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

[Liaw, Andy]

See ?tapply or ?by.

Andy

> From: 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
> 
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