In R devel, function 'factor' has been changed, allowing and merging
duplicated 'labels'.
Issue 1: Handling of specified 'labels' without duplicates is slower
than before.
Example:
x <- rep(1:26, 40000)
system.time(factor(x, levels=1:26, labels=letters))
Function 'factor' is already rather slow because of conversion to
character. Please don't add slowdown.
Issue 2: While default 'labels' is 'levels', not specifying
'labels' may be different from specifying 'labels' to be the
same as 'levels'.
Example 1:
as.integer(factor(c(NA,2,3), levels = c(2, NA), exclude = NULL))
is different from
as.integer(factor(c(NA,2,3), levels = c(2, NA), labels = c(2, NA), exclude =
NULL))
File reg-tests-1d.R indicates that 'factor' behavior with NA is slightly
changed, for the better. NA entry (because it is unmatched to 'levels'
argument or is in 'exclude') is absorbed into NA in "levels"
attribute (comes from 'labels' argument), if any. The issue is that it
happens only when 'labels' is specified.
Function 'factor' could use match(xlevs, nlevs)[f]. It doesn't match
NA to NA level. When 'f' is long enough, longer than 'xlevs', it
is faster than match(xlevs[f], nlevs).
Example 2:
With
levs <- c("A","A")? ,
factor(levs, levels=levs)
gives error, but
factor(levs, levels=levs, labels=levs)
doesn't.
Note: In theory, if function 'factor' merged duplicated 'labels'
in all cases, at least in
factor(c(sqrt(2)^2, 2))? ,
function 'factor' could do matching on original 'x' (without
conversion to character), as in R before version 2.10.0. If function
'factor' did it,
factor(c(sqrt(2)^2, 2), levels = c(sqrt(2)^2, 2), labels =
c("sqrt(2)^2", "2"))
could take sqrt(2)^2 and 2 as distinct.
Another thing: Function 'factor' in R devel uses 'order' instead
of 'sort.list'.
The case of as.factor(x) for
x <- as.data.frame(character(0))
in tests/isas-tests.Rout.save reveals that 'order' on data frame is
strange.
x <- as.data.frame(character(0))
y <- unique(x)
length(y)? # 1
length(order(y))? # 0
length(as.character(y))? # 1
order(y) is not as long as as.character(y).
Another example:
length(mtcars)? # 11
length(order(mtcars))? # 352
>>>>> Suharto Anggono Suharto Anggono via R-devel <r-devel at r-project.org> >>>>> on Sun, 15 Oct 2017 16:03:48 +0000 writes:> In R devel, function 'factor' has been changed, allowing and merging duplicated 'labels'. Indeed. That had been asked for and discussed a bit on this list from June 14 to June 23, starting at https://stat.ethz.ch/pipermail/r-devel/2017-June/074451.html > Issue 1: Handling of specified 'labels' without duplicates is slower than before. > Example: > x <- rep(1:26, 40000) > system.time(factor(x, levels=1:26, labels=letters)) > Function 'factor' is already rather slow because of conversion to character. Please don't add slowdown. Indeed, I doo see a ~ 20% performance loss for the example above, and I may get to look into this. However, in R-devel there have been important internal changes (ALTREP additions) some of which are currently giving some performance losses in some cases (but they have the potential to give big performance _gains_ e.g. for simple indexing into large vectors which may apply here !). For factor(), these C level "ALTREP" changes may not be the reason at all for the slow down; I may find time to investigate further. {{ For the ALTREP-change slowdowns I've noticed in some indexing/subset operations, we'll definitely have time to look into before R-devel is going to be released next spring... and as mentioned, these operations may even become considerably faster *thanks* to ALTREP ... }} > Issue 2: While default 'labels' is 'levels', not specifying 'labels' may be different from specifying 'labels' to be the same as 'levels'. > Example 1: > as.integer(factor(c(NA,2,3), levels = c(2, NA), exclude = NULL)) > is different from > as.integer(factor(c(NA,2,3), levels = c(2, NA), labels = c(2, NA), exclude = NULL)) You are right. But this is not so exceptional and part of the new feature of 'labels' allowing to "fix up" things in such cases. While it would be nice if this was not the case the same phenomenon happens in other functions as well because of lazy evaluation. I think I had noticed that already and at the time found "not easy" to work around. (There are many aspects about changing such important base functions: 1. not breaking back compatibility ((unless in rare border cases, where we are sure it's worth)) 2. Keeping code relatively transparent 3. Keep the semantics "simple" to document and as intuitive as possible ) > File reg-tests-1d.R indicates that 'factor' behavior with NA is slightly changed, for the better. NA entry (because it is unmatched to 'levels' argument or is in 'exclude') is absorbed into NA in "levels" attribute (comes from 'labels' argument), if any. The issue is that it happens only when 'labels' is specified. I'm not sure anymore, but I think I had noticed that also in June, considered to change it and found that such a changed factor() would be too different from what it has "always been". So, yes, IIRC, this current behavior is on purpose, if only for back compatibility. > Function 'factor' could use match(xlevs, nlevs)[f]. It doesn't match NA to NA level. When 'f' is long enough, longer than 'xlevs', it is faster than match(xlevs[f], nlevs). > Example 2: > With > levs <- c("A","A")? , > factor(levs, levels=levs) > gives error, but > factor(levs, levels=levs, labels=levs) > doesn't. yes, again that is a consequence of what you said above (before 'Example 1') > Note: In theory, if function 'factor' merged duplicated 'labels' in all cases, at least in > factor(c(sqrt(2)^2, 2))? , > function 'factor' could do matching on original 'x' (without conversion to character), as in R before version 2.10.0. If function 'factor' did it, > factor(c(sqrt(2)^2, 2), levels = c(sqrt(2)^2, 2), labels = c("sqrt(2)^2", "2")) > could take sqrt(2)^2 and 2 as distinct. Well, that may be interesting.. but I doubt if that's somewhere we should go, easily, because factor() has been documented to do what it does now (with very slightly rounding such numbers via as.character(.)) and hence such a change would typically lead to much work for too many people. I do see that indeed the as.character(.) inside factor() takes most of the CPU time used in largish factor() examples [as your first], and indeed, for the case of integer 'x', we really could be much faster in factor construction. > Another thing: Function 'factor' in R devel uses 'order' instead of 'sort.list'. This has been by a change on purpose --- well documented as new feature in NEWS --- to allow using *methods* for order(), i.e. for the workhorse of order, xtfrm() so that factor(OB) works for more general objects OB. > The case of as.factor(x) for > x <- as.data.frame(character(0)) > in tests/isas-tests.Rout.save reveals that 'order' on data frame is strange. > x <- as.data.frame(character(0)) > y <- unique(x) > length(y)? # 1 > length(order(y))? # 0 > length(as.character(y))? # 1 > order(y) is not as long as as.character(y). > Another example: > length(mtcars)? # 11 > length(order(mtcars))? # 352 I agree that order(<data.frame>) may look a bit strange; I've spent more than an hour into looking into it, and making it [actually, rank(<data.frame>,..) ] an error, but ended up finding much evidence that there's too much related code, sometimes even in base R which assumes that a numeric data frame behaves the same as a numeric matrix. And also, if you carefully read the help files, of order(), xtfrm(), rank() there's always mentioned that these work for R object 'x' basically as long as x[!is.na(x)] returns a "nice" (typically atomic) vector .. which is the case for such data frames. The consequence, that in R-devel, currently factor(mtcars) just "works", is indeed unexpected or even "shocking", and I still don't know what the most elegant and reasonable way would be to make this an error -- as it used to be when sort.list() was used instead of order(). I'd find it ugly (and even more time consuming!) if factor() itself would have to check its argument and signal an error for a data.frame. The relevant call tree is factor() -> order() -> xtfrm() -> xtfrm.default() -> rank() and as I said, rank(x,*) works when x[!is.na(x)] is an atomic "numeric-like" vector which is the case for a numeric data frame such as 'mtcars'. Martin Maechler ETH Zurich
Martin, Suharto, et al., On Wed, Oct 18, 2017 at 9:54 AM, Martin Maechler <maechler at stat.math.ethz.ch> wrote:> *<snip>* > > > Note: In theory, if function 'factor' merged duplicated 'labels' in > all cases, at least in > > factor(c(sqrt(2)^2, 2)) , > > function 'factor' could do matching on original 'x' (without > conversion to character), as in R before version 2.10.0. If function > 'factor' did it, > > factor(c(sqrt(2)^2, 2), levels = c(sqrt(2)^2, 2), labels > c("sqrt(2)^2", "2")) > > could take sqrt(2)^2 and 2 as distinct. > > Well, that may be interesting.. but I doubt if that's somewhere > we should go, easily, because factor() has been documented to do > what it does now (with very slightly rounding such numbers via > as.character(.)) > and hence such a change would typically lead to much work for > too many people. > > I do see that indeed the as.character(.) inside factor() takes > most of the CPU time used in largish factor() examples [as your > first], and indeed, for the case of integer 'x', we really could > be much faster in factor construction. >Indeed; the ALTREP framework already has an alternative string (character vector) implementation which defers conversion from another vector type. Luke implemented it to drastically speed up the creation of default row labels (1:n, I believe) on design matrices within lm/glm, under the assumption that no one is ever going to look at the design matrix row labels most of the time, and there's no reason to pay the cost of creating them until they do. In principle, we could look at doing the same for levels of a factor. Furthermore, while I haven't put in the hooks yet to utilize them yet, even in my local copy, ALTREP classes are allowed to include a custom unique method, so in cases where unique(as.character(x)) == as.character(unique(x)) we could avoid the conversion even when calling unique() (at the R level) on such a deferred vector. This would include, I think, the case where inferring levels from the vector. I believe the point Martin made earlier about as.character potentially doing some rounding means that the required identity would hold when factors are being generated from integer vectors but would not be guaranteed when factors are generated from (non-integer) numeric vectors. Best, ~G> > > Another thing: Function 'factor' in R devel uses 'order' instead of > 'sort.list'. > > This has been by a change on purpose --- well documented as new > feature in NEWS --- to allow using *methods* for order(), > i.e. for the workhorse of order, xtfrm() so that factor(OB) > works for more general objects OB. > > > > The case of as.factor(x) for > > x <- as.data.frame(character(0)) > > in tests/isas-tests.Rout.save reveals that 'order' on data frame is > strange. > > > x <- as.data.frame(character(0)) > > y <- unique(x) > > length(y) # 1 > > length(order(y)) # 0 > > length(as.character(y)) # 1 > > > order(y) is not as long as as.character(y). > > > Another example: > > length(mtcars) # 11 > > length(order(mtcars)) # 352 > > I agree that order(<data.frame>) may look a bit strange; > I've spent more than an hour into looking into it, and making it > [actually, rank(<data.frame>,..) ] > an error, but ended up finding much evidence that there's too > much related code, sometimes even in base R which assumes that a > numeric data frame behaves the same as a numeric matrix. > > And also, if you carefully read the help files, of > order(), > xtfrm(), > rank() > > there's always mentioned that these work for R object 'x' > basically as long as x[!is.na(x)] returns a "nice" > (typically atomic) vector .. which is the case for such data frames. > > The consequence, that in R-devel, currently > > factor(mtcars) > > just "works", is indeed unexpected or even "shocking", and I > still don't know what the most elegant and reasonable way would > be to make this an error -- as it used to be when sort.list() > was used instead of order(). I'd find it ugly (and even more > time consuming!) if factor() itself would have to check its > argument and signal an error for a data.frame. > > The relevant call tree is > > factor() -> order() -> xtfrm() -> xtfrm.default() -> rank() > > and as I said, rank(x,*) works when x[!is.na(x)] is an atomic > "numeric-like" vector which is the case for a numeric data > frame such as 'mtcars'. > > Martin Maechler > ETH Zurich > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel >-- Gabriel Becker, PhD Scientist (Bioinformatics) Genentech Research [[alternative HTML version deleted]]