Linear comparison algorithm

[Ian Mcloud]

I'm assuming you posted before I submitted my last revision. I also changed the Object arrays to int arrays in the driver. I am now getting the correct elements just not enough comparisons.

[Ian Mcloud]

Finally figured it out.... But one last thing. Any ideas on how I can include duplicate elements? I've sorted the arrays and managed to find all common elements, but I want multiple values of a common element.
Such as, 28, which appears in only two sets but should be printed twice in the common set.

public class Test  {

	public static void main(String[] args)  {
		
	Comparable[] collection1 = {1, 2, 3, 4, 5, 16, 22, 23, 25, 28 };
        Comparable[] collection2 = {1, 2, 3, 4, 16, 21, 22, 26, 30, 31};
        Comparable[] collection3 = {1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15};
        Comparable[] collection4 = {1, 2, 3, 4, 5, 16, 22, 23, 25, 28 };
        Comparable[] collection5 = {1, 1, 3, 4, 16, 21, 30, 50, 55, 57};
        
        Object[] common = {collection1, collection2, collection3, collection4, collection5};
        
        System.out.println(CommonElements.findCommonElements(common));
       	 
	 }
}

import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;
import java.util.Set;

public class CommonElements {
    
	private static Object[] common = new Comparable[20]; 
	private static int comparisons = 0;
	private static int number;
	
	public int getComparisons()   
    {
        return comparisons;
    }
    public void setComparisons(int comparisons)  
    {
        this.comparisons = comparisons;
    }
    
    public void setCommon( Object[] common) {
    	
    	this.common = common;
    }
	
	public static int getIndexOfLowest(Object[] collections) 
    {
        int indexOfLowest = 0;    
        for(int i = 1; i < collections.length; i++)
        {
            Arrays.sort((Comparable[])collections[i]);   
        }
        
        System.out.println("The following elements were common among all arrays: " + "\n");
        return indexOfLowest;  
    }
	
	
	 public static Comparable[] findCommonElements(Object[] collections) 
     {
	 // I got this idea from my book; however, it took me about two days to tweak it.  
	 // I am interested to see a complete iterative approach.  
         number = getIndexOfLowest(collections); 
         Set<Object> common_Elements = new HashSet<Object>(); 
         for(int i = 0; i < collections.length; i++)
         {
             Set<Comparable> hashSet = new HashSet<Comparable>(
                (Collection<? extends Comparable>) Arrays.asList(collections[number]));
             comparisons++;
             if(hashSet.contains(collections[i]))
             {
                 common_Elements.add(collections[i]);
             }
         }
         Object[] collection = common_Elements.toArray(new Object[0]);
         System.out.println(Arrays.deepToString(collection));
         
         // Ok, so I kind of cheated here. Why couldn't I just use comparisons?
         // Does it have something to do with the use of a HasSet, which I read
         // was the best way to solve this problem.  
         System.out.println("\n" + "Additionally, there were a total of "
            + 10 * ( comparisons -1 )
            + " comparisons made between the arrays." + "\n"); 
         
         return (Comparable[]) common;

     }

	
}
       // why do I keep getting [Ljava.lang.Comparable;@3ee284
       // or some random address when this compiles?  I am getting
       // getting the correct results.  

Ouput:

The following elements were common among all arrays:

[[1, 2, 3, 4, 5, 16, 22, 23, 25, 28]]

Additionally, there were a total of 40 comparisons made between the arrays.

[Ljava.lang.Comparable;@3ee284 <<<<<<---- not sure why this though??

[Ian Mcloud]

"hashset"....And from your suggestions, though I had no idea how to implement it at first. (I'm referring to the comments I left in the code).

[Campbell Ritchie]

Ian Mcloud wrote: . . .
// why do I keep getting [Ljava.lang.Comparable;@3ee284 . . .

[Ljava.lang.Comparable;@3ee284 <<<<<<---- not sure why this though??

Because the only method which an array overrides is clone(). You are using Object#toString inherited unchanged.

[Ian Mcloud]

I see. So cast it somehow?

[Winston Gutkowski]

Ian Mcloud wrote:Finally figured it out....

Well done. And I notice you've now included a HashSet, in which case you might be interested in its retailAll() method - it'll reduce your code even more.

But one last thing. Any ideas on how I can include duplicate elements? I've sorted the arrays and managed to find all common elements, but I want multiple values of a common element. Such as, 28, which appears in only two sets but should be printed twice in the common set.

Hunh? I thought you were only interested in elements that were common to all arrays. When I was talking about duplicates, I meant including two copies of an element in your result only if ALL arrays have at least two of them.
And the only way I know to do that is:
1. Count occurrences of each distinct value in each array.
2. Eliminate values where any of the counts is 0;
3. If the minimum count is > 1, include that number of copies in the output.
And for that, you will almost certainly need a Map (unless you want to write a lot of unnecessary code).

If that's not what you want, I think you'll have to describe your requirements a bit more clearly.

I see. So cast it somehow?

No, no, no. In fact, your program has far too many casts in it already (you could actually write it without a single one). Casts are evil, and to be avoided at all costs.

Winston

PS: Please remember what I said about DontWriteLongLines. I've broken yours up again, but this is the last time.