Java : Cartesian Product of a List of Lists

Question

I have a problem that is really kind of a general programming question, but my implementation is in Java, so I will provide my examples that way

I have a class like

Answer 1

Recursive solution:

public <T> List<List<T>> cartesianProduct(int i, List<T>... a) {
    if(i == a.length ) {
        List<List<T>> result = new ArrayList<>();
        result.add(new ArrayList());
        return result;
    }
    List<List<T>> next = cartesianProduct(i+1, a);
    List<List<T>> result = new ArrayList<>();
    for(int j=0; j < a[i].size(); j++) {
        for(int k=0; k < next.size(); k++) {
            List<T> concat = new ArrayList();
            concat.add(a[i].get(j));
            concat.addAll(next.get(k));
            result.add(concat);
        }
    }
    return result;
}

Answer 2

Try something like this:

public static void generate(int[][] sets) {
    int solutions = 1;
    for(int i = 0; i < sets.length; solutions *= sets[i].length, i++);
    for(int i = 0; i < solutions; i++) {
        int j = 1;
        for(int[] set : sets) {
            System.out.print(set[(i/j)%set.length] + " ");
            j *= set.length;
        }
        System.out.println();
    }
}

public static void main(String[] args) {
    generate(new int[][]{{1,2,3}, {3,2}, {5,6,7}});
}

which will print:

1 3 5
2 3 5
3 3 5
1 2 5
2 2 5
3 2 5
1 3 6
2 3 6
3 3 6
1 2 6
2 2 6
3 2 6
1 3 7
2 3 7
3 3 7
1 2 7
2 2 7
3 2 7

I've implemented the algorithm above based on (I believe) one of Knuth's TAOCP books (in the comments @chikitin has a more specific reference: it is in

Answer 3

I'm late to the party but I followed Shiomi's link and translated the functions into Java. The result is an easy to follow and understand algorithm (I may be a little slow since I had a hard time understanding Bart Kiers' solution).

Here it is (the key is an int, replacing to String should be straightforward):

Usage

    public void testProduct(){
        Map<Integer, List<String>> data =   new LinkedHashMap<Integer, List<String>>(){{                
            put(0, new ArrayList<String>(){{
                add("John"); add("Sarah");                      
            }});                
            put(1, new ArrayList<String>(){{
                add("Red"); add("Green"); add("Blue"); add("Orange");
            }});
            put(2, new ArrayList<String>(){{
                add("Apple"); add("Tomatoe"); add("Bananna");                   
            }});
    }};

        List<String[]> product =  GetCrossProduct(data);
        for(String[] o : product)
            System.out.println(Arrays.toString(o));

    }

Result

[John, Red, Apple]
[John, Red, Tomatoe]
[John, Red, Bananna]
[John, Green, Apple]
[John, Green, Tomatoe]
[John, Green, Bananna]
[John, Blue, Apple]
[John, Blue, Tomatoe]
[John, Blue, Bananna]
[John, Orange, Apple]
[John, Orange, Tomatoe]
[John, Orange, Bananna]
[Sarah, Red, Apple]
[Sarah, Red, Tomatoe]
[Sarah, Red, Bananna]
[Sarah, Green, Apple]
[Sarah, Green, Tomatoe]
[Sarah, Green, Bananna]
[Sarah, Blue, Apple]
[Sarah, Blue, Tomatoe]
[Sarah, Blue, Bananna]
[Sarah, Orange, Apple]
[Sarah, Orange, Tomatoe]
[Sarah, Orange, Bananna]

Cartesian Product Functions

    public static List<String[]> GetCrossProduct(Map<Integer, List<String>> lists)
    {
        List<String[]> results = new ArrayList<String[]>();
        GetCrossProduct(results, lists, 0, new String[(lists.size())]);
        return results;
    }

    private void GetCrossProduct(List<String[]> results, Map<Integer, List<String>> lists, int depth, String[] current)
    {
        for (int i = 0; i < lists.get(depth).size(); i++)
        {
            current[depth] = lists.get(depth).get(i);            
            if (depth < lists.keySet().size() - 1)
                GetCrossProduct(results, lists, depth + 1, current);
            else{
                results.add(Arrays.copyOf(current,current.length));                
            }
        }
    }       

Answer 4

How about generating the product lazily, ie. only create the tuple when you're accessing it?

/**
* A random access view of tuples of a cartesian product of ArrayLists
*
* Orders tuples in the natural order of the cartesian product
*
* @param T the type for both the values and the stored tuples, ie. values of the cartesian factors are singletons
* While the type of input sets is List<T> with elements being treated as singletons
*
*/

abstract public class CartesianProductView<T> extends AbstractList<T> {

private final List<List<T>> factors;
private final int size;

/**
 * @param factors the length of the factors (ie. the elements of the factors argument) should not change,
 *  otherwise get may not return all tuples, or throw exceptions when trying to access the factors outside of range
 */
public CartesianProductView(List<List<T>> factors) {
    this.factors = new ArrayList<>(factors);
    Collections.reverse(this.factors);
    int acc = 1;
    for (Iterator<List<T>> iter = this.factors.iterator(); iter.hasNext(); ) {
        acc *= iter.next().size();
    }
    this.size = acc;
}

@Override
public T get(int index) {
    if (index < 0 || index >= size()) {
        throw new IndexOutOfBoundsException(String.format("index %d > size() %d", index, size()));
    }

    T acc = null;
    for (Iterator<List<T>> iter = factors.iterator(); iter.hasNext();) {
        List<T> set = iter.next();
        acc = makeTupleOrSingleton(set.get(index % set.size()), acc);
        index /= set.size();
    }
    return acc;
}

@Override
public int size() {
    return size;
}

private T makeTupleOrSingleton(T left, T right) {
    if (right == null) {
        return left;
    }
    return makeTuple(left, right);
}

/**
 *
 * @param left      a singleton of a value
 * @param right     a tuple of values taken from the cartesian product factors, with null representing the empty set
 * @return          the sum of left and right, with the value of left being put in front
 */
abstract protected T makeTuple(T left, T right);
}

and use it like this

final List<List<String>> l1 = new ArrayList<List<String>>() {{ add(singletonList("a")); add(singletonList("b")); add(singletonList("c")); }};
final List<List<String>> l2 = new ArrayList<List<String>>() {{ add(singletonList("X")); add(singletonList("Y")); }};
final List<List<String>> l3 = new ArrayList<List<String>>() {{ add(singletonList("1")); add(singletonList("2")); add(singletonList("3")); add(singletonList("4")); }};


List<List<List<String>>> in = new ArrayList<List<List<String>>>() {{ add(l1); add(l2); add(l3); }};

List<List<String>> a = new CartesianProductView<List<String>>(in) {

    @Override
    protected List<String> makeTuple(final List<String> left, final List<String> right) {
        return new ArrayList<String>() {{ add(left.get(0)); addAll(right); }};
    }

};

System.out.println(a);

The result:

[[a, X, 1], [a, X, 2], [a, X, 3], [a, X, 4], [a, Y, 1], [a, Y, 2], [a, Y, 3], [a, Y, 4], [b, X, 1], [b, X, 2], [b, X, 3], [b, X, 4], [b, Y, 1], [b, Y, 2], [b, Y, 3], [b, Y, 4], [c, X, 1], [c, X, 2], [c, X, 3], [c, X, 4], [c, Y, 1], [c, Y, 2], [c, Y, 3], [c, Y, 4]]

As an added bonus, you can use it join strings all with all:

final List<String> l1 = new ArrayList<String>() {{ add("a"); add("b"); add("c"); }};
final List<String> l2 = new ArrayList<String>() {{ add("X"); add("Y"); }};
final List<String> l3 = new ArrayList<String>() {{ add("1"); add("2"); add("3"); add("4"); }};


List<List<String>> in = new ArrayList<List<String>>() {{ add(l1); add(l2); add(l3); }};

List<String> a = new CartesianProductView<String>(in) {

    @Override
    protected String makeTuple(String left, String right) {
        return String.format("%s%s", left, right);
    }

};

System.out.println(a);

The result:

[aX1, aX2, aX3, aX4, aY1, aY2, aY3, aY4, bX1, bX2, bX3, bX4, bY1, bY2, bY3, bY4, cX1, cX2, cX3, cX4, cY1, cY2, cY3, cY4]

Answer 5

Here is a link, its c#, but i am sure you could work with that!

Answer 6

Thanks to Vitalii Fedorenko, I could achive the same for list by using

Lists.cartesianProduct(..)

from https://guava.dev/releases/23.5-jre/api/docs/com/google/common/collect/Lists.html#cartesianProduct-java.util.List...-

I think, if it is required for a production code then it is better to rely on tried and tested library like Guava rather than building our own.