Finding cartesian product in Java

问题

I want to find cartesian product of set of elements. Here's an example

example 1 :
sets :(ab) (bc) (ca)

cartesian product is,

abc aba acc aca bbc bba bcc bca

example 2 :
sets : (zyx) b c

cartesian product is,

zbc ybc xbc

So I am thinking of an algorithm to execute in java which can find cartesian product of particular amount of groups defined at compile time at the start.

回答1:

You can use the Sets.cartesianProduct() method from Google's Guava libraries to generate Cartesian products:

com.google.common.collect.Sets.cartesianProduct(Set[] yourSets)

If only everything was that easy!

回答2:

Define your own Iterator/Iterable:

import java.util.*;

class CartesianIterator <T> implements Iterator <List <T>> {

    private final List <List <T>> lilio;    
    private int current = 0;
    private final long last;

    public CartesianIterator (final List <List <T>> llo) {
        lilio = llo;
        long product = 1L;
        for (List <T> lio: lilio)
            product *= lio.size ();
        last = product;
    } 

    public boolean hasNext () {
        return current != last;
    }

    public List <T> next () {
        ++current;
        return get (current - 1, lilio);
    }

    public void remove () {
        ++current;
    }

    private List<T> get (final int n, final List <List <T>> lili) {
        switch (lili.size ())
        {
            case 0: return new ArrayList <T> (); // no break past return;
            default: {
                List <T> inner = lili.get (0);
                List <T> lo = new ArrayList <T> ();
                lo.add (inner.get (n % inner.size ()));
                lo.addAll (get (n / inner.size (), lili.subList (1, lili.size ())));
                return lo;
            }
        }
    }
}

class CartesianIterable <T> implements Iterable <List <T>> {

    private List <List <T>> lilio;  

    public CartesianIterable (List <List <T>> llo) {
        lilio = llo;
    }

    public Iterator <List <T>> iterator () {
        return new CartesianIterator <T> (lilio);
    }
}

And test it with your Data:

class CartesianIteratorTest {

    public static void main (String[] args) {
        List <Character> la = Arrays.asList (new Character [] {'a', 'b'});
        List <Character> lb = Arrays.asList (new Character [] {'b', 'c'});      
        List <Character> lc = Arrays.asList (new Character [] {'c', 'a'});
        List <List <Character>> llc = new ArrayList <List <Character>> ();
        llc.add (la);
        llc.add (lb);
        llc.add (lc);

        CartesianIterable <Character> ci = new CartesianIterable <Character> (llc);
        for (List<Character> lo: ci)
            show (lo);

        la = Arrays.asList (new Character [] {'x', 'y', 'z'});
        lb = Arrays.asList (new Character [] {'b'});    
        lc = Arrays.asList (new Character [] {'c'});
        llc = new ArrayList <List <Character>> ();
        llc.add (la);
        llc.add (lb);
        llc.add (lc);

        ci = new CartesianIterable <Character> (llc);
        for (List<Character> lo: ci)
            show (lo);    
    }

    public static void show (List <Character> lo) {
        System.out.print ("(");
        for (Object o: lo)
            System.out.print (o);
        System.out.println (")");
    }
}

Result:

(abc)
(bbc)
(acc)
(bcc)
(aba)
(bba)
(aca)
(bca)
(xbc)
(ybc)
(zbc)

回答3:

A purely functional approach to this can be found in this paper (a "functional pearl")... It might not be easily translatable to Java, though.

来源：https://stackoverflow.com/questions/6563589/finding-cartesian-product-in-java

标签

java

math

cartesian-product