# Re: [R] To many NA's from mean(..., na.rm=T) when a column is all NA's

From: Achim Zeileis <Achim.Zeileis_at_wu-wien.ac.at>
Date: Tue 14 Jun 2005 - 03:24:32 EST

On Mon, 13 Jun 2005 11:05:46 -0600 (MDT) Jim Robison-Cox wrote:

> Dear R-help folks,
>
> I am seeing unexpected behaviour from the function mean
> with option na.rm =TRUE (which is removing a whole column of a data
> frame or matrix.
>
> example:
>
> testcase <- data.frame( x = 1:3, y = rep(NA,3))

In addition to what Sundar already wrote: In the code above x is numeric and y logical, hence as.matrix() will not do what you want (create a "character" matrix). Probably it is more appropriate to do

testcase <- data.frame( x = 1:3, y = as.numeric(rep(NA,3)))

hth,
Z

> mean(testcase[,1], na.rm=TRUE)
> [1] 2
> mean(testcase[,2], na.rm = TRUE)
> [1] NaN
>
> OK, so far that seems sensible. Now I'd like to compute both means
> at
> once:
>
> lapply(testcase, mean, na.rm=T) ## this works
> \$x
> [1] 2
>
> \$y
> [1] NaN
>
> But I thought that this would also work:
>
> apply(testcase, 2, mean, na.rm=T)
> x y
> NA NA
> Warning messages:
> 1: argument is not numeric or logical: returning NA in:
> mean.default(newX[, i], ...)
> 2: argument is not numeric or logical: returning NA in:
> mean.default(newX[, i], ...)
>
> Summary:
> If I have a data frame or a matrix where one entire column is NA's,
> mean(x, na.rm=T) works on that column, returning NaN, but fails using
> apply, in that apply returns NA for ALL columns.
> lapply works fine on the data frame.
>
> If you wonder why I'm building data frames with columns that could
> be
> all missing -- they arise as output of a simulation. The fact that
> the entire column is missing is informative in itself.
>
>
> I do wonder if this is a bug.
>
> Thanks,
> Jim
>
> Jim Robison-Cox ____________
> Department of Math Sciences | | phone: (406)994-5340
> 2-214 Wilson Hall \ BZN, MT | FAX: (406)994-1789
> Montana State University | *_______|
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> jimrc@math.montana.edu
>
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