// 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
}