Question

I have a Map<Integer, List<Float>> that represent a position with some scores, and I would like to generate all possible combinations of scores per position (essentially the cross-product of each list from the original map) into a List<List<Float>>. So let’s say I have the following map:

{ 1 => [0.1f, 0.2f], 2 => [0.3f, 0.4f] }

I would to get the following list of lists of floats:

[[0.1f, 0.3f], [0.1f, 0.4f], [0.2f, 0.3f], [0.2f, 0.4f]]

I am pretty sure I have to use recursion, just not sure how to go on about it..

Answer 1

any time you have a recursive algorithm to code, you need an inductive definition to help design it.

base case: {} ↦ []
inductive step: M ↦ L ⇒ ( M ∪ { n ↦ S } ) ↦ L × S

For each element in S, you add it to the end of every list in L, and add the resulting list to your output.

List<List<Float>> llfNext = new LinkedList<List<Float>>();
for (float f : entEach.getValue())
    for (List<Float> lfEach : llfInductive)
        llfNext.add(copyAndAddToEnd(lfEach, f));
return llfNext;

It should be pretty easy from here, and then you can make it non-recursive and reduce some of the copying going on.