提取列表中向量的选定元素

Question

Let's consider list following:

listed_list <- list(
  list("something", 2), list("cool", 3),
  list(c("big", "small"), c(2, 3)),
  list(c("huge", "not", "small"), c(3, 4, 5))
)

As you can see first element of the list is a string or vector of strings. What I want to have is to extract specific element, regardless it's in a vector or not.

In this example I would like to have for extracting first element:

"something", "cool", "big", "huge"

If second then

"small", "not"

If third then

"small"

because I'm always extracting the very first element.

And I'm not sure how to do it in the simpler way. I tried with sapply:

unlist(sapply(listed_list, "[[", 1))

but pure use of sapply is not solving the issue (which is logical - it output first element of the first regardless it's simple string or vector).

Could you please give me a hand how it can be done?

Answer 1

foo <- function(L, i) {
  res <- lapply(L, "[[", 1)
  res <- res[lengths(res) >= i]
  vapply(res, "[[", i, FUN.VALUE = character(1))
}

foo(listed_list, 1)
#[1] "something" "cool"      "big"       "huge"   
foo(listed_list, 2)
#[1] "small" "not" 
foo(listed_list, 3)
#[1] "small"