// EX6_09.CPP
  // A program to implement a calculator
  
  include <iostream>                   // For stream input/output
  include <cstdlib>                    // For the exit() function
  include <cctype>                     // For the isdigit() function
  using std::cin;
  using std::cout;
  using std::endl;
  
  void eatspaces(char* str);            // Function to eliminate blanks
  double expr(char* str);               // Function evaluating an expression
  double term(char* str, int& index);   // Function analyzing a term
  double number(char* str, int& index); // Function to recognize a number
  
  const int MAX = 80;                   // Maximum expression length, 
                                        // including '\0'
  int main()
  {
     char buffer[MAX] = {0};    // Input area for expression to be evaluated
  
     cout << endl
          << "Welcome to your friendly calculator."
          << endl
          << "Enter an expression, or an empty line to quit."
          << endl;
  
     for(;;)
     {
        cin.getline(buffer, sizeof buffer);   // Read an input line
        eatspaces(buffer);                    // Remove blanks from input
  
        if(!buffer[0])                        // Empty line ends calculator
           return 0;
  
        cout << "\t= " << expr(buffer)        // Output value of expression
             << endl << endl;
     }
  }
  
  // Function to eliminate spaces from a string
  void eatspaces(char* str)
  {
     int i = 0;                             // 'Copy to' index to string
     int j = 0;                             // 'Copy from' index to string
  
     while((*(str + i) = *(str + j++)) != '\0')  // Loop while character
                                                 // copied is not \0
        if(*(str + i) != ' ')                    // Increment i as long as
           i++;                                  // character is not a space
     return;
  }
  
  // Function to evaluate an arithmetic expression
  double expr(char* str)
  {
    double value = 0.0;                  // Store result here
    int index = 0;                       // Keeps track of current character position
  
    value = term(str, index);            // Get first term
  
    for(;;)                              // Indefinite loop, all exits inside
    {
      switch(*(str + index++))           // Choose action based on current character
      {
        case '\0':                       // We're at the end of the string
           return value;                 // so return what we have got
  
        case '+':                        // + found so add in the
           value += term(str, index);    // next term
           break;
  
        case '-':                        // - found so subtract
           value -= term(str, index);    // the next term
           break;
  
        default:                         // If we reach here the string
           cout << endl                  // is junk
                << "Arrrgh!*#!! There's an error"
                << endl;
           exit(1);
      }
    }
  }
  
  // Function to get the value of a term
  double term(char* str, int& index)
  {
    double value = 0.0;                  // Somewhere to accumulate 
                                         // the result
  
    value = number(str, index);          // Get the first number in the term
  
    // Loop as long as we have a good operator
    while((*(str + index) == '*') || (*(str + index) == '/'))
    {
  
      if(*(str + index) == '*')          // If it's multiply,
        value *= number(str, ++index);   // multiply by next number
  
      if(*(str + index) == '/')          // If it's divide,
        value /= number(str, ++index);   // divide by next number
    }
    return value;                        // We've finished, so return what 
                                         // we've got
  }
  
  // Function to recognize a number in a string
  double number(char* str, int& index)
  {
    double value = 0.0;                  // Store the resulting value
  
    while(isdigit(*(str + index)))       // Loop accumulating leading digits
      value = 10*value + (*(str + index++) - '0');
  
                                         // Not a digit when we get to here
    if(*(str + index) != '.')            // so check for decimal point
      return value;                      // and if not, return value
  
    double factor = 1.0;                 // Factor for decimal places
    while(isdigit(*(str + (++index))))   // Loop as long as we have digits
    {
      factor *= 0.1;                     // Decrease factor by factor of 10
      value = value + (*(str + index) - '0')*factor;   // Add decimal place
    }
  
    return value;                        // On loop exit we are done
  }