e3.cpp

//
// This is example code from Chapter 6.7 "Trying the second version" of
// "Software - Principles and Practice using C++" by Bjarne Stroustrup
//

/*
    This file is known as calculator02buggy.cpp

    I have inserted 5 errors that should cause this not to compile
    I have inserted 3 logic errors that should cause the program to give wrong results

    First try to find and remove the bugs without looking in the book.
    If that gets tedious, compare the code to that in the book (or posted source code)

    Happy hunting!

*/

#include "../std_lib_facilities.h"

double expression ();    // declaration so that primary() can call expression()
double primary ();
double term ();
int factorial (double);

//------------------------------------------------------------------------------

class Token
{
  public:
	  char kind;        // what kind of token
	  double value;     // for numbers: a value
	  Token (char ch)    // make a Token from a char
		  : kind(ch), value(0)
	  {}
	  Token (char ch, double val)     // make a Token from a char and a double
		  : kind(ch), value(val)
	  {}
};

//------------------------------------------------------------------------------

class Token_stream
{
  public:
	  Token_stream ();   // make a Token_stream that reads from cin
	  Token get ();      // get a Token (get() is defined elsewhere)
	  void putback (Token t);    // put a Token back
  private:
	  bool full;        // is there a Token in the buffer?
	  Token buffer;     // here is where we keep a Token put back using putback()
};

//------------------------------------------------------------------------------

// The constructor just sets full to indicate that the buffer is empty:
Token_stream::Token_stream ()
	: full(false), buffer(0)    // no Token in buffer
{
}

//------------------------------------------------------------------------------

// The putback() member function puts its argument back into the Token_stream's buffer:
void Token_stream::putback (Token t)
{
  if (full)
  {
	error("putback() into a full buffer");
  }
  buffer = t;       // copy t to buffer
  full = true;      // buffer is now full
}

//------------------------------------------------------------------------------

Token Token_stream::get ()
{
  if (full)
  {       // do we already have a Token ready?
	full = false; // remove token from buffer
	return buffer;
  }

  char ch;
  cin >> ch;    // note that >> skips whitespace (space, newline, tab, etc.)

  Token token('@');

  switch (ch)
  {
	case 'q':    // for "quit"
	  exit(EXIT_SUCCESS);
	case '=':    // for "print"
	case '(':
	case ')':
	case '{':
	case '}':
	case '+':
	case '-':
	case '*':
	case '/':
	  token.kind = ch;
	  return token;        // let each character represent itself
	case '.':
	case '0':
	case '1':
	case '2':
	case '3':
	case '4':
	case '5':
	case '6':
	case '7':
	case '9':
	{
	  cin.putback(ch);         // put digit back into the input stream
	  double val;
	  cin >> val;              // read a floating-point number
	  cin >> ch;
	  if (ch == '!')
	  {
		val = factorial(val);
	  }
	  else
	  {
		cin.putback(ch);
	  }
	  token.kind = '8';
	  token.value = val;
	  return token;   // let '8' represent "a number"
	}
	default:
	  error("Bad token");
  }
  return token;
}

//------------------------------------------------------------------------------

Token_stream ts;        // provides get() and putback()

//------------------------------------------------------------------------------


int main ()
{
  cout << "Welcome to our simple calculator." << endl;
  cout << "Please enter expressions using floating-point numbers." << endl;

  try
  {
	while (cin)
	{
	  cout << "> ";   // print prompt
	  Token t = ts.get();

	  while (t.kind == '=')
	  {
		t = ts.get();
	  }

	  ts.putback(t);
	  cout << "= " << expression() << endl;
	}
  }
  catch (exception &e)
  {
	cerr << "error: " << e.what() << '\n';
	return 1;
  }
  catch (...)
  {
	cerr << "Oops: unknown exception!\n";
	return 2;
  }
}
//------------------------------------------------------------------------------
//
// deal with numbers and parentheses
double primary ()
{
  Token t = ts.get();

  switch (t.kind)
  {
	case '(':    // handle '(' expression ')'
	case '{':
	{
	  double d = expression();
	  t = ts.get();
	  if (t.kind != ')' && t.kind != '}')
	  {
		error("'" + to_string(t.kind) + " ' expected");
	  }
	  return d;
	}
	case '8':            // we use '8' to represent a number
	  return t.value;  // return the number's value
	default:
	  error("primary expected");
  }
  return -1;
}

//------------------------------------------------------------------------------

// deal with *, /, and %
double term ()
{
  double left = primary();
  Token t = ts.get();        // get the next token from token stream

  while (true)
  {
	switch (t.kind)
	{
	  case '*':
		left *= primary();
		t = ts.get();
		break;
	  case '/':
	  {
		double d = primary();
		if (d == 0)
		{
		  error("divide by zero");
		}
		left /= d;
		t = ts.get();
		break;
	  }
	  default:
		ts.putback(t);     // put t back into the token stream
		return left;
	}
  }
}

//------------------------------------------------------------------------------

// deal with + and -
double expression ()
{
  double left = term();      // read and evaluate a Term
  Token t = ts.get();        // get the next token from token stream

  while (true)
  {
	switch (t.kind)
	{
	  case '+':
		left += term();    // evaluate Term and add
		t = ts.get();
		break;
	  case '-':
		left -= term();    // evaluate Term and subtract
		t = ts.get();
		break;
	  default:
		ts.putback(t);     // put t back into the token stream
		return left;       // finally: no more + or -: return the answer
	}
  }
}

//------------------------------------------------------------------------------

int factorial (double value)
{
  int max = int(value);
  if (max == 0)
  {
	return 1;
  }

  int frac {1};
  for (int i {1} ; i <= max ; ++i)
  {
	frac *= i;
  }
  return frac;
}