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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