[R] NAs and row/column calculations
Jim Bouldin
jrbouldin at ucdavis.edu
Fri Mar 12 00:20:39 CET 2010
>
> On 12/03/2010, at 11:25 AM, Jim Bouldin wrote:
>
> >
> > I continue to have great frustrations with NA values--in particular
> making
> > summary calculations on rows or cols of a matrix containing them. For
> > example, why does:
> >
> >> a = matrix(1:30,nrow=5)
> >> is.na(a[c(1:2),c(3:4)]);a
> > [,1] [,2] [,3] [,4] [,5] [,6]
> > [1,] 1 6 NA NA 21 26
> > [2,] 2 7 NA NA 22 27
> > [3,] 3 8 13 18 23 28
> > [4,] 4 9 14 19 24 29
> > [5,] 5 10 15 20 25 30
> >> apply(a[!is.na(a)],2,sum)
> >
> > give me this:
> >
> > "Error in apply(a[!is.na(a)], 2, sum) : dim(X) must have a positive
> length"
> >
> > when
> >
> >> dim(a)
> > [1] 5 6
> >
> > What is the trick to calculating summary values from rows or columns
> > containing NAs? Drives me nuts. More nuts that is.
>
> When you do a[!is.na(a)] you get a ***vector*** --- not a matrix.
> ``Obviously''!!!
Well, obvious to you maybe, or someone who's done it before, but not to me.
The non-missing values of a cannot be arranged in
> a 5 x 6 matrix; there are only 26 of them. So (as my late Uncle
> Stanley would have said) ``What the hell do you expect?''.
Silly me, I expected, based on (1) previous experience doing summary calcs
on subsets of a matrix using exactly that style of command, and (2) the
fact that dim(a) returns: [1] 5 6, and (3) the fact that a help search
under the "apply" function gives NO INDICATION of any possible use of the
na.rm command, AND (4) a help search on "na.action" does not even mention
na.rm, that:
> apply(a[!is.na(a)],2,sum)
would sum the non-NA elements of matrix a, by columns. Terribly faulty
reasoning on my part, obviously.
>
> The ``trick'' is to remove the NAs at the summing stage:
>
> apply(a,2,sum,na.rm=TRUE)
>
> Not all that tricky.
>
> cheers,
>
> Rolf Turner
>
> ######################################################################
> Attention:
> This e-mail message is privileged and confidential. If you are not the
> intended recipient please delete the message and notify the sender.
> Any views or opinions presented are solely those of the author.
>
> This e-mail has been scanned and cleared by MailMarshal
> www.marshalsoftware.com
> ######################################################################
>
Jim Bouldin, PhD
Research Ecologist
Department of Plant Sciences, UC Davis
Davis CA, 95616
530-554-1740
More information about the R-help
mailing list