Like this?
library(plyr)
ddply(df,.(comn,mi),summarise,stDEV=sd(x))
Felipe D. Carrillo
Supervisory Fishery Biologist
Department of the Interior
US Fish & Wildlife Service
California, USA
http://www.fws.gov/redbluff/rbdd_jsmp.aspx
From: Aurélien PHILIPPOT <aurelien.philip...@gmail.com>
>To: R-help@r-project.org
>Sent: Sunday, December 4, 2011 12:32 PM
>Subject: [R] Group several variables and apply a function to the group
>
>Dear R-experts,
>I am struggling with the following problem, and I am looking for advice
>from more experienced R-users: I have a data frame with 2 identifying
>variables (comn and mi), and an output variable (x). comn is a variable for
>a company and mi is a variable for a month.
>
>comn<-c("abc", "abc", "abc", "abc", "abc", "abc", "xyz", "xyz","xyz", "xyz")
>mi<- c("1", "1","1", "2", "2", "2", "1", "1", "3", "3")
>x<- c("-0.0031", "0.0009", "-0.007", "0.1929","0.0087", "0.099","-0.089",
>"0.005", "-0.0078", "0.67" )
>df<- data.frame(comn=comn, mi=mi, x=x)
>
>
>For each company, within a particular month, I would like to compute the
>standard deviation of x: for example, for abc, I would like to compute the
>sd of x for month1 (when mi=1) and for month2 (when mi=2).
>
>In other languages (Stata for instance), I would create a grouping variable
>(group comnn and mi) and then, apply the sd function for each group.
>
>However, I don't find an elegant way to do the same in R:
>
>I was thinking about the following: I could subset my data frame by mi and
>create one file per month, and then make a loop and in each file, use a
>"by" operator for each comn. I am sure it would work, but I feel that it
>would be like killing an ant with a tank.
>
>I was wondering if anyone knew a more straightforward way to implement that
>kind of operation?
>
>Thanks a lot,
>
>Best,
>Aurelien
>
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>
>______________________________________________
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>PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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>
>
>
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