Just adding here that [[ also is equipped for recursive indexing.
This was hinted at in the answer by @JijoMatthew but not explored.
As noted in ?"[[", syntax like x[[y]], where length(y) > 1, is interpreted as:
x[[ y[1] ]][[ y[2] ]][[ y[3] ]] ... [[ y[length(y)] ]]
Note that this doesn't change what should be your main takeaway on the difference between [ and [[ -- namely, that the former is used for subsetting, and the latter is used for extracting single list elements.
For example,
x <- list(list(list(1), 2), list(list(list(3), 4), 5), 6)
x
# [[1]]
# [[1]][[1]]
# [[1]][[1]][[1]]
# [1] 1
#
# [[1]][[2]]
# [1] 2
#
# [[2]]
# [[2]][[1]]
# [[2]][[1]][[1]]
# [[2]][[1]][[1]][[1]]
# [1] 3
#
# [[2]][[1]][[2]]
# [1] 4
#
# [[2]][[2]]
# [1] 5
#
# [[3]]
# [1] 6
To get the value 3, we can do:
x[[c(2, 1, 1, 1)]]
# [1] 3
Getting back to @JijoMatthew's answer above, recall r:
r <- list(1:10, foo=1, far=2)
In particular, this explains the errors we tend to get when mis-using [[, namely:
r[[1:3]]
Error in
r[[1:3]]: recursive indexing failed at level 2
Since this code actually tried to evaluate r[[1]][[2]][[3]], and the nesting of r stops at level one, the attempt to extract through recursive indexing failed at [[2]], i.e., at level 2.
Error in
r[[c("foo", "far")]]: subscript out of bounds
Here, R was looking for r[["foo"]][["far"]], which doesn't exist, so we get the subscript out of bounds error.
It probably would be a bit more helpful/consistent if both of these errors gave the same message.