tapply huge speed difference if X has names

Hi all,

Apologies if this has been raised before ... R's tapply is very fast, but if
X has names in this example, there seems to be a huge slow down: under 1
second compared to 151 seconds.  The following timings are repeatable and
are timed properly on a single user machine :

X = 1:100000
names(X) = X
system.time(fast<<-tapply(as.vector(X), rep(1:10000,each=10), mean))	#

as.vector() to drop the names
[1] 0.36 0.00 0.35 0.00 0.00

system.time(slow<<-tapply(X, rep(1:10000,each=10), mean))

[1] 149.95   1.83 151.79   0.00   0.00

head(fast)

  1    2    3    4    5    6
5.5 15.5 25.5 35.5 45.5 55.5

head(slow)

  1    2    3    4    5    6
5.5 15.5 25.5 35.5 45.5 55.5

identical(fast,slow)

[1] TRUE

Looking inside tapply, which then calls split, it seems there is an
is.null(names(x)) which prevents R's internal fast version from being
called. Why is that there? Could it be removed?  I often do something like
tapply(mat[,"colname"],...) where mat has rownames. Therefore the rownames
of mat become the names of the vector mat[,"colname"], and this seems to
slow down tapply a lot. Perhaps other functions which call split also suffer
this problem?

split.default

function (x, f)
{
   if (is.list(f))
       f <- interaction(f)
   f <- factor(f)
   if (is.null(attr(x, "class")) && is.null(names(x)))
       return(.Internal(split(x, f)))
   lf <- levels(f)
   y <- vector("list", length(lf))
   names(y) <- lf
   for (k in lf) y[[k]] <- x[f %in% k]
   y
}
<environment: namespace:base>

version

        _
platform x86_64-redhat-linux-gnu
arch     x86_64
os       linux-gnu
system   x86_64, linux-gnu
status
major    2
minor    0.1
year     2004
month    11
day      15
language R

Thanks and regards,
Matthew



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