Count matching characters between two strings using LINQ

Question

A friend asked me how to improve some code with LINQ. How would you do a character by character comparison between two strings to count the number of matches at an index? Here's the original code, could it be improved with LINQ?

private int Fitness(string individual, string target)
   {
       int sum = 0;
       for (int i = 0; i < individual.Length; i++)
           if (individual[i] == target[i]) sum++;
       return sum;
   }

Answer 1

return Enumerable.Range(0, individual.Length)
                 .Count(i => individual[i] == target[i]);

A more fool-proof way would be (the above snippet will fail if target is shorter than individual):

return Enumerable.Range(0, Math.Min(individual.Length, target.Length))
                 .Count(i => individual[i] == target[i]);

---

I believe the code is correct as is. Enumerable.Range method takes two arguments. The first of which is the start index (should be 0), the second is the count of items. The complete code snippet to test and make sure:

class Program {
  static void Main(string[] args) {
      Console.WriteLine(Fitness("hello", "world"));
  }
  static int Fitness(string individual, string target) {
      return Enumerable.Range(0, Math.Min(individual.Length, target.Length))
                       .Count(i => individual[i] == target[i]);
  }
}

Answer 2

You could write something similar with LINQ, but since "Zip" isn't built in until .NET 4.0 it will be more code than we'd like, and/or not as efficient. I'd be tempted to leave it "as is", but I'd probably check target.Length to avoid an out-of-range exception.

Perhaps I'd make an extension method, though:

public static int CompareFitness(this string individual, string target)
{
   int sum = 0, len = individual.Length < target.Length
        ? individual.Length : target.Length;
   for (int i = 0; i < len; i++)
       if (individual[i] == target[i]) sum++;
   return sum;
}

Then you can use:

string s = "abcd";
int i = s.CompareFitness("adc"); // 2

Answer 3

How about a join, or did I misunderstand?

    static void Main(string[] args)
    {
        var s1 = "abcde";
        var s2 = "hycdh";
        var count = s1.Join(s2, c => c, c => c, (a,b) => a).Count();
        Console.WriteLine(count);
        Console.ReadKey();
    }