[R] tapply

[Chuck Cleland]

On 4/8/2008 5:14 AM, Manuel Montes wrote:
> Does anyone have something like tapply for matrices? I would like to average the contents of columns 
> according to factors but tapply works only with one column at each time. Is something available to do 
> everything in one step?
> Many thanks.
> Manuel Montes

   How about aggregate()?  For example:

aggregate(iris[,c("Sepal.Length", "Sepal.Width", "Petal.Length", 
"Petal.Width")], by = list(Species = iris$Species), FUN = mean)

      Species Sepal.Length Sepal.Width Petal.Length Petal.Width
1     setosa        5.006       3.428        1.462       0.246
2 versicolor        5.936       2.770        4.260       1.326
3  virginica        6.588       2.974        5.552       2.026

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-- 
Chuck Cleland, Ph.D.
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