How can I calculate the difference between two ArrayLists?

Question

Hi,

I have two ArrayLists .(IN JAVA Programming Language)

ArrayList A contains ['2009-05-18','2009-05-19','2009-05-21']

ArrayList B Contains ['2009-05-18','2009-05-18','2009-05-19','2009-05-19','2009-05-20','2009-05-21','2009-05-21','2009-05-22']

I have to compare ArrayLst A and ArrayLst B . The result ArrayList should contain the List which does not exist in ArrayList A. ArrayList result should be ['2009-05-20','2009-05-22']

how to compare ?

Please help me,

Thanks in advance.

Answer 1

EDIT: Original question did not specify language. My answer is in C#.

You should instead use HashSet for this purpose. If you must use ArrayList, you could use the following extension methods:

var a = arrayListA.Cast<DateTime>();
var b = arrayListB.Cast<DateTime>();    
var c = b.Except(a);

var arrayListC = new ArrayList(c.ToArray());

using HashSet...

var a = new HashSet<DateTime>(); // ...and fill it
var b = new HashSet<DateTime>(); // ...and fill it
b.ExceptWith(a); // removes from b items that are in a

Answer 2

I guess you're talking about C#. If so, you can try this

    ArrayList CompareArrayList(ArrayList a, ArrayList b)
    {
        ArrayList output = new ArrayList();
        for (int i = 0; i < a.Count; i++)
        {
            string str = (string)a[i];
            if (!b.Contains(str))
            {
                if(!output.Contains(str)) // check for dupes
                    output.Add(str);
            }
        }
        return output;
    }

Answer 3

In Java, you can use the Collection interface's removeAll method.

// Create a couple ArrayList objects and populate them
// with some delicious fruits.
Collection firstList = new ArrayList() {{
    add("apple");
    add("orange");
}};

Collection secondList = new ArrayList() {{
    add("apple");
    add("orange");
    add("banana");
    add("strawberry");
}};

// Show the "before" lists
System.out.println("First List: " + firstList);
System.out.println("Second List: " + secondList);

// Remove all elements in firstList from secondList
secondList.removeAll(firstList);

// Show the "after" list
System.out.println("Result: " + secondList);

The above code will produce the following output:

First List: [apple, orange]
Second List: [apple, orange, banana, strawberry]
Result: [banana, strawberry]

Answer 4

You are just comparing strings.

Put the values in ArrayList A as keys in HashTable A.
Put the values in ArrayList B as keys in HashTable B.

Then, for each key in HashTable A, remove it from HashTable B if it exists.

What you are left with in HashTable B are the strings (keys) that were not values in ArrayList A.

C# (3.0) example added in response to request for code:

List<string> listA = new List<string>{"2009-05-18","2009-05-19","2009-05-21'"};
List<string> listB = new List<string>{"2009-05-18","2009-05-18","2009-05-19","2009-05-19","2009-05-20","2009-05-21","2009-05-21","2009-05-22"};

HashSet<string> hashA = new HashSet<string>();
HashSet<string> hashB = new HashSet<string>();

foreach (string dateStrA in listA) hashA.Add(dateStrA);
foreach (string dateStrB in listB) hashB.Add(dateStrB);

foreach (string dateStrA in hashA)
{
    if (hashB.Contains(dateStrA)) hashB.Remove(dateStrA);
}

List<string> result = hashB.ToList<string>();

Answer 5

You already have the right answer. And if you want to make more complicated and interesting operations between Lists (collections) use apache commons collections (CollectionUtils) It allows you to make conjuction/disjunction, find intersection, check if one collection is a subset of another and other nice things.