How to calculate the stratified means in a data frame?

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

One way is to use by(). Using the 'iris' dataset to get the means for
Sepal.Length by Species:

with(iris, by(Sepal.Length, Species, mean))

INDICES: setosa
[1] 5.006
------------------------------------------------------ 
INDICES: versicolor
[1] 5.936
------------------------------------------------------ 
INDICES: virginica
[1] 6.588

See ?by, also ?tapply and ?aggregate.

Note also the use of with() as a wrapper, in lieu of attach() here.

HTH,

Marc Schwartz