Explain R tapply description

Question

I understand what tapply() does in R. However, I cannot parse this description of it from the documentaion:

Apply a Function Over a \"Ragged\" Array

Description:

         


        

Answer 1

@joran's great answer helped me understand it (so please vote for his - I would have added it as comment if it wasn't too long for that), but this may be of help to some:

In quite a few languages, you have twodimensional arrays. Depending on the language, these arrays have fixed dimensions (i.e.: each row has the same number of columns), or some languages allow the number of items per row to differ. So instead of:

A: 1  2  3
B: 4  5  6
C: 7  8  9

You could get something like

A: 1  3
B: 4  5  6
C: 8

This is called a ragged array because, well, the right side of it looks ragged. In typical R-style, we might represent this as two vectors:

values<-c(1,3,4,5,6,8)
names<-c("A", "A", "B", "B", "B", "C")

So tapply with these two vectors as the first parameters indeed allows us to apply this function to each 'row' of our ragged array.