R devel - Jun 2013 - col2rgb, factors & documentation

Hello Core Folk.

Quite by accident I discovered today that col2rgb, when fed factors, acts on the
integer representation of those factors:

td <- as.factor(rainbow(5)) # not how I discovered the issue!
td
res <- col2rgb(td)
res
# but
col2rgb("#FF0000FF") # this is td[1] but does not produce the same
answer
res[,1]
# moreover
as.integer(td)
col2rgb(5) # the integer value of the first level
# and this is res[,1] which by the way is palette()[5], aka "cyan"

?col2rgb says:
col	
vector of any of the three kinds of R color specifications, i.e., either a color
name (as listed by colors()), a hexadecimal string of the form
"#rrggbb" or "#rrggbbaa" (see rgb), or a positive integer i
meaning palette()[i]. Non-numeric values are coerced to character.

So in the example the factor is being taken as its integer representation, and
finding an integer as the input value, col2rgb goes to palette().

Perhaps this is a nuance that belongs in the documentation?

Thanks, Bryan
***********
Bryan Hanson
Professor of Chemistry & Biochemistry
DePauw University

On 07/06/2013 18:59, Bryan Hanson wrote:
> Hello Core Folk.
This is the R-devel list.  Bug reports/enhancement suggestions should go 
to bugs.r-project.org.
> Quite by accident I discovered today that col2rgb, when fed factors, acts
on the integer representation of those factors:
>
> td <- as.factor(rainbow(5)) # not how I discovered the issue!
> td
> res <- col2rgb(td)
> res
> # but
> col2rgb("#FF0000FF") # this is td[1] but does not produce the
same answer
> res[,1]
> # moreover
> as.integer(td)
> col2rgb(5) # the integer value of the first level
> # and this is res[,1] which by the way is palette()[5], aka
"cyan"
>
> ?col2rgb says:
> col	
> vector of any of the three kinds of R color specifications, i.e., either a
color name (as listed by colors()), a hexadecimal string of the form
"#rrggbb" or "#rrggbbaa" (see rgb), or a positive integer i
meaning palette()[i]. Non-numeric values are coerced to character.
>
> So in the example the factor is being taken as its integer representation,
and finding an integer as the input value, col2rgb goes to palette().
>
> Perhaps this is a nuance that belongs in the documentation?
It has been like that since at least R 2.0.0.  I'll rephrase the help 
and change to coercion to character.

>
> Thanks, Bryan
> ***********
> Bryan Hanson
> Professor of Chemistry & Biochemistry
> DePauw University
>
> ______________________________________________
> 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