Extract specific list elements from a nested list in R?

I have a nested list that looks like this:

x <- list(list(a = c(1,10), b=c(2,10)), list(c=c(3,10), d=c(6,10)), list(e= c(3,10)))
> x
[[1]]
[[1]]$a
[1]  1 10

[[1]]$b
[1]  2 10


[[2]]
[[2]]$c
[1]  3 10

[[2]]$d
[1]  6 10


[[3]]
[[3]]$e
[1]  3 10

I’m trying to extract just the first value from each list element. However, if the length of a list is greater than 1, then I want to add the extracted values together.

For example, in my list x above, we can see that the first list (i.e., x[[1]]) contains a and b… so I want to extract the 1st values from this list (in this case, 1 and 2) and since they belong to the same list, they get added together.

At the end, Im just trying to return a vector of the extracted (and where necessary, added) values. So in my example, I would expect a vector like: [1] 3 9 3.

>Solution :

Using nested sapply –

sapply(x, function(y) sum(sapply(y, head, 1)))
#[1] 3 9 3

Or combine each list into a matrix and take sum of first column/row.

sapply(x, function(y) sum(do.call(rbind, y)[, 1]))
#sapply(x, function(y) sum(do.call(cbind, y)[1, ]))