R: split elements of a list into sublists

Question

I have a data frame with following structure:

pat <- c(rep(1,50), rep(2,50), rep(3,50))
inc <- rep(c(rep(1,5), rep(2,5), rep(3,5), rep(4,5), rep(5,5),
             rep(6,5), rep(7,5), rep(8,5), rep(9,5), rep(10,5)), 3)
df <- data.frame(cbind(pat, inc))

df is split into a list of elements:

all.inc = split(df, inc)

Now I want to split each element of this list into sub-lists. Something like:

all.pat = split(all.inc, pat)

This doesn't work, obviously. I've already tried the plyr functions and lapply, but didn't get it to work.

Any ideas?

Answer 1

Use lapply:

lapply(all.inc, function(x) split(x, x$pat))

Answer 2

If you'd like to split your data frame all at once, you could use

split(df, interaction(df$pat,df$inc))

However, the returned value will be a single list of data frames, which is slightly different from what you would get by splitting list elements.