R devel - Oct 2018 - Relevel confusing with numeric value

Something that bit me:
The function relevel takes a factor, and a reference level to be promoted to the
first place.
If ?ref? is a character this level is promoted, if it?s a numeric the ?ref?-th
level is promoted.
Which turns out to be very confusing if you have factor with numeric values
(e.g. when reading in a csv with some dirty numeric columns and stringsAsFactors
TRUE)
For example:

set.seed(1)
test <- data.frame(n=sample(c(1:100, letters[1:10]), size=90))
test$n <- relevel(test$n, 50)
print(levels(test$n))

gives ?62? as the first level.

Could we make something like this an error, or at least issue a warning?
Also because some other functions automatically coerce, factor(?, levels=1:100)
and levels(test$n) <- 1:100 works fine.
So this is maybe the most confusing: relevel(factor(1:10, levels = -10:20), 15)
gives ?4? as the first level

For now I?ve thought of 2 possible implementations, that could be inserted in
stats::relevel.factor(), just before is.character(ref):

if(is.numeric(ref) && ref %in% lev)
    warning('Provided numeric reference, note that this will promote the
', ref, 'th value, not level with value "', ref,
'"!')

or

if(is.numeric(ref) && any(!is.na(suppressWarnings(as.numeric(lev)))))
    warning('Provided numeric reference, note that this will promote the
', ref, 'th value, not level with value "', ref,
'"!')


Best regards,
Emil Bode

Data-analyst

+31 6 43 83 89 33
emil.bode at dans.knaw.nl<mailto:emil.bode at dans.knaw.nl>

DANS: Netherlands Institute for Permanent Access to Digital Research Resources
Anna van Saksenlaan 51 | 2593 HW Den Haag | +31 70 349 44 50 | info at
dans.knaw.nl<mailto:info at dans.kn> |
dans.knaw.nl<applewebdata://71F677F0-6872-45F3-A6C4-4972BF87185B/www.dans.knaw.nl>
DANS is an institute of the Dutch Academy KNAW<http://knaw.nl/nl> and
funding organisation NWO<http://www.nwo.nl/>.

	[[alternative HTML version deleted]]

In a word, no. It is behaving as documented and adding a warning would just
confuse others who have been using the feature as intended.

This belongs in the same bin as "as.integer(f) vs
as.integer(as.character(f))" and "x[f] vs. x[as.character(f)]"

-pd

> On 2 Oct 2018, at 17:18 , Emil Bode <emil.bode at dans.knaw.nl>
wrote:
> 
> Something that bit me:
> The function relevel takes a factor, and a reference level to be promoted
to the first place.
> If ?ref? is a character this level is promoted, if it?s a numeric the
?ref?-th level is promoted.
> Which turns out to be very confusing if you have factor with numeric values
(e.g. when reading in a csv with some dirty numeric columns and stringsAsFactors
TRUE)
> For example:
> 
> set.seed(1)
> test <- data.frame(n=sample(c(1:100, letters[1:10]), size=90))
> test$n <- relevel(test$n, 50)
> print(levels(test$n))
> 
> gives ?62? as the first level.
> 
> Could we make something like this an error, or at least issue a warning?
> Also because some other functions automatically coerce, factor(?,
levels=1:100) and levels(test$n) <- 1:100 works fine.
> So this is maybe the most confusing: relevel(factor(1:10, levels = -10:20),
15) gives ?4? as the first level
> 
> For now I?ve thought of 2 possible implementations, that could be inserted
in stats::relevel.factor(), just before is.character(ref):
> 
> if(is.numeric(ref) && ref %in% lev)
>    warning('Provided numeric reference, note that this will promote the
', ref, 'th value, not level with value "', ref,
'"!')
> 
> or
> 
> if(is.numeric(ref) &&
any(!is.na(suppressWarnings(as.numeric(lev)))))
>    warning('Provided numeric reference, note that this will promote the
', ref, 'th value, not level with value "', ref,
'"!')
> 
> 
> Best regards,
> Emil Bode
> 
> Data-analyst
> 
> +31 6 43 83 89 33
> emil.bode at dans.knaw.nl<mailto:emil.bode at dans.knaw.nl>
> 
> DANS: Netherlands Institute for Permanent Access to Digital Research
Resources
> Anna van Saksenlaan 51 | 2593 HW Den Haag | +31 70 349 44 50 | info at
dans.knaw.nl<mailto:info at dans.kn> |
dans.knaw.nl<applewebdata://71F677F0-6872-45F3-A6C4-4972BF87185B/www.dans.knaw.nl>
> DANS is an institute of the Dutch Academy KNAW<http://knaw.nl/nl> and
funding organisation NWO<http://www.nwo.nl/>.
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> 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
Office: A 4.23
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com