#include <iostream>
#include <fstream>
#include <cctype>
#include <string>

using namespace std;

bool isanumber(string s)
{ for (int i = 0; i < s.length(); i += 1)
    if (! isdigit(s[i]))
      return false;
  return true; }

int stringtoint(string s)
{ int value = 0;
  for (int i = 0; i < s.length(); i += 1)
    value = value * 10 + digittoint(s[i]);
  return value; }

int sympos = 0;
string input;

string getnextsymbol()
{ if (sympos >= input.length())
    return "*";
  char c = input[sympos];
  sympos += 1;
  if (isupper(c))
  // what goes here?
  else if (isdigit(c))
  // and what goes here?
  else
  { cout << "Improper character " << c << " in input\n";
    return "*"; }

int main()
{ string symbols[120];
  string names[120];
  double atwts[120];

  string filename = "/home/www/class/een118/a3010b.txt";
  ifstream fin;
  fin.open(filename);
  if (fin.fail())
  { cout << "Couldn't open " << filename << "\n";
    exit(1); }

  symbols[0] = "Xx";
  names[0] = "Does not exist";
  atwts[0] = 0.0;

  int num = 1;
  while (num < 120)
  { fin >> symbols[num] >> names[num] >> atwts[num];
    if (fin.fail())
      break;
    num += 1; }
  fin.close();

  bool errors = false;
  double molwt = 0.0;
  double prevatwt = 0.0;
  cout << "Enter the formula: ";
  cin >> input;
  while (true)
  { string sym = getnextsymbol();

    if (sym == "*")
      break;

    if (isanumber(sym))
    { int value = stringtoint(sym);
      molwt += (value - 1) * prevatwt;
      continue; }

    int pos = -1;
    for (int i = 0; i < num; i += 1)
      if (symbols[i] == sym)
      { pos = i;
        break; }

    if (pos == -1)
    { cout << "symbol " << sym << " Not found\n";
      errors = true;
      break; }

    prevatwt = atwts[pos];
    cout << "Adding " << names[pos] << ", at " << atwts[pos] << "\n";
    molwt += atwts[pos]; }

  if (! errors)
    cout << "Total molecular weight = " << molwt << "\n"; }