0/1 vector for indexing leads to funny behaviour (PR#8389) (maybe a documentation deficiency?)

?"[" says

See Also:

     'list', 'array', 'matrix'.

     '[.data.frame' and '[.factor' for the behaviour when applied to
     data.frame and factors.

     'Syntax' for operator precedence, and the _R Language_ reference
     manual about indexing details.

and the `indexing details' are indeed where it says they are.

This is not an introductory topic, and it makes sense to have the 
details in only one place and refer to it.  That help page is already 
over-loaded.


On Tue, 13 Dec 2005, Tony Plate wrote:

Yes, 0/1 (numeric) are intended to be used as index vectors -- and they
have the semantics of numeric indices, which is that 0 elements in the
index are omitted from the result.  This can be a very useful mode of
operation in many situations.

I was going to write "This is described in both the introduction to R,
and in the documentation for '['", except that I checked before I wrote
and was surprised to be unable to any discussion of zeros in indexing in
any of the first three places I looked:

(1) help page for '[' (There is discussion of zero indices here, but
only in the context of using matrices to index matrices, not in the
context of ordinary vector indices).

(2) Section 2.7 "Index vectors: selecting and modifying subsets of a
data set" in "An Introduction to R", which does say this about numeric
indices:
    2. A vector of positive integral quantities. In
       this case the values in the index vector must
       lie in the set {1, 2, . . . , length(x)}
(This seems to commit the sin of not telling the whole truth.)

No. Zero is not a positive integer.

(3) Section 5.5 "Array Indexing.  Subsections of an array" (In "An
Introduction to R")

Question for others: did I miss something obvious, or is this a
documentation deficiency that zeros in indices are not discussed in 3 of
some obvious first places to look?

If indeed this is a documentation deficiency, I'm happy to contribute
documentation patch, but I await other opinions before spending any time
on that.

-- Tony Plate

rasche at molgen.mpg.de wrote:

Full_Name: Axel Rasche
Version: 2.2.0
OS: Linux
Submission from: (NULL) (141.14.21.81)


Dear Debuggers,

This is not a serious problem. Are 0/1 vectors intended to be used as 
index
vectors? If yes, there is a bug. If not, it leads just to some funny 
behaviour
rather than an error message.

In the appendix is some simple code to reproduce the problem. A 
logical vector
as.logic(a) helps by indexing the vector b. The 0/1 vector a just 
returns the
first value "a". But as many times as there is a 1 in a.

Best regards,
Axel


Appendix:

b = c("a","b","c","d")
a = c(0,1,1,0)
b[as.logical(a)]
b[a]
a = c(1,0,1,0)
b[as.logical(a)]
b[a]
a = c(0,1,1,1)
b[as.logical(a)]
b[a]