I want to interleave interElem after every 2 list elements.

Data:

listi     <- c(rbind(letters[1:4], list(c(13,37))))

interElem <- c("inter","leavistan")

looks like:

> listi
[[1]]
[1] "a"

[[2]]
[1] 13 37

[[3]]
[1] "b"

[[4]]
[1] 13 37

[[5]]
[1] "c"

[[6]]
[1] 13 37

[[7]]
[1] "d"

[[8]]
[1] 13 37

> 

Desired result (list element numbers are not accurate)

> listi
[[1]]
[1] "a"

[[2]]
[1] 13 37

[[XXX]]
[1] "inter" "leavistan"

[[3]]
[1] "b"

[[4]]
[1] 13 37

[[XXX]]
[1] "inter" "leavistan"

[[5]]
[1] "c"

[[6]]
[1] 13 37

[[XXX]]
[1] "inter" "leavistan"

[[7]]
[1] "d"

[[8]]
[1] 13 37

> 

Here's the length of the new list

len = length(listi) + max(0, floor((length(listi) - 1) / 2))

and the index of the elements that should be the original values

idx = seq_len(len) %% 3 != 0

Use these to create a new list and insert the old and interstitial values

res = vector("list", len)
res[idx] = l
res[!idx] = list(v)

Package as a function for robustness and reuse.

fun = function(l, v, n = 2) {
    ## validate arguments
    stopifnot(
        is.numeric(n), length(n) == 1, !is.na(n),
        is.list(l), is.vector(v)
    )

    ## length and index for original values
    len = length(l) + max(0, floor((length(l) - 1) / n))
    idx = seq_len(len) %% (n + 1) != 0

    ## create and populate result
    res = vector("list", len)
    res[idx] = l
    res[!idx] = list(v)
    res
}

with

> str(fun(listi, interElem))
List of 11
 $ : chr "a"
 $ : num [1:2] 13 37
 $ : chr [1:2] "inter" "leavistan"
 $ : chr "b"
 $ : num [1:2] 13 37
 $ : chr [1:2] "inter" "leavistan"
 $ : chr "c"
 $ : num [1:2] 13 37
 $ : chr [1:2] "inter" "leavistan"
 $ : chr "d"
 $ : num [1:2] 13 37