[R] How to capture the expression corresponding to the i param in the [ function

[David Winsemius]

On May 13, 2013, at 6:38 PM, Nhan Vu Lam Chi wrote:

> Hi everyone,
> I currently work on a S4 class that has the [ function. I want to capture
> the unevaluated expression corresponding to the i param using substitute()
> function and do a non-standard evaluation. However R automatically
> evaluates the expression and give me its value.
> For example:
> Given mydf[mydf$V1 > 1,] with mydf is an object of my custom S4 dataframe
> class and V1 is one of its columns, I want to get the unevaluated
> expression mydf$V1 > 1.
> 
> My questions are:
> 1. Is it possible to do that in R?
> 2. If yes, how to do?

Doesn't this cry out for the S4 class definition of "[" to be answerable?. Because "[" is generic, it could have almost any definition at the whim of the package author.

> My R version and OS info are:
> R version 2.15.3 (2013-03-01) -- "Security Blanket"
> Copyright (C) 2013 The R Foundation for Statistical Computing
> ISBN 3-900051-07-0
> Platform: x86_64-pc-linux-gnu (64-bit)
> 
> This is the first time I post to the mailing list, so please forgive any
> mistakes and/or advise me if possible.
> Regards,
> Nhan Vu
> 
> 	[[alternative HTML version deleted]]

You are forgiven, but  this once, for posting in HTML.

-- 
David Winsemius
Alameda, CA, USA