[R] Remark on tapply().

[Rolf Turner]

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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