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

You are using a split-apply strategy when you use by. The objects being passed to the function are dataframes and you are getting the warning and error because of the non-existence of median.data.frame and the impending non-existence of mean.data.frame. It might work better if you used aggregate:

> aggregate(iris[,1:3], iris["Species"], function(x) mean(x,na.rm=T))
     Species Sepal.Length Sepal.Width Petal.Length
1     setosa        5.006       3.428        1.462
2 versicolor        5.936       2.770        4.260
3  virginica        6.588       2.974        5.552
> aggregate(iris[,1:3], iris["Species"], function(x) median(x,na.rm=T))
     Species Sepal.Length Sepal.Width Petal.Length
1     setosa          5.0         3.4         1.50
2 versicolor          5.9         2.8         4.35
3  virginica          6.5         3.0         5.55

aggregate works on the column vectors individually and then tabulates the results.