[Rd] How I() works in a formula

Lorenz, David lorenz at usgs.gov
Fri Oct 3 14:55:28 CEST 2014

  Basically, the I() function, as it is used in a regression model, allows
the user to perform arithmetic operations on a variable that would
otherwise be interpreted by the formula. It is not trapped as a special
function as Error() is in aov().
  There may be other applications where the class "AsIs" is needed, so
there are support functions for subscripting, formatting, printing and so

On Fri, Oct 3, 2014 at 7:32 AM, Joris Meys <jorismeys at gmail.com> wrote:

> Dear all,
> I'm updating a package regarding a new type of models, and I'm looking to
> extend the formula interface with two functions (L() and R() ) for
> construction of these models. I want to use as much of the formula
> interface as possible, and hoped to do something similarly to I().
> I know the I() function does nothing more than add the class "AsIs". I've
> been browsing the source code of R for a couple of days now trying to
> locate where this class assignment gets translated into a specific action,
> but i couldn't locate it. I've been as far as the internal C function
> modelframe.
> Any pointers on how I() is processed internally are greatly appreciated.
> Cheers
> Joris
> --
> Joris Meys
> Statistical consultant
> Ghent University
> Faculty of Bioscience Engineering
> Department of Mathematical Modelling, Statistics and Bio-Informatics
> tel : +32 9 264 59 87
> Joris.Meys at Ugent.be
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