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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