From b.rowlingson at lancaster.ac.uk  Fri Sep 19 12:07:25 2008
From: b.rowlingson at lancaster.ac.uk (Barry Rowlingson)
Date: Fri, 19 Sep 2008 11:07:25 +0100
Subject: [Rd] indexing by character with identical names
In-Reply-To: <1221815747.7356.83.camel@slipstream>
References: <1221815747.7356.83.camel@slipstream>
Message-ID: <d8ad40b50809190307s70139cbah57bdd59c27591d2a@mail.gmail.com>

2008/9/19 Jake Michaelson <jake.michaelson at gmail.com>

> ... 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.

 Some people may think it sensible that indexing a vector with an
index of length N returns a vector of length N:

 > x[c("a","b","a")]
 a b a
 1 2 1

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

 Changing this would probably (no, definitely!) break stuff. Breaking
stuff is a bad thing. Perhaps the upcoming R version Aleph-null
release would be the right place for it...

 R is not like other programming languages, so your perspective of
sensibility of minimal use here. You will get bitten by these gotchas.
I bet everyone on this mailing list has done a X[1,] and got bitten by
not having drop=FALSE....

 Ouch. Writing tests on edge cases is a good thing, since it catches
these problems (if you write your tests well!).

Barry



[Rd] indexing by character with identical names
Barry Rowlingson