create nested list from 2 or more vectors in R?

Question

Say I have 2 (or more) vectors:

v1 <- letters[1:2]
v2 <- letters[24:26]

> v1
[1] "a" "b"

> v2
[1] "x" "y" "z"

How could I programmatically construct a nested list like this, that contains lists of all of the combinations of my input vectors? Edit: I'd like to do this without inefficient looping structures.

> l1 <- list(list(v1[1], v2[1]), list(v1[1], v2[2]), 
        list(v1[1], v2[3]), list(v1[2], v2[1]), 
        list(v1[2], v2[2]), list(v1[2], v2[3]))

> str(l1)
List of 6
$ :List of 2
..$ : chr "a"
..$ : chr "x"
$ :List of 2
..$ : chr "a"
..$ : chr "y"
$ :List of 2
..$ : chr "a"
..$ : chr "z"
$ :List of 2
..$ : chr "b"
..$ : chr "x"
$ :List of 2
..$ : chr "b"
..$ : chr "y"
$ :List of 2
..$ : chr "b"
..$ : chr "z"

Answer 1

A simple way is to use loops:

l1 <- list()
list_pos <- 1L

for (i in 1:length(v1)) {
  for (j in 1:length(v2)) {
    l1[[list_pos]] <- list(v1[i], v2[j])
    list_pos <- list_pos + 1L
  }  
}

Edit :

I think loops get a bit of unfair rep but here is some (ugly) code that avoids them:

l2 <- apply(expand.grid(v1, v2), 1, as.list)

Btw. The loop is way faster. I suspect there are some much prettier and more creative ways to do this still.

Answer 2

Here is a solution that generates the required combinations into a data frame.

> v1 <- letters[1:2]
> v2 <- letters[24:26]
> 
> aResult <- expand.grid(v2,v1)
> View(aResult)
> aResult
  Var1 Var2
1    x    a
2    y    a
3    z    a
4    x    b
5    y    b
6    z    b
> 

If the data frame must be split into a list() of one combination per list entry, we can accomplish this with split() .

#If the data frame must be split into a list, one entry per row
aResult$id <- as.factor(1:nrow(aResult))
theList <- split(aResult,aResult$id)

Printing the first element in the list, we see the following.

> theList[[1]]
  Var1 Var2 id
1    x    a  1
>