From: Sean Davis <sdavis2_at_mail.nih.gov>

Date: Sat 05 Mar 2005 - 00:27:59 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Mar 05 02:59:50 2005

Date: Sat 05 Mar 2005 - 00:27:59 EST

On Mar 4, 2005, at 6:55 AM, Clint Harshaw wrote:

> vito muggeo wrote:

*>> Hi,
**>> if I understand correctly, tapply() is your friend here,
**>> vito
**>> dvrecko@sfu.ca wrote:
**>>>
**>>> Hi. I'm a student at SFU in Canada. The basic thing I want to do is
**>>> calculate means of different strata. I have 2 vectors. One has the
**>>> values I
**>>> want to take the means from, the other is the four strata I am
**>>> interested
**>>> in. So I essentially want to break up the information vector into
**>>> the four
**>>> strata and calculate four means, one for each stratum. How can I do
**>>> this in
**>>> a reasonable way?
**>>>
**>>>
**>>> Thanks very much.
**>>> Dean Vrecko
**>>>
**>>> PS: Incidentally I forget how to see the code of functions. Does
**>>> anyone
**>>> remember the command to do this?
**>>>
**>>> ______________________________________________
**>>> R-help@stat.math.ethz.ch mailing list
**>>> https://stat.ethz.ch/mailman/listinfo/r-help
**>>> PLEASE do read the posting guide!
**>>> http://www.R-project.org/posting-guide.html
**>>>
**>
**> Would split(y,f) be of some help here? And then you could find the
**> mean for each level of f:
**>
**> y.split <- split(y,f)
**> mean(y.split$"0")
**> mean(y.split$"1")
**> mean(y.split$"2")
**> mean(y.split$"3")
*

You can automate this using aggregate:

> a <- rep(c('A','B','C','D'),25) > b <- rnorm(100) > aggregate(b,by=list(a),mean) Group.1 x 1 A 0.1409995 2 B -0.1524387 3 C 0.3329184 4 D 0.1354157

Sean

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Mar 05 02:59:50 2005

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