[R] simple question - mean of a row of a data.frame

[Matthew Keller]

Hi all,

Simple question I should know: I'm unclear on the logic of why the sum of a
row of a data.frame returns a valid sum but the mean of a row of a
data.frame returns NA:

sum(rock[2,])
[1] 10901.05

mean(rock[2,],trim=0)
[1] NA
Warning message:
In mean.default(rock[2, ], trim = 0) :
  argument is not numeric or logical: returning NA

I get that rock[2,] is itself a data.frame of mode list, but why the
inconsistency between functions? How can you figure this out from, e.g.,
?mean
?sum

Thanks in advance,

Matt


-- 
Matthew C Keller
Asst. Professor of Psychology
University of Colorado at Boulder
www.matthewckeller.com

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