Select first element of nested list

Let's say I have a list like this:

x = list(list(1,2), list(3,4), list(5,6))

I would like a list that contains only the first elements of the nested list. I can do this by returning another list like so

x1 = lapply(x, function(l) l[[1]])

Is there shortcut notation for this?

Not much of a shortcut, but you can do this:

lapply(x, `[[`, 1)
# [[1]]
# [1] 1
#
# [[2]]
# [1] 3
#
# [[3]]
# [1] 5

Another possibility uses the nice purrr library:

library(purrr)
map(x, 1)

For your example list you can just do:

unlist(x)[ c(TRUE,FALSE) ]

but that depends on each sublist having exactly 2 elements.

If there are different numbers of elements then you could first do an sapply to calculate the lengths, then compute the corresponding 1st element positions (see cumsum), then select those values from the unlisted list. But by that time the accepted answer is probably much simpler.

If all the sublists have the same length (but could be different from 2) then you could do something like:

do.call( rbind, x)[,1]

or some other cast to a common object. But I doubt that this would be as efficient as the lapply approach.