Figuring out the number corresponding to a letter of the alphabet?

I'm trying to duplicate the way excel provides labels its columns where

A = 1
B = 2

so on and so forth, so that it eventually reaches

AB
AC
AD

etc, etc.

How do I algorithmically take a number (like 52) and convert it to its equivalent alphabet representation?

string get(int a) {
  a--; // Reduce by one to make the values with 1 letter 0..25, 
       // with two - 26.. 26^2-1 and so on
  int val = 0; // number of columns with no more then given number of letters
  int number = 0;
  while (val < a) {
    val = val*26 + 26;
    number++;
  }
  val = (val - 26)/26;
  a -= val; // subtract the number of columns with less letters 
  string res;
  for (int i = 0; i < number; ++i) {
    res.push_back(a%26 + 'A');
    a /= 26;
  }
  reverse(res.begin(), res.end());
  return res;
}

Hope that helps.