One way for doing it would be to combine the columns using paste and then
use tapply to get the means.

For example:

set.seed(32341)
a1 = sample(c("a","b"), 100,replace = T)
a2 = sample(c("a","b"), 100,replace = T)
y = rnorm(100)
tapply(y,paste(a1,a2), mean)



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On Thu, Oct 6, 2011 at 8:40 AM, SML <parallax@lafn.org> wrote:

> Hello:
>
> Is there a way to get a mean from values stored in different rows?
>
> The data looks like this:
>  YEAR-1, JAN, FEB, ..., DEC
>  YEAR-2, JAN, FEB, ..., DEC
>  YEAR-3, JAN, FEB, ..., DEC
>
> What I want is the mean(s) for just the consecutive winter months:
>  YEAR-1.DEC, YEAR-2.JAN, YEAR-2.FEB
>  YEAR-2.DEC, YEAR-3.JAN, YEAR-3.FEB
>  etc.
>
> Thanks.
>
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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