Given a nested list of numeric vectors like
l = list( a = list(1:2, 3:5), b = list(6:10, 11:16)).
If I want to apply a function, say length, of the "index 1 / first" numeric vectors I can do it using the subset function [[:
> sapply(lapply(l, "[[", 1), length)
a b
2 5
I can't figure how to supply arbitrary indices to [[ in order to get length of (in this example) both vectors in every sub-list (a naive try: sapply(lapply(l, "[[", 1:2), length)).
The [[ can only subset a single one. Instead, we need [ for more than 1 and then use lengths
sapply(lapply(l, "[", 1:2), lengths)
# a b
#[1,] 2 5
#[2,] 3 6