How to create all combinations from a nested list while preserving the structure using R?

Question

Given a nested list, how to create all possible lists from its elements, while preserving the structure of the nested list?

Nested list:

Answer 1

The relist function from utils seems to be designed for this task:

rl <- as.relistable(l)
r <- expand.grid(data.frame(rl), KEEP.OUT.ATTRS = F)
> head(r, 5)
   b c.d.e c.d.f g
1  1     3     5 7
2  2     3     5 7
3  1     4     5 7
4  2     4     5 7
5  1     3     6 7

It saves the structure of the list (skeleton). This means one can now manipulate the data within the nested list and re-assign it into the structure (flesh). Here with the first row of the expanded matrix.

r <- rep(unname(unlist(r[1,])),each = 2)
l2 <- relist(r, skeleton = rl)
> l2
$a
$a$b
[1] 1 1


$c
$c$d
$c$d$e
[1] 3 3

$c$d$f
[1] 5 5



$g
[1] 7

attr(,"class")
[1] "relistable" "list"  

Note that since the structure stays the same, I need to supply the same amount of elements as in the original list. This is why used rep to repeat the element twice. One could also fill it with NA, I guess.

For every possible combination iterate through r (expanded):

lapply(1:nrow(r), function(x) 
          relist(rep(unname(unlist(r[x,])),each = 2), skeleton = rl))