Create a calculator

- support operations such as +,-,*,/,^
- Input is going to be a postfix expression ending with a =
- Each token will be separated by a space
- Parse operation reads tokens and puts them in the q


- Write the token class
class Token
{
	friend class BXTree;
	char a[28];
	public:
	bool IsOperator();
	double value();
	friend istream& operator>>(istream&,Token&);
};


IsOperator()
	if(strcmp(a,"+") == 0) return true;
	if(strcmp(a,"-") == 0) return true;
	.
	.
	.
	return false;

Value()
	return atod(a); // if there is no atod try atof

Create a class called calculator with two private data members:

-	DoubleList<Token> q
-	Doublestack<double> s

Two public member functions:

-	void parse(istream& in)

	- Read tokens and put them in the list q

-	double evaluate()

	while q is not empty
	{
		delete tok from position 1 in the q
		if tok is not an operator
			add tok.value() to s
		else
		{
			delete op2 from s
			delete op1 from s

			switch(tok.a[0])
			case '+':
				add (op1 + op2) to s
				break;
			case '-':
				add (op1 - op2) to s
				break;
			...
			...
		}
	}
	return Top of s

-	Try your program for the following expressions

3 3 ^ =
answer: 27
9 3 / =
answer: 3
8 9 + =
answer: 17
8 7 9.8 2.3 - * + =
answer: 60.5