[Rd] indexing by character with identical names

[Jake Michaelson]

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

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Re: [Rd] indexing by character with identical names

[Jake Michaelson]

On Fri, 2008-09-19 at 11:07 +0100, Barry Rowlingson wrote:

>  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 thought of this side of it after I sent the original email, and I
realized that this probably would break a lot of stuff.

I think at the very least the documentation for '[' could be updated to
emphasize that only first matches are returned when indexing with a
character vector.

Thanks,

Jake

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