Hi,
It's been my experience that when you combine or aggregate vectors of factors
using a function, you should be prepared for surprises, as it's not obvious
what the "right" way to combine factors is (ordered or not), especially if two
vectors of factors have different levels or (if ordered) are ordered in a
different way.
For instance, what would you expect to get from unlist() if each element of the
list had different levels, or were both ordered, but in a different way, or if
some elements of the list were factors and others were ordered factors?
> unlist(list(ordered(c("a","b")), ordered(c("b","a"))))
[1] ?
Honestly, my biggest surprise from your question was that unlist even returned
a factor at all. For example, the c() function just converts factors to
integers.
> c(ordered(c("a","b")), ordered(c("a","b")))
[1] 1 2 1 2
And here's one that's especially weird. When rbind() data frames with an
ordered factor, you still get an ordered factor back, but the order may be
different from either of the original orders:
> x1 <- data.frame(a=ordered(c("b","c")))
> x2 <- data.frame(a=ordered(c("a","b","c")))
> str(rbind(x1,x2)) # Note b < a
'data.frame': 5 obs. of 1 variable:
$ a: Ord.factor w/ 3 levels "b"<"c"<"a": 1 2 3 1 2
Should rbind just have returned an integer like c(), or returned a factor like
unlist(), or should it kept the result as an ordered factor, but ordered the
result in a different way? I have no idea.
So in short, IMO, there are definitely inconsistencies in how ordered/factors
are handled across functions, but I think it would be hard to point to any
single function and say it is wrong or needs to be changed. My best advice, is
to just be careful when combining or aggregating factors.
--Robert
-----Original Message-----
From: R-devel [mailto:[email protected]] On Behalf Of "Jens
Oehlschlägel"
Sent: Friday, June 16, 2017 9:04 AM
To: [email protected]
Cc: [email protected]
Subject: [Rd] 'ordered' destroyed to 'factor'
Dear all,
I don't know if you consider this a bug or feature, but it breaks reasonable
code: 'unlist' and 'sapply' convert 'ordered' to 'factor' even if all levels
are equal. Here is a simple example:
o <- ordered(letters)
o[[1]]
lapply(o, min)[[1]] # ordered factor
unlist(lapply(o, min))[[1]] # no longer ordered
sapply(o, min)[[1]] # no longer ordered
Jens Oehlschlägel
P.S: The above examples are silly for simple reproduction. The current behavior
broke my use-case which had a structure like this
# have some data
x <- 1:20
# apply some function to each element
somefunc <- function(x){
# do something and return an ordinal level
sample(o, 1)
}
x <- sapply(x, somefunc)
# get minimum result
min(x)
# Error in Summary.factor(c(2L, 26L), na.rm = FALSE) :
# ‘min’ not meaningful for factors
> version
_
platform x86_64-pc-linux-gnu
arch x86_64
os linux-gnu
system x86_64, linux-gnu
status
major 3
minor 4.0
year 2017
month 04
day 21
svn rev 72570
language R
version.string R version 3.4.0 (2017-04-21)
nickname You Stupid Darkness
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