From jake.michaelson at gmail.com  Fri Sep 19 11:15:47 2008
From: jake.michaelson at gmail.com (Jake Michaelson)
Date: Fri, 19 Sep 2008 11:15:47 +0200
Subject: [Rd] indexing by character with identical names
Message-ID: <1221815747.7356.83.camel@slipstream>

Hi all,

I've been using R for a while, but was puzzled when I just barely
noticed this behavior:

> x <- 1:10
> names(x) <- c("a","b","a",letters[1:7])
> x
 a  b  a  a  b  c  d  e  f  g 
 1  2  3  4  5  6  7  8  9 10 

> x["a"]
a 
1 

... that is, if a vector has identical (and therefore ambiguous) names,
it only returns the first match when the vector is indexed by a
non-unique name.  From my perspective, a more sensible behavior would be
to return *all* elements which carry the non-unique name (e.g.
x[names(x)=="a"] ), since it's ambiguous as to which element the user
actually wanted to extract.  It just seems safer than quietly returning
only the first matched element with no warning or other indication as to
what's actually happening.

I couldn't find that this behavior is explicitly documented in the help
file for '[' or 'names'.

I also couldn't find any prior discussion of this behavior.  Is this
something that could be addressed in an upcoming release of R? 

Thanks,

jake

R version 2.7.2 (2008-08-25) 
x86_64-unknown-linux-gnu



[Rd] indexing by character with identical names
Jake Michaelson