R help - Nov 2004 - 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

On Thu, 18 Nov 2004, Frank Duan wrote:
> 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(y, x, mean)  # which _is_ in `An Introduction to R', BTW

?by
?aggregate

for more sophisticated packaging of such ideas.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Got it. Many thanks.


On Thu, 18 Nov 2004 13:10:53 -0800, Jeff Laake <jeff.laake at noaa.gov>
wrote:
> look at "by"
> 
> 
> 
> Frank Duan wrote:
> 
> > 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
> >
> > ______________________________________________
> > R-help at stat.math.ethz.ch mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>

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
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

On Thu, 2004-11-18 at 15:34 -0500, Frank Duan wrote:
> 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

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
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>