converting output of R's “by” command to data frame

Question

I\'m trying to use R\'s by command to get column means for subsets of a data frame. For example, consider this data frame:

> z = data.frame(         


        

Answer 1

The output of by is a list so you can use do.call to rbind them and then convert this:

as.data.frame(do.call("rbind",by(z[,2:5],z$labels,colMeans)))
  data.1 data.2 data.3 data.4
a    1.5    6.5   11.5   16.5
b    3.0    8.0   13.0   18.0
c    4.5    9.5   14.5   19.5