equivalent of Stata "by construct"

I am evaluating a switch from Stata to R. I don't need to extensive Statistical methods, but the main reason I am exploring the switch is the coding flexibility in R (e.g. Stata does not support linear/quadratic programming). I have been going over the R syntax and I had a quick question:

In Stata, one has a very useful construction called "by", e.g.

by month signal: gen xxx = myfunction(yyy zzz) if cond1==cond2

Similarly, in SAS, one can write

by monthn signal;
if cond1=cond2 then xxx = myfunction(yyy zzz);
run;


I looked at a lot of R tutorials, but I haven't come across the equivalent of the Stata or SAS "by" construct in R?

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Pradipto Banerjee
Sent: Thursday, September 27, 2012 9:13 AM
To: r-help at r-project.org
Subject: [R] equivalent of Stata "by construct"

I am evaluating a switch from Stata to R. I don't need to extensive
Statistical methods, but the main reason I am exploring the switch is the
coding flexibility in R (e.g. Stata does not support linear/quadratic
programming). I have been going over the R syntax and I had a quick
question:

In Stata, one has a very useful construction called "by", e.g.

by month signal: gen xxx = myfunction(yyy zzz) if cond1==cond2

Similarly, in SAS, one can write

by monthn signal;
if cond1=cond2 then xxx = myfunction(yyy zzz);
run;


I looked at a lot of R tutorials, but I haven't come across the equivalent
of the Stata or SAS "by" construct in R?