How do I do an integer list intersection while keeping duplicates?

Question

I\'m working on a Greatest Common Factor and Least Common Multiple assignment and I have to list the common factors. Intersection() won\'t work because that removes duplicat

Answer 1

You could use this generic extension I wrote for another answer, it is essentially a single Linq statement. Note that it uses Zip to avoid the needless full enumeration of matched groups.

public static IEnumerable Commom(
        this IEnumerable source,
        IEnumerable sequence,
        IEqualityComparer comparer = null)
{
    if (sequence == null)
    {
        return Enumerable.Empty();
    }

    if (comparer == null)
    {
        comparer = EqualityComparer.Default;
    }

    return source.GroupBy(t => t, comparer)
        .Join(
            sequence.GroupBy(t => t, comparer),
            g => g.Key,
            g => g.Key,
            (lg, rg) => lg.Zip(rg, (l, r) => l),
            comparer)
        .SelectMany(g => g);
}

this enables,

new[] {1, 2, 2, 2, 3, 3, 4, 5}.Common(
    new[] {1, 1, 2, 2, 3, 3, 3, 4, 4}).ToArray()

maintaining the order of the source sequence, as desired.