[R] Iterative sampling with restrictions

[David A. Kim]

Hi all,

I'm working on a seemingly trivial problem that I can't figure out how
to implement in R.  I'd be most grateful for any help here.

I want to do the following: first, randomly assign each of n units to
one of g groups of size n/g. Then, randomly re-assign each of the n
units to a different group (i.e., same as the first step, but the unit
can't be assigned to a group to which it's already belonged).  Then
repeat this step until each unit has at some point been assigned to
every group.

More concretely, say I have 60 units and 3 groups into which to divide
the units.  I could first do something like:

group1<-sample(1:60,20)
group2<-sample(setdiff(1:60,group1),20)
group3<-sample(setdiff(1:60,c(group1,group2)),20)

But then how to randomly re-assign group membership such that all
units are assured a different group assignment in the second "wave" of
grouping?  Just narrowing the sampling pool to those units that
weren't previously assigned to a given group won't work (consider the
case where groups 1 and 2 swap units: in the second wave, there would
be no units to assign to group 3 as all the remaining units had
already been in group 3 in the first wave).

Most grateful for any assistance,

David