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

[Peter Dalgaard]

Frank Duan <fhduan at gmail.com> writes:

> 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.

tapply() will do that. (help(tapply), look at the "presidents" example).

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
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