Question

I’d like to create a list of tuples from each value within a set of lists. The set of lists can be open, but for the example I have the following three lists of Strings.

L1: (one, two three)
L2: (a, b, c)
L3: (yes, no)

I would like to return a list of tuples, where in each tuple I have on element from each list. In this case, I will have 18 combinations (3 x 3 x 2)

T1: (one, a, yes)
T2: (one, a, no)
T3: (one, b, yes)
T4: (one, b, no)
T5: (one, c, yes)
T6: (one, c, no)
T7: (two, a, yes) 

and so on. In this case we’re using Java.

List<List<String>> list = getInput();
List<List<String> tuples = combinations(list);

where getInput() returns my input (L1, L2, L3), and combinations creates my output (T1, T2, T3…)

Answer 1

This should be pretty easy with a recursive function. This is untested:

List<List<String>> listOfTuples(<List<List<String>> list) {
    ArrayList<List<String>> result = new ArrayList<List<String>>();
    List<String> prefix = new ArrayList<String>();
    recurse(0, list, prefix, result);
    return result;
}

void recurse(int index,
             List<List<String>> input,
             List<String> prefix,
             List<List<String>> output)
{
    if (index >= input.size()) {
        output.add(new ArrayList<String>(prefix));
    } else {
        List<String> next = input.get(index++);
        for (String item : next) {
            prefix.add(item);
            recurse(index, input, prefix, output);
            prefix.remove(item);
        }
    }
}