[R] By processing on two variables at once?

[zwarren]

Hello!

I'm trying to runs stats on two vars at a time in a big data frame.  I knew
how to do this in SAS many years ago, but have half-forgotten that as well!

I need, for instance, mean(value) by x-y combination:
x   y   z   value
1   1   1    10
1   1   2    20
1   2   1    30

with results:
x   y   mean(value)
1   1    15
1   2    30

Any help?

Thanks,

~Zack
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