copying and
> propagating them.)
>
> The "model" component of the regression object doesn't serve this purpose:
>
>> head(z1$model)
> weight poly(Time, 2).1 poly(Time, 2).2
> 1 42 -0.066020938 0.072002235
> 2 51 -0.053701293 0.031099018
> 3 59 -0.041381647 -0.001334588
> 4 64 -0.029062001 -0.025298582
> 5 76 -0.016742356 -0.040792965
> 6 93 -0.004422710 -0.047817737
>
> The following awkward workaround seems to do it when variable names contain
> only "word characters" as defined by regex:
>
> minimalvariablesfrommodel20161120 <-function(object, originaldata){
> # stopifnot(!missing(originaldata))
> stopifnot(!missing(object))
> intersect(
> unique(unlist(strsplit(format(object$call$formula), split="\\W",
> perl=TRUE)))
> , names(originaldata)
> )
> }
>
>> minimalvariablesfrommodel20161120(z1, ChickWeight)
> [1] "weight" "Time"
>>
>
> But if a variable has a space in its name, my workaround fails:
>
>> ChickWeight$"dog tail"<-ChickWeight$Time
>> z1<-lm(weight~poly(`dog tail`,2), data=ChickWeight)
>> head(z1$model)
> weight poly(`dog tail`, 2).1 poly(`dog tail`, 2).2
> 1 42 -0.066020938 0.072002235
> 2 51 -0.053701293 0.031099018
> 3 59 -0.041381647 -0.001334588
> 4 64 -0.029062001 -0.025298582
> 5 76 -0.016742356 -0.040792965
> 6 93 -0.004422710 -0.047817737
>> minimalvariablesfrommodel20161120(z1, ChickWeight)
> [1] "weight"
>>
>
> Is there a more elegant, and hence more reliable, approach?
>
> Thanks
>
> Jacob A. Wegelin
Jacob,
In general, if you have a model object 'm', you can use the following syntax:
all.vars(terms(m))
See ?terms and ?all.vars, the latter also includes all.names().
Regards,
Marc Schwartz
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