Iterative Cartesian Product in Java

Question

I want to compute the cartesian product of an arbitrary number of nonempty sets in Java.

I\'ve wrote that iterative code...

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Answer 1

I believe this is correct. It is not seeking efficiency, but a clean style through recursion and abstraction.

The key abstraction is to introduce a simple Tuple class. This helps the generics later:

class Tuple {
    private List list = new ArrayList();

    public void add(T t) { list.add(t); }

    public void addAll(Tuple subT) {
        for (T t : subT.list) {
            list.add(t);
        }
    }

    public String toString() {
        String result = "(";

        for (T t : list) { result += t + ", "; }

        result = result.substring(0, result.length() - 2);
        result += " )";

        return result;
    } 
}

With this class, we can write a class like so:

public class Example {

public static  List> cartesianProduct(List> sets) {
    List> tuples = new ArrayList>();

    if (sets.size() == 1) {
        Set set = sets.get(0);
        for (T t : set) {
            Tuple tuple = new Tuple();
            tuple.add(t);    
            tuples.add(tuple);
        }
    } else {
        Set set = sets.remove(0);
        List> subTuples = cartesianProduct(sets);
        System.out.println("TRACER size = " + tuples.size());
        for (Tuple subTuple : subTuples) {
            for (T t : set) {
                Tuple tuple = new Tuple();
                tuple.addAll(subTuple);
                tuple.add(t);
                tuples.add(tuple);
            }
        }
    }

    return tuples;
}

}

I have a decent example of this working, but it is omitted for brevity.