From pdalgd at gmail.com  Sat May  9 09:55:44 2015
From: pdalgd at gmail.com (peter dalgaard)
Date: Sat, 9 May 2015 09:55:44 +0200
Subject: [Rd] R Language Definition: Subsetting matrices with negative
	indices is *not* an error
In-Reply-To: <CAFDcVCQdx0XQaBgw8rNrN0ayB8fxbiFPanXRPgfwys93zpBG8A@mail.gmail.com>
References: <CAFDcVCQdx0XQaBgw8rNrN0ayB8fxbiFPanXRPgfwys93zpBG8A@mail.gmail.com>
Message-ID: <FFEC09E0-0FF0-48F0-8238-4FD01E5E6A6E@gmail.com>


> 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]
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

-- 
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



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