How to merge two lists based on object indices - keeping attributes?

Question

I want to merge two lists keeping the index of each object:

mylist<-list(1,NULL,2)
otherlist<-list(NULL,3,NULL,4,5,6)

# Desired

list(1,3,2,4,5,6)

# my try:

suppressWarnings(mapply(c, mylist, otherlist) )

Answer should be universal

EDIT: In order to avoid proliferation of similar questions. I decided to request here also the possibility of keeping attributes (preferably with base).

mylist<-list(1,NULL,2)
attr(mylist[[1]],"at")<-"a"
attr(mylist[[3]],"at")<-"c"

otherlist<-list(NULL,3,NULL,4,5,6)

attr(otherlist[[2]],"at")<-"b"
attr(otherlist[[4]],"at")<-"d"
attr(otherlist[[5]],"at")<-"e"
attr(otherlist[[6]],"at")<-"f"

Answer 1

Here is an option where we create a logical index with lengths (which will return 0 when there is NULL) and use to assign the elements with mylist unlisted

otherlist[lengths(otherlist) == 0] <- unlist(mylist)
otherlist
#[[1]]
#[1] 1

#[[2]]
#[1] 2

#[[3]]
#[1] 3

#[[4]]
#[1] 4

#[[5]]
#[1] 5

#[[6]]
#[1] 6

---

If we need to use Map, make sure the lengths are the same for the corresponding elements

otherlist[seq_along(mylist)] <- Map(c, otherlist[seq_along(mylist)], mylist)

Update

For the updated example

i1 <- sapply(otherlist, is.null)
i2 <- !sapply(mylist, is.null)
otherlist[i1] <- mylist[i2]

otherlist
#[[1]]
#[1] 1
#attr(,"at")
#[1] "a"

#[[2]]
#[1] 3
#attr(,"at")
#[1] "b"

#[[3]]
#[1] 2
#attr(,"at")
#[1] "c"

#[[4]]
#[1] 4
#attr(,"at")
#[1] "d"

#[[5]]
#[1] 5
#attr(,"at")
#[1] "e"

#[[6]]
#[1] 6
#attr(,"at")
#[1] "f"

Answer 2

foo <- function(l1, l2) {
  out <- vector(mode = "list", length = max(length(l1), length(l2)))
  out[seq_along(l1)] <- l1
  out[!lengths(out)] <- l2[!lengths(out)]
  out
}

foo(mylist, otherlist2)

# [[1]]
# [1] 1
# attr(,"at")
# [1] "a"
# 
# [[2]]
# [1] 3
# attr(,"at")
# [1] "b"
# 
# [[3]]
# [1] 2
# attr(,"at")
# [1] "c"
# 
# [[4]]
# [1] 5
# attr(,"at")
# [1] "e"
# 
# [[5]]
# [1] 6
# attr(,"at")
# [1] "f"