具有位置向量的向量的子集列表

Question

I have a list of vectors ( mylist ):

a <- c(1,2,3,4)
b <- c(5,6,7,8)
c <- c(9,10,11,12)
mylist <- list(a,b,c)

I also have a vector of positions ( mypos ):

mypos <- c(1,2,3)

I would like to use mypos to give the position of elements to subset each vector of mypos so that it returns:

 [1] 1 6 11

I have tried using lapply like this:

lapply(mylist, "[", mypos) 

but this returns elements 1, 2 and 3 of each vector:

[[1]]
 [1] 1 2 3

[[2]]
 [1] 5 6 7

[[3]] 
 [1] 9 10 11

I have also tried:

lapply(mylist, subset, mypos)

But this returns an error that the subset must be logical

Answer 1

We can use Map to extract the corresponding elements of 'mylist' from the index of 'mypos'

Map(`[`, mylist, mypos)

---

In the OP's code, the 'mypos' is repeated in each of list elements resulting in extracting all the elements from the index. Instead it could be looped on sequence

lapply(seq_along(mylist), function(x) mylist[[x]][mypos[[x]]])