C++ Error about parenthesis

Heres my code:

//The convert function takes a string of numbers and returns its integer value.
int convert(string strNum)
{
      char c;
      const int Size = strNum.size();
      int I;
      int J;
      int X;
      int Y;
      int total;
      int tmpint1;
      int tmpint2;
      const int plusone = Size + 1;
      
      for (I = 1, I < plusone, I++)
      {
            X = Size - I;
            if (X > 0)
            {
                  Y = 10;
                  for(J = 1, J < X, J++)
                  {
                        Y = Y * 10;
                  }
            }
            else Y = 1;
            c = strNum.substr(I,I);
            tmpint1 = c - '0';
            tmpint2 = tmpint1 * Y;
            total = total + tmpint2;
      }
      return total;
}


Heres the odd errors im getting:

sr1 $ CC -o Program08 Program08.cpp
"Program08.cpp", line 170: Error: Unexpected ")" -- Check for matching parenthesis.
"Program08.cpp", line 171: Error: Use ";" to terminate statements.
"Program08.cpp", line 171: Error: Expected an expression.
"Program08.cpp", line 176: Error: Unexpected ")" -- Check for matching parenthesis.
"Program08.cpp", line 177: Error: Use ";" to terminate statements.
"Program08.cpp", line 177: Error: Expected an expression.
"Program08.cpp", line 182: Error: Cannot assign std::string  to char.
7 Error(s) detected.

Can anyone tell me why im getting these errors... If you need more code I can post it but I commented out this function and the errors disappeared