[R] Aggregation and the meaning of class

[Chip Barnaby]

Dear R-ers,

My aggregation saga continues.

Using the following sequence, I can calculate any statistic for row 
groups and merge the result back to all associated rows ...

 > WM = by( D60, D60[ "KeyProfA"], FUN=function(x) weighted.mean( x$IAC, x$Wt))

 > D60$IAC.WM = as.numeric( WM[ D60$KeyProfA])

 > class( WM)
[1] "by"

Questions ...

1) Is this a reasonable way to obtain the desired result?

2) What can one glean by knowing the class of WM ("by")?  It appears 
to me that class is a pretty shallow attribute in R ... just an 
associated string that selects among methods in some contexts.  Is 
that really all there is to it?  Is there a way to discover what 
generic methods are aware of a given class?  In other words, who 
cares if WM is a "by" ... what does that do for me?

In my traditional universe (C++) I can grep and discover what methods 
are virtual, who inherits from whom, etc.  In R, the documentation 
appears silent on what is a "by" (correct me if I'm wrong).  In 
addition, I have found no way to broadly search code to learn 
things.  (Displaying single functions is useful but hardly broad.)

How does one learn R more efficiently than randomly discovering how 
to avoid error messages?  (For example, I now know that a "by" cannot 
be coerced into a data.frame (although it seems to me that such a 
conversion could be usefully defined), so now I don't hit myself on 
the head with that particular hammer.)

Chip Barnaby






---------------------------------------------------------
Chip Barnaby                   cbarnaby at wrightsoft.com
Vice President of Research
Wrightsoft Corp.               781-862-8719 x118 voice
131 Hartwell Ave               781-861-2058 fax
Lexington, MA 02421         www.wrightsoft.com