extract indep vars from formula
On Nov 7, 2012, at 4:03 PM, Alexander Shenkin <ashenkin at ufl.edu> wrote:
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