extract indep vars from formula

Hello,

I'm trying to extract the independent variables from a formula.  The
closest I've been able to come, aside from rolling my own, is the following:
a = y ~ b * x
attr(terms(formula(a)),"variables")
The reason I'm doing this is that I'm building a grid of points that I
use to construct a 3-d model prediction surface in rgl.  If there are
more than two independent variables, I need to set the other ones to 0
in the dataframe for the predict() routine.  In order to do that, I need
to know what those variables are.

I suspect there's a better way to go about doing what I'm doing (perhaps
using the model.* family of functions).  But, in the meantime, just
extracting the independent variables will move me forward.

Thanks,
Allie
Hi,
Hello,

I'm trying to extract the independent variables from a formula.
This is somewhat ambiguous. Do you want the actual columns of values?
The quoted variable names? Including the interaction term or not?

If you just want the quoted column names

names(as.quoted(a))[-1]

will do it. If you want to include the interaction term you can use

labels(terms(a))
closest I've been able to come, aside from rolling my own, is the following:

a = y ~ b * x
attr(terms(formula(a)),"variables")
a is already a formula, so you can shorten this to attr(terms(a),"variables")

Hope this helps!

Ista
The reason I'm doing this is that I'm building a grid of points that I
use to construct a 3-d model prediction surface in rgl.  If there are
more than two independent variables, I need to set the other ones to 0
in the dataframe for the predict() routine.  In order to do that, I need
to know what those variables are.

I suspect there's a better way to go about doing what I'm doing (perhaps
using the model.* family of functions).  But, in the meantime, just
extracting the independent variables will move me forward.

Thanks,
Allie

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Hello,

I'm trying to extract the independent variables from a formula.  The
closest I've been able to come, aside from rolling my own, is the following:

a = y ~ b * x
attr(terms(formula(a)),"variables")
The reason I'm doing this is that I'm building a grid of points that I
use to construct a 3-d model prediction surface in rgl.  If there are
more than two independent variables, I need to set the other ones to 0
in the dataframe for the predict() routine.  In order to do that, I need
to know what those variables are.

I suspect there's a better way to go about doing what I'm doing (perhaps
using the model.* family of functions).  But, in the meantime, just
extracting the independent variables will move me forward.

Thanks,
Allie
You might want to look at ?all.vars as one possible approach. The nice thing about all.vars() is that if you have transformations on the IV's in the formula (eg. poly(x, 3)), you will get the base variable names (eg. 'x', rather than 'poly(x, 3)').

A general approach would be:

  all.vars(formula(MODEL))

where the response variable would typically be the first element in the returned vector. So, if you just want the IV's you could use:

  all.vars(formula(MODEL))[-1]

An example:

# from ?cars
fm1 <- lm(log(dist) ~ log(speed), data = cars)
all.vars(formula(fm1))
[1] "dist"  "speed"  

Regards,

Marc Schwartz
...perhaps ?all.vars

as in
form <- y~x+z
all.vars(form)
[1] "y" "x" "z"
all.vars(form)[-1]
[1] "x" "z"

-- Bert
Hello,

I'm trying to extract the independent variables from a formula.  The
closest I've been able to come, aside from rolling my own, is the following:

a = y ~ b * x
attr(terms(formula(a)),"variables")
The reason I'm doing this is that I'm building a grid of points that I
use to construct a 3-d model prediction surface in rgl.  If there are
more than two independent variables, I need to set the other ones to 0
in the dataframe for the predict() routine.  In order to do that, I need
to know what those variables are.

I suspect there's a better way to go about doing what I'm doing (perhaps
using the model.* family of functions).  But, in the meantime, just
extracting the independent variables will move me forward.

Thanks,
Allie

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
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