On 1/12/2009, at 8:32 PM, Karl Ove Hufthammer wrote:
<SNIP>
Exercise to the reader:
Note that
sapply(split(x, ff, drop=TRUE), sum)
gives you the values of (just) the non-empty levels.
Now, why does
sapply(split(x, ff), sum, drop=TRUE)
give the wrong value (1) for these levels, while
sapply(split(x, ff), sum, drop=FALSE)
gives the the correct value?
(The answer should be fairly obvious, but it's an easy mistake to
make.)
That is ***CUTE***. I had to think long and hard to see what was
going on.
(But then I'm never very quick out of the blocks. :-) )
cheers,
Rolf Turner
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