[R] Ask if an object will respond to a function or method

Paul Johnson pauljohn32 at gmail.com
Thu Mar 31 22:00:28 CEST 2016

In the rockchalk package, I want to provide functions for regression
objects that are "well behaved." If an object responds to the methods
that lm or glm objects can handle, like coef(), nobs(), and summary(),
I want to be able to handle the same thing.

It is more difficult than expected to ask a given fitted model object
"do you respond to these functions: coef(), nobs(), summary()." How
would you do it?

I tried this with the methods() function but learned that all methods
that a class can perform are not listed.  I'll demonstrate with a
regression "zz" that is created by the example in the plm package.
The coef() function succeeds on the zz object, but coef is not listed
in the list of methods that the function can carry out.

> library(plm)
> example(plm)

> class(zz)
[1] "plm"        "panelmodel"
> methods(class = "plm")
 [1] ercomp          fixef           has.intercept   model.matrix
 [5] pFtest          plmtest         plot            pmodel.response
 [9] pooltest        predict         residuals       summary
[13] vcovBK          vcovDC          vcovG           vcovHC
[17] vcovNW          vcovSCC
see '?methods' for accessing help and source code
> methods(class = "panelmodel")
 [1] deviance      df.residual   fitted        has.intercept index
 [6] nobs          pbgtest       pbsytest      pcdtest       pdim
[11] pdwtest       phtest        print         pwartest      pwfdtest
[16] pwtest        residuals     terms         update        vcov
see '?methods' for accessing help and source code
> coef(zz)
   log(pcap)      log(pc)     log(emp)        unemp
-0.026149654  0.292006925  0.768159473 -0.005297741

I don't understand why coef(zz) succeeds but coef is not listed as a method.

Right now, I'm contemplating this:

zz1 < - try(coef(zz))
if (inherits(zz1, "try-error")) stop("Your model has no coef method")

This seems like a bad workaround because I have to actually run the
function in order to find out if the function exists. That might be
time consuming for some summary() methods.


Paul E. Johnson
Professor, Political Science        Director
1541 Lilac Lane, Room 504      Center for Research Methods
University of Kansas                 University of Kansas
http://pj.freefaculty.org              http://crmda.ku.edu

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