How to apply a function to a subset of columns in r?

Question

I am using by to apply a function to a range columns of a data frame based on a factor. Everything works perfectly well if I use mean()

Answer 1

The original question is answered. If, however, the range happens to be (instead) all columns except those specified as the independent variable in the formula, the dot formula notation works, and represents a nifty alternative:

> aggregate(. ~ Species, data = iris, 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

> aggregate(. ~ Species, data = iris, median)
     Species Sepal.Length Sepal.Width Petal.Length Petal.Width
1     setosa          5.0         3.4         1.50         0.2
2 versicolor          5.9         2.8         4.35         1.3
3  virginica          6.5         3.0         5.55         2.0