How I() works in a formula

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

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Joris Meys
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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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