Number of loop recursions as a parameter

Question

I'm building a list of strings that contains all permutations of 2-letter strings, for instance "aa" to "zz". This is what I have:

public List<string> SomeMethod(int NumberOfChars) {

    for (var i = 0; i < 26; i++)
    {
        char character1 = (char)(i + 97);
        var Letter1 = character1.ToString();

        for (var j = 0; j < 26; j++)
        {
            char character2 = (char)(j + 97);
            var Letter2 = character2.ToString();

            string TheString = Letter1 + Letter2;
            TheList.Add(TheString);
        }
    }

    return TheList;
}

Basically, it's a loop inside a loop that combines characters from the alphabet. Now suppose I want to include NumberOfChars as a parameter that determines the length of each string. For instance, if I pass in 2 it would return all 676 2-letter strings from "aa" to "zz" and if I pass in 3 it would return all 17,576 3-letter strings from "aaa" to "zzz".

For the moment, the easiest way to do it would be to have two different methods, one that returns 2-letter strings with two nested loops and another one that returns 3-letter strings with 3 nested loops.

What's a cleaner way to do this?

Thanks.

Answer 1

How about a method like this:

public IEnumerable<string> SomeMethod(int NumberOfChars) 
{
    if (NumberOfChars == 0)
    {
        yield return string.Empty;
    }
    else 
    {
        for (var i = 'a'; i <= 'z'; i++)
        {
            foreach (var s in SomeMethod(NumberOfChars - 1)) 
            {
                yield return i + s;
            }
        }
    }
}

And just for fun, here's another solution using Linq:

public IEnumerable<string> SomeMethod(int n) 
{
    var r = Enumerable.Range('a', 26).Select(x => ((char)x).ToString());
    return (n > 1) ? r.SelectMany(x => SomeMethod(n - 1), (x, y) => x + y) : r;
}

Answer 2

Here's an alternative that uses a loop instead of recursion:

public static List<string> SomeMethod(int numberOfChars)
{
    IEnumerable<string> results = new List<string> { "" };

    for (int i = 0; i < numberOfChars; ++i)
        results = from s in results
                  from c in Enumerable.Range('a', 26)
                  select s + (char)c;

    return results.ToList();
}