C# Exam April 2014 - Programer DNA

using System;

class ProgramerDNA
{
    static void Main()
    {
        char[] dna = { 'A', 'B', 'C', 'D', 'E', 'F', 'G' };

        int length = int.Parse(Console.ReadLine());
        char startingLetter = char.Parse(Console.ReadLine());
        int dnaindex = startingLetter - 65;
        for (int i = 0; i < length; i++)
        {
            int row = i % 7 + 1;

            switch (row)
            {
                case 1: Console.WriteLine("{0}{1}{0}", new string('.', 3), dna[dnaindex % 7]); dnaindex++; break;
                case 2: Console.WriteLine("{0}{1}{2}{3}{0}", new string('.', 2), dna[dnaindex % 7], dna[(dnaindex + 1) % 7], dna[(dnaindex + 2) % 7]); dnaindex += 3; break;
                case 3: Console.WriteLine("{0}{1}{2}{3}{4}{5}{0}", new string('.', 1), dna[dnaindex % 7], dna[(dnaindex + 1) % 7], dna[(dnaindex + 2) % 7], dna[(dnaindex + 3) % 7], dna[(dnaindex + 4) % 7]); dnaindex = dnaindex + 5; break;
                case 4: Console.WriteLine("{0}{1}{2}{3}{4}{5}{6}", dna[dnaindex % 7], dna[(dnaindex + 1) % 7], dna[(dnaindex + 2) % 7], dna[(dnaindex + 3) % 7], dna[(dnaindex + 4) % 7], dna[(dnaindex + +5) % 7], dna[(dnaindex + 6) % 7]); dnaindex = dnaindex + 7; break;
                case 5: Console.WriteLine("{0}{1}{2}{3}{4}{5}{0}", new string('.', 1), dna[dnaindex % 7], dna[(dnaindex + 1) % 7], dna[(dnaindex + 2) % 7], dna[(dnaindex + 3) % 7], dna[(dnaindex + 4) % 7]); dnaindex = dnaindex + 5; break;
                case 6: Console.WriteLine("{0}{1}{2}{3}{0}", new string('.', 2), dna[dnaindex % 7], dna[(dnaindex + 1) % 7], dna[(dnaindex + 2) % 7]); dnaindex += 3; break;
                case 7: Console.WriteLine("{0}{1}{0}", new string('.', 3), dna[dnaindex % 7]); dnaindex = dnaindex + 1; break;
                default: ; break;
            }
        }
    }
}