R devel - Mar 2008 - Subsetting vectors/arrays using factors can be seen as misleading

Dear list,

Subsetting vectors/arrays using factors can be seen as misleading, and
I was thinking that it could be discouraged (at least by issuing a
warning).
I could not find whether this was discussed earlier, but I can be
pointed to a reference if I missed any.

The "extract" operator "[" can take as arguments either
vectors of
integers or vectors of characters in order to subset a data structure.
For example:
> x <- seq(1, 5)
> names(x) <- letters[1:5]
>
> x[1]
a
1
> x["a"]
a
1

Using a factor caused some confusion to someone here, and I have to
admit that it can indeed appear misleading:
> f <- factor("a", levels=c("b", "a",
"c"))
> f
[1] a
Levels: b a c
> x[f]  # here the integer is used, rather than the level
b
2

The dual nature of the factor (vector of integers, with an attached
vector of levels), is not always clear to many users, especially since
factors are treated differently in other situations.
Example:
> f == 1
[1] FALSE
> f == "a" #here the level is used, not the integer
[1] TRUE

This is making me suggest that indexing using a factor could issue a
warning, and the user should explicitly wrap the vector with either
"as.integer" or "as.character".


L.

PS: All examples above were run with
platform       x86_64-unknown-linux-gnu
arch           x86_64
os             linux-gnu
system         x86_64, linux-gnu
status         Under development (unstable)
major          2
minor          7.0
year           2008
month          03
day            12
svn rev        44742
language       R
version.string R version 2.7.0 Under development (unstable) (2008-03-12 r44742)

This is long established and documented on the basic help page for '['.
Further, the convention is widely used in R itself: running 'make check'
would give a few hundred warnings and then fail.  Working around those 
warnings would be inefficient (involving unnecessary copying of large 
objects).

One place where this matters is the advice to use levels(x)[x] as in 
as.character.factor() -- that construction is widespread, perhaps so 
widespread as to make it worthwhile making that an internal operation.

On Thu, 13 Mar 2008, Laurent Gautier wrote:
> Dear list,
>
> Subsetting vectors/arrays using factors can be seen as misleading, and
> I was thinking that it could be discouraged (at least by issuing a
> warning).
> I could not find whether this was discussed earlier, but I can be
> pointed to a reference if I missed any.
>
> The "extract" operator "[" can take as arguments either
vectors of
> integers or vectors of characters in order to subset a data structure.
> For example:
>> x <- seq(1, 5)
>> names(x) <- letters[1:5]
>>
>> x[1]
> a
> 1
>> x["a"]
> a
> 1
>
> Using a factor caused some confusion to someone here, and I have to
> admit that it can indeed appear misleading:
>> f <- factor("a", levels=c("b", "a",
"c"))
>> f
> [1] a
> Levels: b a c
>> x[f]  # here the integer is used, rather than the level
> b
> 2
>
> The dual nature of the factor (vector of integers, with an attached
> vector of levels), is not always clear to many users, especially since
> factors are treated differently in other situations.
> Example:
>> f == 1
> [1] FALSE
>> f == "a" #here the level is used, not the integer
> [1] TRUE
>
> This is making me suggest that indexing using a factor could issue a
> warning, and the user should explicitly wrap the vector with either
> "as.integer" or "as.character".
>
>
> L.
>
> PS: All examples above were run with
> platform       x86_64-unknown-linux-gnu
> arch           x86_64
> os             linux-gnu
> system         x86_64, linux-gnu
> status         Under development (unstable)
> major          2
> minor          7.0
> year           2008
> month          03
> day            12
> svn rev        44742
> language       R
> version.string R version 2.7.0 Under development (unstable) (2008-03-12
r44742)
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595