/* File: proj4.c
 *
 *  This is a sample solution for project 4.  There are 
 *  many other solutions; some better than this one.
 *  For example, this program makes heavy use of global 
 *  variables.  I have seen several nice programs that
 *  do not use any global variables.
 *
 *                                Richard Chang, 12/8/94
 */

#include <stdio.h>
#include "genlib.h"
#include "simpio.h"
#include "strlib.h"

/* Global Variables */

string input_line ;
int input_length ;
int input_pos ;

/* Function Prototypes */
void ReadNewLine() ;
void SkipBlanks() ;
double ReadDouble() ;

/* Read in a new line and update position 
*/
void ReadNewLine() {
   input_line = GetLine() ;
   input_length = StringLength(input_line) ;
   input_pos = 0 ;
}

/* Skip over blank spaces until an interesting
   character is encountered 
*/
void SkipBlanks() {
   char c ;

   while (TRUE) {
      /* Get non-empty line */
      while (input_pos >= input_length) {
	 ReadNewLine() ;
      }
      /* Get next character */
      c = IthChar(input_line, input_pos) ;

      /* if non-white space character found, return */
      if (!isspace(c)) return ;

      input_pos++ ;
   }
}

/* Read in the next operand and return as a double.
*/
double ReadDouble() {
   char c ;
   int start, stop ;
   string result ;

   /* Skip over white space */
   SkipBlanks() ;

   start = input_pos ;

   /* Look for the end of the number */
   while(input_pos < input_length) {
      c = IthChar(input_line, input_pos) ;
      if (!isdigit(c) && (c != '.')) break ; 
      input_pos++ ;
   }

   /* Extract operand and return as double */
   stop = input_pos - 1 ;
   result = SubString(input_line, start, stop) ;
   return( StringToReal(result) ) ;
}

main() {
   char operator ;
   double operand, result ;

   printf("Enter an expression ending with '='\n") ;

   /* Initialize Global variables */
   ReadNewLine() ;

   /* Get first operand */
   result = ReadDouble() ;

   while(TRUE) {

      /* Get the operator */
      SkipBlanks() ;
      operator = IthChar(input_line, input_pos) ;
      input_pos++ ;

      /* End of expression??? */
      if (operator == '=') {
	 printf("Answer = %g\n", result) ;
	 break ;
      }

      /* Process operator */
      switch(operator) {
	 case '+' : 
            operand = ReadDouble() ;
	    result = result + operand ; 
	    break ;

	 case '-' : 
            operand = ReadDouble() ;
	    result = result - operand ; 
	    break ;

	 case '*' : 
            operand = ReadDouble() ;
	    result = result * operand ; 
	    break ;

	 case '/' : 
            operand = ReadDouble() ;
	    result = result / operand ; 
	    break ;

         default : Error("Unknown operator: %c", operator) ;
      }
   }
}