Find number of characters mutual between two strings in C#

Question

I am looking for a method that will take two strings and return the number of characters that are common to both e.g.:

"G010" & "G1820A" should return 3 as the G, 0 and 1 chars exist in both.

If a char exists twice in both they should be counted separately as follows:

"G12AA" & "GAA2" should return 4 as the G, A, A and 2 characters exist in both.

Any help with this? Google searches haven't been too helpful thus far.

Answer 1

Okay, how about this, it has the advantage of maximising lazy evaluation and minimising string manipulation.

public int CommonChars(string left, string right)
{
    return left.GroupBy(c => c)
        .Join(
            right.GroupBy(c => c),
            g => g.Key,
            g => g.Key,
            (lg, rg) => lg.Zip(rg, (l, r) => l).Count())
        .Sum(); 
}

essentialy, it groups each side by char, then finds chars which have a group on both sides. The matched groups are counted in tandem, until either runs out. These counts are summed to produce the result.

---

It would be trivial to perform this generically for any two sequences. See below,

public static int CommomCount<T>(
        this IEnumerable<T> source,
        IEnumerable<T> sequence,
        IEqualityComparer<T> comparer = null)
{
    if (sequence == null)
    {
        return 0;
    }

    if (comparer == null)
    {
        comparer = EqualityComparer<T>.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).Count(),
            comparer)
        .Sum();
}

Which you would use like this.

"G12AA".CommonCount("GAA2")

The optional comparer parameter may prove useful if you require case insensitivity or other special treatment.

---

In the interest of resuability, I'd be tempted to remove the Sum() and return an IEnumerable<T>, and then add sum to the call, like this,

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

    if (comparer == null)
    {
        comparer = EqualityComparer<T>.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);
}

so you could easily do

Console.WriteLine(new string("G12AA".Common("GAA2").ToArray()));

or just the orgininal

"G12AA".Common("GAA2").Count();