[R] Averaging over columns

Sean Davis sdavis2 at mail.nih.gov
Mon Mar 6 13:31:15 CET 2006

On 3/6/06 7:13 AM, "michael watson (IAH-C)" <michael.watson at bbsrc.ac.uk>

> Hi
> I've been reading the help for by and aggregate but can't get my head
> round how to do this.
> I have a data frame - the first three columns are replicate
> measurements, then the next 3 are replicates etc up to 36 (so 12
> variables with 3 replicate measurements each).  I want to compute the
> mean for each of the 12 variables, so that, for each row, I have 12
> means.
> A grouping variable across columns can easily be created by
> rep(1:12,each=3), but I can't figure out which function to use to get R
> to calculate the means I want.

Hi, Mick.  How about this?

 a <- matrix(rnorm(360),nr=10)
 b <- rep(1:12,each=3)
 avgmat <- aggregate(a,by=list(b))


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