How can I find all of the permutations consisting of 1 element from a variable number of arrays of variable length?

Question

I have an array U of arrays D that vary in length. I need to be able to return all permutations of array indices that would select a different perm

Answer 1

So ... what about this isn't straightforward?

You want an iterator. You want it to iterate over the last array. When it gets to the end of that array, increment its current position in the second-last array and go back to the start of the last array.

psuedocode using C#s yield return syntax:

foreach n1 in a1
    foreach n2 in a2
        foreach n3 in a3
            yield return (n1, n2, n3)

EDIT: If the number of sets varies, you could use some form of recursion:

function next(list)
    firstArray = list.first
    iterator = iterator(list.rest)
    if !iterator
        foreach i in firstArray
            yield return i
    else
        foreach i in firstArray
            while (iterator.hasNext)
                yield return (i, iterator.next)

Consider the behaviour when a list of length 1 is passed in, then consider the behaviour for a list of length 2, and satisfy yourself that it does in fact work.