Transpose a List of Lists

Question

I have a list which contains list entries, and I need to transpose the structure. The original structure is rectangular, but the names in the sub-lists do not match.

Here is an example:

ax <- data.frame(a=1,x=2)
ay <- data.frame(a=3,y=4)
bw <- data.frame(b=5,w=6)
bz <- data.frame(b=7,z=8)
before <- list(  a=list(x=ax, y=ay),   b=list(w=bw, z=bz))

What I want:

after  <- list(w.x=list(a=ax, b=bw), y.z=list(a=ay, b=bz))

I do not care about the names of the resultant list (at any level).

Clearly this can be done explicitly:

after <- list(x.w=list(a=before$a$x, b=before$b$w), y.z=list(a=before$a$y, b=before$b$z))

but this is ugly and only works for a 2x2 structure. What's the idiomatic way of doing this?

Answer 1

The following piece of code will create a list with i-th element of every list in before:

lapply(before, "[[", i)

Now you just have to do

n <- length(before[[1]]) # assuming all lists in before have the same length
lapply(1:n, function(i) lapply(before, "[[", i))

and it should give you what you want. It's not very efficient (travels every list many times), and you can probably make it more efficient by keeping pointers to current list elements, so please decide whether this is good enough for you.

Answer 2

The purrr package now makes this process really easy:

library(purrr)

before %>% transpose()

## $x
## $x$a
##   a x
## 1 1 2
## 
## $x$b
##   b w
## 1 5 6
## 
## 
## $y
## $y$a
##   a y
## 1 3 4
## 
## $y$b
##   b z
## 1 7 8