发表新帖
发表新帖

Cartesian product of arbitrary sets in Java

后端 未结
关注
9 2224
小鲜肉
小鲜肉 2020-11-22 07:28

Do you know some neat Java libaries that allow you to make cartesian product of two (or more) sets?

For example: I have three sets. One with objects of class Person

9条回答
  • 情歌与酒
    情歌与酒 (楼主)
    2020-11-22 07:37

    Here is an Iterable, which allows you to use a simplified for-loop: 

    import java.util.*;

// let's begin with the demo. Instead of Person and Gift, 
// I use the well known char and int. 
class CartesianIteratorTest {

    public static void main (String[] args) {
        List  lc = Arrays.asList (new Object [] {'A', 'B', 'C', 'D'});
        List  lC = Arrays.asList (new Object [] {'a', 'b', 'c'});   
        List  li = Arrays.asList (new Object [] {1, 2, 3, 4});
            // sometimes, a generic solution like List >
            // might be possible to use - typically, a mixture of types is 
            // the common nominator 
        List > llo = new ArrayList > ();
        llo.add (lc);
        llo.add (lC);
        llo.add (li);

        // Preparing the List of Lists is some work, but then ...    
        CartesianIterable  ci = new CartesianIterable  (llo);

        for (List  lo: ci)
            show (lo);
    }

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



    How is it done? We need an Iterable, to use the simplified for-loop, and an Iterator has to be returned from the Iterable. 
We return a List of Objects - this could be a Set instead of List, but Set has no indexed access, so it would be a bit more complicated, to implement it with Set instead of List. Instead of a generic solution, Object would have been fine for many purposes, but generics allow for more restrictions. 
    


    class CartesianIterator  implements Iterator > {

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

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

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

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

    public void remove () {
        ++current;
    }

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

    


    The mathematical work is done in the 'get'-method. Think about 2 sets of 10 elements. You have a total of 100 combinations, enumerated from 00, 01, 02, ... 10, ... to 99. For 5 X 10 elements 50, for 2 X 3 elements 6 combinations. The modulo of the sublist size helps to pick one element for each iteration. 
    


    Iterable i the least interesting thing here: 
    


    class CartesianIterable  implements Iterable > {

    private List > lilio;  

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

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

    


    To implement Iterable, which allows the for-each kind of loop, we have to implement iterator (), and for Iterator we have to implement hasNext (), next () and remove (). 
    


    Result: 
    


    (A, a, 1, )
(B, a, 1, )
(C, a, 1, )
(D, a, 1, )
(A, b, 1, )
(B, b, 1, )
(C, b, 1, )
(D, b, 1, )
...
(A, a, 2, )
...
(C, c, 4, )
(D, c, 4, )

    
    
    
             
    
                                                        
    
            
            
    
              
                
                0
              
                   
                
               讨论(0)
              
                                                  
              
              
    
                          
             
       
          
              
    
                                       
    

    

                
    
                         
          
                              			
        
        
        
    
          
    
            
            
    
              
              
    
            
    


                                 
              
    
            
    
                          
    

        
         
    
                
                
    
                  
                
    
                
    
                   看不清?
                
    
              
    
                                  
    
                    
   
                 
             
              
    提交回复
    
            
    
          
    
        
    
      
       
      
    
    
    
          
 
     
 
    
        
    热议问题