// Massingill, Berna L. and Oldham, Jeffrey D.
// 2000 Mar 29
// CS1321

// Boolean-expression evaluator.

// Input:
//
//   A boolean expression, read from standard input (terminated by
//   EOF), in postfix notation.  Terms are separated by whitespace,
//   and each term is either an operator (&&, ||, !, =>, or ==) or
//   a constant (true or false).  
//
// Output:
//
//   A message to standard output giving the value of the expression
//   if it is well-formed, or an error message if it is not.

#include <iostream>
#include <cassert>		// has assert()
#include <cstdlib>		// has EXIT_SUCCESS, EXIT_FAILURE
#include <string>
#include <vector>

typedef vector<string> expr;

// ADD CODE HERE?

// ---- Function prototypes. ----
void readInput(istream & in, expr & inputExpression);
bool evaluate(const expr & expression, string & expressionValue);
// ADD CODE HERE?

// ---- Main program. ----

int main(void)
{
  cout << "Enter the expression to evaluate "
    "(multiple lines okay, control-D to end):\n";

  expr inputExpression;
  readInput(cin, inputExpression);

  string result;
  if (evaluate(inputExpression, result)) 
    cout << "Result = " << result << endl;
  else
    cout << "Expression is not well-formed\n";

  return EXIT_SUCCESS;
}

// ---- Function definitions. ----

// Pre:  "in" contains a sequence of strings.
// Post:  strings read from inputExpression have been appended to
//          inputExpression.
void readInput(istream & in, expr & inputExpression) {
  string word;
  while (in >> word)
    inputExpression.push_back(word);
}

// Pre:  inputExpression contains a boolean expression (as described in
//         header comments).
// Post:  if input expression is well-formed, expressionValue 
//          contains its value and return value is "true";
//          otherwise return value is "false".

// ADD CODE HERE.