Accessing selected elements of a list of lists in R - r

Question

I have a list of list subgame[[i]]$Weight of this type:
[[1]]
[1] 0.4720550 0.4858826 0.4990469 0.5115899 0.5235512 0.5349672 0.5458720
[8] 0.5562970 0.5662715 0.5758226 0.5849754 0.5937532 0.6021778 0.6102692
[15] 0.6180462 0.6255260 0.6327250 0.6396582 0.6463397 0.6527826
[[2]]
[1] 0.4639948 0.4779027 0.4911519 0.5037834 0.5158356 0.5273443 0.5383429
[8] 0.5488623 0.5589313 0.5685767 0.5778233 0.5866943 0.5952111 0.6033936
[15] 0.6112605 0.6188291 0.6261153 0.6331344 0.6399002 0.6464260
[[3]]
[1] 0.4629488 0.4768668 0.4901266 0.5027692 0.5148329 0.5263534 0.5373639
[8] 0.5478953 0.5579764 0.5676339 0.5768926 0.5857755 0.5943041 0.6024984
[15] 0.6103768 0.6179568 0.6252543 0.6322844 0.6390611 0.6455976
What I am looking for is to access all the j-th elements of every list. Example if j=1 I must get:
>0.4720550 0.4639948 0.4629488
How can I do it?
I found
sapply(1:length(subgame[[i]]$Weight),function(k) subgame[[i]]$Weight[[k]][1])
But seems too tricky to me.
There is a more elegant way?

Answer 1

If j=1, then you're interested in subgame[[i]]$Weight[[1]][1], subgame[[i]]$Weight[[2]][1], and subgame[[i]]$Weight[[3]][1]. In other words, you want to use [1] on each list element.
But what happens when you subset a vector? For example:
(x <- rnorm(5))
# [1] -1.8965529 0.4688618 0.6588774 0.2749539 0.1829046
x[3]
# [1] 0.6588774
[ is actually a function, and it gets called in this situation. You can read a bit more about it with ?"[", but the point is that you can call it like any other function. Its first argument will be the object to subset, then you can pass it the index (or indices) you're interested in (along with some other arguments that the help page discusses):
x[3]
# [1] 0.6588774
`[`(x, 3)
# [1] 0.6588774
Note the backticks surrounding the name. A bare [ will throw an error, so you need to quote it. The same goes for other functions like +.
So if you want to get the first element of each list element, you can apply [ to each element of the list, passing it 1 or whatever j is:
sapply(subgame[[i]]$Weight, `[`, 1)

Answer 2

I would like to add a solution which returns the result you want for the Weight list of each elements of your subgame list.
> subgame <- list(list(weight = list(c(1, 2), c(3, 4), c(5, 6))), list(weight = list(c(7, 8), c(9, 10), c(11, 12))))
>
> j = 1
>
> do.call(rbind, subgame[[1]]$weight)[,j]
[1] 1 3 5
>
> lapply(subgame, function(x) {do.call(rbind, x$weight)[,j]})
[[1]]
[1] 1 3 5
[[2]]
[1] 7 9 11