[R] How to calculate the stratified means in a data frame?

[Frank Duan]

Dear R people,

I have a simple question to ask. Suppose I have a data.frame with two
variables: one factor (x) and one numeric (y), I want to calculate the
mean of y for each value of x. Although it's easy to do it within a
for a loop, I believe there may be a concise way by using some kinds
of "apply" functions. Could anyone tell me how to do that? Thank you.

Frank