[Rd] R Language Definition: Subsetting matrices with negative indices is *not* an error

peter dalgaard
2015-05-09T07:55:44Z

> On 09 May 2015, at 02:53 , Henrik Bengtsson <henrik.bengtsson at
ucsf.edu> wrote:
>
> Hi,
>
> I spotted what looks like another(*) mistake in 'R Language
> Definition' on how subsetting should work.  In Section 'Indexing
> matrices and arrays'
>
[http://cran.r-project.org/doc/manuals/r-release/R-lang.html#Indexing-matrices-and-arrays]
> one can read
>
>   "Negative indices are not allowed in indexing matrices."

Parse error: I believe that this is intended to mean

"Indexing matrices may not contain negative indices"

not

"You cannot use negative indices when indexing matrices".

This is consistent with the help page:

"
     A third form of indexing is via a numeric matrix with the one
     column for each dimension: each row of the index matrix then
     selects a single element of the array, and the result is a vector.
     Negative indices are not allowed in the index matrix.
"

Rephrasing would seem to be in order....

-pd

>
> but this is not true, e.g.
>
>> x <- matrix(1:12, nrow=4)
>> x
>     [,1] [,2] [,3]
> [1,]    1    5    9
> [2,]    2    6   10
> [3,]    3    7   11
> [4,]    4    8   12
>
>> x[c(-2,-4),]
>     [,1] [,2] [,3]
> [1,]    1    5    9
> [2,]    3    7   11
>
> /Henrik
>
> (*) https://stat.ethz.ch/pipermail/r-devel/2015-May/071091.html [docs
> have been fixed]
>
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--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com