[R] more on apply on data frame

[Göran Broström]

On Fri, Aug 20, 2004 at 11:40:16PM -0500, Laura Holt wrote:
> Hi R People:
> 
> Several of you pointed out that using "tapply" on a data frame will work on 
> the iris data frame.
> 
> I'm still having a problem.
> 
> The iris data frame has 150 rows, 5 variables.  The first 4 are numeric, 
> while the last is a factor, which has the Species names.
> 
> I can use tapply for 1 variable at a time:
> >tapply(iris[,1],iris[,5],mean)
>    setosa versicolor  virginica
>     5.006      5.936      6.588
> >
> but if I try to use this for all of the first 4, I get an error:
> >tapply(iris[,1:4],iris[,5],mean)
> Error in tapply(iris[, 1:4], iris[, 5], mean) :
>        arguments must have same length
> >
> Any ideas of what I'm doing wrong, please?

You are not reading the help page:

Usage:

     tapply(X, INDEX, FUN = NULL, ..., simplify = TRUE)

Arguments:

       X: an atomic object, typically a vector.

iris[, 1:4] is a data frame of length 4; iris[, 5] is a vector of length 150.

You probably need to loop over the four first columns and apply tapply(!)
four times, but I'm sure there is a smarter way. Others will tell you.

Göran
-- 
 Göran Broström                    tel: +46 90 786 5223
 Department of Statistics          fax: +46 90 786 6614
 Umeå University                   http://www.stat.umu.se/egna/gb/
 SE-90187 Umeå, Sweden             e-mail: gb at stat.umu.se