The opposite of Intersect()

Intersect can be used to find matches between two collections, like so:

// Assign two arrays.
int[] array1 = { 1, 2, 3 };
int[] array2 = { 2, 3, 4 };
// Call Intersect extension method.
var intersect = array1.Intersect(array2);
// Write intersection to screen.
foreach (int value in intersect)
{
    Console.WriteLine(value); // Output: 2, 3
}

However what I'd like to achieve is the opposite, I'd like to list items from one collection that are missing from the other:

// Assign two arrays.
int[] array1 = { 1, 2, 3 };
int[] array2 = { 2, 3, 4 };
// Call "NonIntersect" extension method.
var intersect = array1.NonIntersect(array2); // I've made up the NonIntersect method
// Write intersection to screen.
foreach (int value in intersect)
{
    Console.WriteLine(value); // Output: 4
}

As stated, if you want to get 4 as the result, you can do like this:

var nonintersect = array2.Except(array1);

If you want the real non-intersection (also both 1 and 4), then this should do the trick:

var nonintersect = array1.Except(array2).Union( array2.Except(array1));

This will not be the most performant solution, but for small lists it should work just fine.

You can use

a.Except(b).Union(b.Except(a));

Or you can use

var difference = new HashSet(a);
difference.SymmetricExceptWith(b);

This code enumerates each sequence only once and uses Select(x => x) to hide the result to get a clean Linq-style extension method. Since it uses HashSet<T> its runtime is O(n + m) if the hashes are well distributed. Duplicate elements in either list are omitted.

public static IEnumerable<T> SymmetricExcept<T>(this IEnumerable<T> seq1,
    IEnumerable<T> seq2)
{
    HashSet<T> hashSet = new HashSet<T>(seq1);
    hashSet.SymmetricExceptWith(seq2);
    return hashSet.Select(x => x);
}

I think you might be looking for Except:

> The Except operator produces the set > difference between two sequences. It > will only return elements in the first > sequence that don't appear in the > second. You can optionally provide > your own equality comparison function.

Check out this link, this link, or Google, for more information.

array1.NonIntersect(array2);

Nonintersect such operator is not present in Linq you should do

except -> union -> except

a.except(b).union(b.Except(a));

I'm not 100% sure what your NonIntersect method is supposed to do (regarding set theory) - is it
B \ A (everything from B that does not occur in A)?
If yes, then you should be able to use the Except operation (B.Except(A)).

/// <summary>
/// Given two list, compare and extract differences
/// http://stackoverflow.com/questions/5620266/the-opposite-of-intersect
/// </summary>
public class CompareList
{
    /// <summary>
    /// Returns list of items that are in initial but not in final list.
    /// </summary>
    /// <param name="listA"></param>
    /// <param name="listB"></param>
    /// <returns></returns>
    public static IEnumerable<string> NonIntersect(
        List<string> initial, List<string> final)
    {
        //subtracts the content of initial from final
        //assumes that final.length < initial.length
        return initial.Except(final);
    }

    /// <summary>
    /// Returns the symmetric difference between the two list.
    /// http://en.wikipedia.org/wiki/Symmetric_difference
    /// </summary>
    /// <param name="initial"></param>
    /// <param name="final"></param>
    /// <returns></returns>
    public static IEnumerable<string> SymmetricDifference(
        List<string> initial, List<string> final)
    {
        IEnumerable<string> setA = NonIntersect(final, initial);
        IEnumerable<string> setB = NonIntersect(initial, final);
        // sum and return the two set.
        return setA.Concat(setB);
    }
}

string left = "411329_SOFT_MAC_GREEN";
string right= "SOFT_MAC_GREEN";

string[] l = left.Split('_');
string[] r = right.Split('_');

string[] distinctLeft = l.Distinct().ToArray();
string[] distinctRight = r.Distinct().ToArray();

var commonWord = l.Except(r, StringComparer.OrdinalIgnoreCase)
string result = String.Join("_",commonWord);
result = "411329"