[Rd] Relevel confusing with numeric value

[Emil Bode]

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
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