/****   CHAPTER 6  -  PROGRAM 10  
 *
 *  Model of a simple calculator
 *     Scans data lines consisting of a binary operator and a right
 *     operand.  Using the current value of the accumulator (initial value
 *     zero) as the left operand, program calculates and displays new
 *     accumulator value.
 *
 *     Recognized operations are addition (+), subtraction (-),
 *     multiplication (*), division (/), and exponentiation (^).
 */

#include <stdio.h>
#include <math.h>
typedef enum {false, true} boolean;

 /*
  *  Input a one-character operator and a number from a line of data.
  *  If a valid number is scanned and the character is one of these--
  *  + - * / ^ -- return 1.  Otherwise return 0.  Discard the rest of the
  *  input line.  
  *  Post-condition: operator contains the symbol for the operation to perform.
                     operand contains the value to be manipulated.
  */
 boolean scan_data (char *operator, /* output - the operator */
                    double *operand); /* output - the operand */

 /*
  *  Apply op using *accum as left operand and operand as right operand,
  *  storing result in *accum.
  *    Pre:  op is + or - or * or / or ^
  */
 void do_next_op (char op, /* input - operator */
                  double operand, /* input - operand */
                  double *accump); /* input/output - accumulated value */

 /*
  *  Verify that ch is one of the characters + - * / ^
  */
 int valid_op (char ch);

 int
 main(void)
 {
    double accum = 0;   /* Accumulator */
    char op;            /* symbol for arithmetic operator to be applied */
    boolean scan_status;    /* Successful scan flag */
    double operand;     /* operand */

    printf("Repeatedly enter operator and operand. Enter \nq 0\nto quit.\n");
    scan_status = scan_data(&op, &operand);
    while (scan_status)
    {
       do_next_op(op, operand, &accum);
       printf("result so far is %.1f\n", accum);
       scan_status = scan_data(&op, &operand);
    }

    printf("final result is %.2f\n", accum);

    return (0);
 }

 int
 valid_op(char ch)
 {
    int valid;

    switch (ch) 
    {
    case '+':
    case '-':
    case '*':
    case '/':
    case '^':  valid = 1;
               break;

    default:   valid = 0;
    }

    return (valid);
 }