How to get Cartesian product from multiple lists?

Question

Say I have several List<T>s, I will put them into another list or other collections, so I don't know how many list<T> I have until I call List<List<T>>.size()

Take below List<Integer> as an example:

list1=[1,2]
list2=[3,4]
list3=[5,6]
....
listn=[2*n-1,2n];

How can I get the result of list1*list2*list3*...listn as a Cartesian product?

For example:

list1*list2*list3

should be:

[1,3,5],[1,3,6],[1,4,5],[1,4,6],[2,3,5],[2,3,6],[2,4,5],[2,4,6]

Answer 1

You can use recursion to achieve it, your base case of recursion is when input is empty then return empty list, else process the remaining elements. E.g.

import java.util.List;
import java.util.ArrayList;
import java.util.Arrays;

public class CartesianProduct {
    public static <T> List<List<T>> calculate(List<List<T>> input) {
        List<List<T>> res = new ArrayList<>();
        if (input.isEmpty()) { // if no more elements to process
            res.add(new ArrayList<>()); // then add empty list and return
            return res;
        } else {
            // we need to calculate the cartesian product
            // of input and store it in res variable
            process(input, res);
        }
        return res; // method completes , return result
    }

    private static <T> void process(List<List<T>> lists, List<List<T>> res) {
        //take first element of the list
        List<T> head = lists.get(0);
        //invoke calculate on remaining element, here is recursion
        List<List<T>> tail = calculate(lists.subList(1, lists.size()));

        for (T h : head) { // for each head
            for (List<T> t : tail) { //iterate over the tail
                List<T> tmp = new ArrayList<>(t.size());
                tmp.add(h); // add the head
                tmp.addAll(t); // and current tail element
                res.add(tmp);
            }
        }
    }

    public static void main(String[] args) {
        //we invoke the calculate method
        System.out.println(calculate(Arrays.asList(
                Arrays.asList(1, 2),
                Arrays.asList(3, 4),
                Arrays.asList(5, 6))));
    }
}

Output

[[1,3,5],[1,3,6],[1,4,5],[1,4,6],[2,3,5],[2,3,6],[2,4,5],[2,4,6]]

Answer 2

The map-and-reduce approach using nested loops

1. Prepare a list of lists List<List<T>> populated with a single empty value. This list is used further as a storage of intermediate results and as a final result.

2. Sequentially append the data from incoming lists List<List<T>> to the intermediate result and obtain the final result. Schematically, this iterative process looks as follows:

   result0: [[]]
     list1: [1,2]
   -------
   result1: [[1],[2]]
     list2: [3,4]
   -------
   result2: [[1,3],[1,4],[2,3],[2,4]]
     list3: [5,6]
   -------
   result3: [[1,3,5],[1,3,6],[1,4,5],[1,4,6],[2,3,5],[2,3,6],[2,4,5],[2,4,6]]
   

Try it online!

/**
 * @param lists an arbitrary number of lists
 * @param <T>   the type of the elements
 * @return the Cartesian product
 */
public static <T> List<List<T>> cartesianProduct(List<List<T>> lists) {
    // check if incoming data is not null
    if (lists == null) return Collections.emptyList();
    // Cartesian product, intermediate result
    List<List<T>> cp = Collections.singletonList(Collections.emptyList());
    // iterate through incoming lists
    for (List<T> list : lists) {
        // non-null and non-empty lists
        if (list == null || list.size() == 0) continue;
        // intermediate result for next iteration
        List<List<T>> next = new ArrayList<>();
        // rows of current intermediate result
        for (List<T> row : cp) {
            // elements of current list
            for (T el : list) {
                // new row for next intermediate result
                List<T> nRow = new ArrayList<>(row);
                nRow.add(el);
                next.add(nRow);
            }
        }
        // pass to next iteration
        cp = next;
    }
    // Cartesian product, final result
    return cp;
}

public static void main(String[] args) {
    List<List<Integer>> lists = prepareLists(3);
    List<List<Integer>> cp = cartesianProduct(lists);
    // output without spaces
    System.out.println(lists.toString().replace(" ", ""));
    System.out.println(cp.toString().replace(" ", ""));
}

// supplementary method, prepares lists for multiplication
public static List<List<Integer>> prepareLists(int n) {
    List<List<Integer>> lists = new ArrayList<>(n);
    for (int i = 1; i <= n; i++)
        lists.add(Arrays.asList(i * 2 - 1, i * 2));
    return lists;
}

Output:

[[1,2],[3,4],[5,6]]
[[1,3,5],[1,3,6],[1,4,5],[1,4,6],[2,3,5],[2,3,6],[2,4,5],[2,4,6]]

---

^{See also: Generate all combinations from multiple lists}

Answer 3

Thanks to @sol4me 's answer using tail recursion, here is another version which is not using tail recursion but I think is easier to understand.

public class CartesianProduct {
    public static <T> List<List<T>> calculate(List<List<T>> input) {
        List<List<T>> result = new ArrayList<List<T>>();
        if (input.isEmpty()) { // If input an empty list
            // add empty list and return
            result.add(new ArrayList<T>());
            return result;
        } else {
            // get the first list as a head
            List<T> head = input.get(0);
            // recursion to calculate a tail list
            List<List<T>> tail = calculate(input.subList(1, input.size()));
            // we merge every head element with every tail list.
            for (T h : head) {
                for (List<T> t : tail) {
                    List<T> resultElement = new ArrayList<T>();
                    resultElement.add(h);
                    resultElement.addAll(t);
                    result.add(resultElement);
                }
            }
        }
        return result;
    }
    public static void main(String[] args) {
        List<List<Integer>> bigList = Arrays.asList(
                Arrays.asList(1, 2),
                Arrays.asList(3, 4),
                Arrays.asList(5, 6),
                Arrays.asList(7, 8));
        System.out.println(calculate(bigList));
    }
}