Split a list of elements into two unique lists in R (and get all combinations)

List 3 elements:

vec <- 1:3

Note that for each element we have two possibilities: whether it is in the first split or in the second. Therefore, we define a matrix of all possible splits (row-wise) with expand.gridwhich all possible combinations can be produced:

groups <- as.matrix(expand.grid(rep(list(1:2), length(vec))))

However, this treats the case where the group is flipped to a different split as a treatment. Also includes scenarios where all observations are in the same group (but only 2 of them).

If we want to remove them, we need to remove only one group of rows (2 such rows) from the groupsmatrix and split all the rows of the vector in the same way, just switch the group.

A set of entries is at the top and bottom, so remove them easily:

groups <- groups[-c(1, nrow(groups)),]

Duplicate entries are a little trickier. But note that we can get rid of them by deleting all the lines where the first group is 2. In effect, this would require that the first element is always assigned to group 1.

groups <- groups[groups[,1]==1,]

The job then is to split the list with each row in the groupsmatrix . To do this, we use to Mapcall functions on each row of lists and matrices :split()vecgroups

splits <- Map(split, list(vec), split(groups, row(groups)))

> splits
[[1]]
[[1]]$`1`
[1] 1 3

[[1]]$`2`
[1] 2


[[2]]
[[2]]$`1`
[1] 1 2

[[2]]$`2`
[1] 3


[[3]]
[[3]]$`1`
[1] 1

[[3]]$`2`
[1] 2 3