Manipulating matrices

Hi,

I'm pretty new to R and I've been having some problems filtering data
in matrices.

I have the following initial dataset:

|| year | name | varA ||

I have multiple values for "varA" for the same "year" and the same "name".
Having this as the input I would like to obtain the following:

|| year | name | {varA mean} ||

Where I only have one line for each "year" and "name" with the mean of
the values of "varA" in "varA mean".

Is there a simple way to achieve this without using control structures
(for or while cycles)?

Thanks in advance for any help.

Best regards,
S??rgio Nunes