[R] Equivalent to a BY command in SAS

[zerfetzen]

Hi,
I'm very new to R and absolutely love it.  Does anyone know how to use
something in R that functions like a BY command in SAS?

For example, let's say you have a variable x, and you want to see the mean. 
Easy...

> mean(x)

But what if you want to see the mean of x conditional on another discrete
variable?  My best attempts so far are something like...

> mean(x, y_cat=1)

...which of course doesn't work.  I have downloaded plenty of R user guides
that are very informative, but am not seeing much on detailed descriptives
or data manipulation (for my life, I can't figure out how to sort an
attached data frame, but that's another issue).  Thanks.
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