Interleave lists in R

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]

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.

interleave(a, b)

# unlist(interleave(a, b))
# [1] "a.1" "b.1" "a.2" "b.2" "a.3" "b.3" "b.4"


interleave <- function(a, b) { 

  shorter <- if (length(a) < length(b)) a else b
  longer  <- if (length(a) >= length(b)) a else b

  slen <- length(shorter)
  llen <- length(longer)


  index.short <- (1:slen) + llen
  names(index.short) <- (1:slen)

  lindex <- (1:llen) + slen
  names(lindex) <- 1:llen


  sindex <- 1:slen
  names(sindex) <- 1:slen

  index <- c(sindex, lindex)
  index <- index[order(names(index))]

  return(c(a, b)[index])

}

Here's one option using the interleave function from ggplot2. I'm sure this can be improved upon, but it's a start:

require(ggplot2)
Interleave <- function(x,y){
    v <- list(x,y)
    lengths <- sapply(v,length)
    mn <- min(lengths)
    v <- v[order(lengths)]
    c(ggplot2:::interleave(v[[1]],v[[2]][seq_len(mn)]),v[[2]][(mn+1):length(v[[2]])])
}

Interleave(a,b)
Interleave(b,a)

In particular, this will do weird things if the lists are actually the same length. Perhaps someone will chime in with a better way to do the indexing for v[[2]] in the last line that avoids that degenerate case.

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

interleave <- function(a, b) {
  mlab <- min(length(a), length(b)) 
  seqmlab <- seq_len(mlab) 
  c(rbind(a[seqmlab], b[seqmlab]), a[-seqmlab], b[-seqmlab]) 
}

interleave(a, b) 

From http://r.789695.n4.nabble.com/Interleaving-elements-of-two-vectors-tp795123p1691409.html

This is a little bit faster than @Arun: