R: Using term.formula on an S4 object that inherits data.frame

Question

I am trying to create a new class that inherits from a data frame:

> setClass('new.frame', representation(colour='character'), contains = 'data.frame') 

This is an instance of this class for testing:

 > test_data = data.frame(cbind(runif(5), runif(5))) > names(test_data) = c('X', 'Y') > test_frame = new('new.frame', test_data, colour='red') 

Just to make sure everything is fine ...

 > data.frame Object of class "new.frame" XY 1 0.8766306 0.4741213 2 0.1221508 0.5117665 3 0.4838761 0.4973627 4 0.7858294 0.4064749 5 0.5147703 0.9135304 Slot "colour": [1] "red" 

... and make sure inheritance is complete

 > is.data.frame(test_frame) [1] TRUE > getClass(class(test_frame)) Class "new.frame" [in ".GlobalEnv"] Slots: Name: .Data colour names Class: list character character Name: row.names .S3Class Class: data.frameRowLabels character Extends: Class "data.frame", directly Class "list", by class "data.frame", distance 2 Class "oldClass", by class "data.frame", distance 2 Class "vector", by class "data.frame", distance 3 

Here is the problem I encountered when trying to use the property to be a data frame:

 > terms.formula(Y ~ X, data = test_frame) Error in terms.formula(Y ~ X, data = test_frame) : 'data' argument is of the wrong type 

Maybe I missed something stupid. If yes, we will indicate in advance for this.

If I'm right about the problem here, is there anyway I can do terms.formula to recognize the fact that I give it data.frame?

Answer 1

Running debug(terms.formula) and then running terms.formula(Y ~ X, data = test_frame) shows that your code does not work on lines 3 and 4 of the specified code block:

 if (!is.null(data) && !is.environment(data) && !is.data.frame(data)) data <- as.data.frame(data, optional = TRUE) terms <- .Internal(terms.formula(x, specials, data, keep.order, allowDotAsName)) 

The problem should be that the .Internal(terms.formula()) call expects a "plain old" data.frame and an object of class new.frame is passed new.frame . As a workaround, why not just pass the terms.formula() type of object it expects (unadorned data.frame)?

Here is an easy way to do this:

 terms.formula(Y ~ X, data = unclass(test_frame)) # Y ~ X # attr(,"variables") # list(Y, X) # attr(,"factors") # X # Y 0 # X 1 # attr(,"term.labels") # [1] "X" # attr(,"order") # [1] 1 # attr(,"intercept") # [1] 1 # attr(,"response") # [1] 1 # attr(,".Environment") # <environment: R_GlobalEnv>