R devel - Oct 2014 - How I() works in a formula

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
-------------------------------
Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php

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Joris,
  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
forth.
Dave

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
> -------------------------------
> Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
	[[alternative HTML version deleted]]

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
> -------------------------------
> Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
-- 
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