On 23 Jun 2017, at 10:42 , Martin Maechler <maechler at stat.math.ethz.ch> wrote:
Martin Maechler <maechler at stat.math.ethz.ch>
on Thu, 22 Jun 2017 11:43:59 +0200 writes:
Paul Johnson <pauljohn32 at gmail.com>
on Fri, 16 Jun 2017 11:02:34 -0500 writes:
On Fri, Jun 16, 2017 at 2:35 AM, Joris Meys <jorismeys at gmail.com> wrote:
To extwnd on Martin 's explanation :
In factor(), levels are the unique input values and labels the unique output
values. So the function levels() actually displays the labels.
I think we agree. Currently, factor insists both levels and labels be unique.
I wish that it would not accept nonunique labels. I also understand it
is impractical to change this now in base R.
I don't think I succeeded in explaining why this would be nicer.
Here's another example. Fairly often, we see input data like
x <- c("Male", "Man", "male", "Man", "Female")
The first four represent the same value. I'd like to go in one step
to a new factor variable with enumerated types "Male" and "Female".
This fails
xf <- factor(x, levels = c("Male", "Man", "male", "Female"),
labels = c("Male", "Male", "Male", "Female"))
Instead, we need 2 steps.
xf <- factor(x, levels = c("Male", "Man", "male", "Female"))
levels(xf) <- c("Male", "Male", "Male", "Female")
I think it is quirky that `levels<-.factor` allows the duplicated
labels, whereas factor does not.
I wrote a function rockchalk::combineLevels to simplify combining
levels, but most of the students here like plyr::mapvalues to do it.
The use of levels() can be tricky because one must enumerate all
values, not just the ones being changed.
But I do understand Martin's point. Its been this way 25 years, it
won't change. :).
Well.. the above is a bit out of context.
Your first example really did not make a point to me (and Joris)
and I showed that you could use even two different simple factor() calls to
produce what you wanted
yc <- factor(c("1",NA,NA,"4","4","4"))
yn <- factor(c( 1, NA,NA, 4, 4, 4))
Your new example is indeed much more convincing !
(Note though that the two steps that are needed can be written
more shortly
The "been this way 25 years" is one a reason to be very
cautious(*) with changes, but not a reason for no changes!
(*) Indeed as some of you have noted we really should not "break behavior".
This means to me we cannot accept a change there which gives
an error or a different result in cases the old behavior gave a valid factor.
I'm looking at a possible change currently
[not promising that a change will happen ...]
In the end, I've liked the change (after 2-3 iterations), and
now been brave to commit to R-devel (svn 72845).
With the change, I had to disable one of our own regression
checks (tests/reg-tests-1b.R, line 726):
The following is now (in R-devel -> R 3.5.0) valid:
factor(1:2, labels = c("A","A"))
I wonder how many CRAN package checks will "break" from
this (my guess is in the order of a dozen), but I hope
that these breakages will be benign, e.g., similar to the above
case where before an error was expected via tools :: assertError(.)
Martin