从嵌套列表中提取列表的第一项

Question

I have the following column in my data table (there are more lists inside, but I just provide it simplified)

n  col1
1  list(c("1", "2", "3", "5"), c("5", "6", "8", "8"))
2  list(c("9", "10", "11", "12"), c("13", "14", "15", "16"))

And I want to extract the first value from each of the lists in the row. So, I want the outcome

n  col2
1  c("1", "5")
2  c("9", "13")

I looked into many examples here and on the internet about nested lists, it seems that it should be easy, but it doesn't seem that it works quite as I intend it to. I use the following variants of code

   1. dt$col2 <-sapply(dt$col1 , "[[", 1)
  `2. dt$col2 <- lapply(dt$col1, function(l) l[[1]])`
  `3. dt$col2 <- rvest::pluck(dt$col1, 1)`

But they all give the same output which is basically the first list

n  col2
1  c("1", "2", "3", "5")
2  c("9", "10", "11", "12")

There must be something wrong with my understanding, but I can't get why it happens this way

Answer 1

Try:

dt$col2 <-sapply(dt$col1 , "[", 1)

(The elements of your lists are not lists, just vectors, so one bracket is fine)

EDIT:

For a column inside a data.table:

 dt$col2 <- sapply(dt$col1,function(x){sapply(x,'[',1)})