Parse int and string

Hi I'm not sure if this is the right place to ask this question. Anyway I have written this code to parse a molecule formula and split it into atoms and amount of each atoms.

For instance if I input "H2O" I will for the atom array get {"H", "O"} and in the amount array I will get {2, 1}. I haven't taken account for amount that is larger than 9, since I don't think there are molecule which can bind to something that is larger than 8.

Anyway I'm quite newbie, so I wonder if this piece of code can be made better?

   string formula = "H2O";
   int no, k = 0, a = 0;
   string atom[10];
   int amount[10];
   bool flag = true;
   stringstream ss(formula);

   for(int i = 0; i < formula.size(); ++i)
   {

      no = atoi(&formula[i]);
      if(no == 0 && (flag || islowe开发者_如何学JAVAr(formula[i]) )  )
      {
         cout << "k = " << k << endl;
         atom[k] += formula[i];
         flag = false;
         cout << "FOO1 " << atom[k] << endl;
         amount[a] = 1;
      }
      else if(no != 0)
      {
         amount[a] = no;
         cout << "FOO2 " << amount[a] << endl;
         a++;
         flag = true;
         k++;
      }
      else
      {
         k++;
         a++;
         atom[k] = formula[i];
         cout << "FOO3 " << atom[k] << endl;
         amount[a] = 1;

         flag = false;
      }

      cout << no << endl;
   }

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Have you considered an approach with regular expressions? Do you have access to Boost or TR1 regular expressions? An individual atom and its count can easily be represented as:

(after edits based on comments)

([A-Z][a-z]{0,2})([0-9]*)

Then you just need to repeatedly find this pattern in your input string and extract the different parts.

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There are many potential improvements that could be made, of course. But as a newbie, I guess you only want the immediate ones. The first improvement is to change this from a program that has a hard coded formula to a program that reads a formula from the user. Then try testing yout program by inputting different formulae, and check that the output is correct.

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What if you modified it to be like this algorithm? This would maybe be less code, but would definitely be more clear:

// while not at end of input
     // gather an uppercase letter
     // gather any lowercase letters
     // gather any numbers
     // set the element in your array

This could be implemented with 3 very simple loops inside of your main loop, and would make your intentions to future maintainers much more obvious.