[Rd] How I() works in a formula

[peter dalgaard]

On 03 Oct 2014, at 14:32 , 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.

It isn't...

> E <- function(x)x
> x <- rnorm(10)
> y <- rnorm(10)
> lm(y~x+E(x^2))

Call:
lm(formula = y ~ x + E(x^2))

Coefficients:
(Intercept)            x       E(x^2)  
     0.2757       0.1725      -0.3823  


The point is that special interpretation of operators never happens inside function calls. I() is just a convenient do-nothing function call.

If you want to add special operators, one place to look is in the handling of specials for survival::coxph.


> 
> 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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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com