Interleave lists in R

Question

Let's say that I have two lists in R, not necessarily of equal length, like:

 a <- list('a.1','a.2', 'a.3')  b <- list('b.1','b.2', 'b.3', 'b.4') 

What is the best way to construct a list of interleaved elements where, once the element of the shorter list had been added, the remaining elements of the longer list are append at the end?, like:

interleaved <- list('a.1','b.1','a.2', 'b.2', 'a.3', 'b.3','b.4') 

without using a loop. I know that mapply works for the case where both lists have equal length.

Answer 1

Here's one way:

idx <- order(c(seq_along(a), seq_along(b))) unlist(c(a,b))[idx]  # [1] "a.1" "b.1" "a.2" "b.2" "a.3" "b.3" "b.4" 

As @James points out, since you need a list back, you should do:

(c(a,b))[idx] 

Answer 2

While investigating a similar question, I came across this beautiful solution by Gabor Grothendieck (i.e. @GGrothendieck?) for certain cases:

c(rbind(a,b)) 

This works equally well when a and b are both lists, or when a and b are both vectors. It's not a precise solution to OP's question, because when a and b have different lengths, it will recycle the elements of the shorter sequence, printing a warning. However, since this solution is simple and elegant, and provides an answer to a very similar question--a question of some people (like me) who find their way to this page as a result--it seemed worth adding as an answer.