As a disclaimer, I cannot say that this is why sum() was designed as it was.
0 is the sum of a set with no elements, the empty set {}. When
na.rm=TRUE, NA values are removed. When the only values are NA (as in
your example c(NA, NA) ), and you remove them all, you are taking the
sum of no elements, which is 0.
Also note the behavior of
sum() # returns 0
sum is one of the few functions that you can simply call that will not
return an error.
HTH,
Josh
On Tue, Apr 20, 2010 at 9:42 AM, <will.ea...@gmx.net> wrote:
> Dear all,
>
> just a stupid R question, since the results puzzle me a bit:
>
>> sum(c(NA,NA), na.rm=TRUE)
> [1] 0
>> NA + NA
> [1] NA
>> NA + 1
> [1] NA
>>
>
> Why does sum(c(NA,NA), na.rm=TRUE) return 0 and not NA?
>
> Thanks in advance,
>
> Will
>
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>
--
Joshua Wiley
Senior in Psychology
University of California, Riverside
http://www.joshuawiley.com/
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