Hello.

This code snippet is a basic calculator.

The general concept is that the calculator accepts infix expressions as strings, converts them to reverse polish notation by way of the shunting-yard algorithm and then evaluates the resulting expression.

I tried to encapsulate the functionality of each piece so that, in theory, they could be pulled out and used independently. The shunting-yard behavior is provided via `class ShuntingYard`

; I used the visitor pattern for the `Token`

; and `RPNExpression`

could be built by hand (or some other process) and still be used with `Calculator`

.

I removed almost all error checking and input validation for the sake of brevity and readibility. Code compiles with MSVC++ and GCC 4.6.3.

Any feedback is appreciated.

```
/*
Author: Jesse Brown
Date: July 7, 2012
*/
#include <iostream>
#include <stack>
#include <vector>
#include <string>
#include <cstdlib>
#include <map>
#include <stdexcept>
class Calculator;
/*
Represents a 'token' in an RPN expression.
An RPN expression looks something like:
2 3 4 + *
This class provides a common interface for handling both
operators and operands.
*/
struct TokenBase {
virtual void evaluate (Calculator *) = 0;
virtual ~TokenBase() {}
};
/*
Concrete 'token' of an RPN expression.
Operators are of type Token< char >
Operands are of type Token< double >
*/
template< class T > class Token : public TokenBase {
T token_;
public:
/* Allow a calculator to consume this token */
void evaluate (Calculator *c);
Token (T t) : token_(t) {}
};
/*
Represents an expression in Reverse Polish Notation.
This object basically acts as a FIFO queue of tokens
*/
class RPNExpression {
std::vector< TokenBase* > stack_;
public:
/* Add a token to the end of the expression */
void push (TokenBase *t) { stack_.push_back (t); }
/* Grab the next token from the front of the expression */
TokenBase* pop () {
TokenBase *t = stack_.front ();
stack_.erase (stack_.begin ());
return t;
}
bool empty () const { return stack_.empty (); }
};
/*
Convert an expression in infix format to RPN format
*/
class ShuntingYard {
const std::string expr_;
RPNExpression rpn_;
std::stack< char > op_stack_;
mutable std::map< char, int > op_precedence_;
/* Returns a precedence value for the given operator */
int precedence (char op) const { return op_precedence_[op]; }
/* Returns the precedence of the top item in the stack */
int stack_precedence () const {
if (op_stack_.empty ()) { return -1; }
return precedence (op_stack_.top ());
}
/* Reset precedence to allow for new scope */
void handle_left_paren () { op_stack_.push ('('); }
/* Consume all operators in current scope and restore previous scope */
void handle_right_paren () {
while ('(' != op_stack_.top ()) {
rpn_.push (new Token< char >(op_stack_.top ()));
op_stack_.pop ();
}
op_stack_.pop ();
}
/* Consume operators with precedence >= than op then add op */
void handle_op (char op) {
while (! op_stack_.empty () &&
precedence (op) <= stack_precedence ()) {
rpn_.push (new Token< char >(op_stack_.top ()));
op_stack_.pop ();
}
op_stack_.push(op);
}
/* Convert infix to RPN via shunting-yard algorithm */
RPNExpression convert(const std::string &infix) {
const char * token = infix.c_str ();
while (token && *token) {
while (*token && isspace (*token)) { ++token; }
if (! *token) { break; }
if (isdigit (*token)) {
char * next_token = 0;
rpn_.push (new Token< double >(strtod (token, &next_token)));
token = next_token;
} else {
char op = *token;
switch (op) {
case '(':
handle_left_paren ();
break;
case ')':
handle_right_paren ();
break;
default:
handle_op (op);
}
++token;
}
}
while (! op_stack_.empty ()) {
rpn_.push (new Token< char >(op_stack_.top ()));
op_stack_.pop ();
}
return rpn_;
}
public:
ShuntingYard (const std::string& infix) : expr_(infix) {
op_precedence_['('] = -1;
op_precedence_['+'] = 2; op_precedence_['-'] = 2;
op_precedence_['*'] = 3; op_precedence_['/'] = 3;
}
RPNExpression to_rpn () { return convert (expr_); }
};
/*
A calculator that evaluates expressions by first converting them to
reverse polish notation then processing the result.
*/
class Calculator {
std::stack< double > operands_;
double pop () {
double d = operands_.top ();
operands_.pop ();
return d;
}
void push (double d) { operands_.push (d); }
/* Returns the most recent operation result (top of the operand stack) */
double result () const { return operands_.top (); }
/* Empty the operand stack */
void flush () {
while (! operands_.empty ()) { operands_.pop (); }
}
protected:
/* Process an operand token from the input stream */
void consume(double value) { push (value); }
/* Process an operator token from the input stream */
void consume(char op) {
switch (op) {
case '+':
push (pop () + pop ());
break;
case '*':
push (pop () * pop ());
break;
case '-':
{
double right = pop ();
push (pop () - right);
}
break;
case '/':
{
double right = pop ();
push (pop () / right);
}
break;
default:
throw std::domain_error("Unknown Operator");
}
}
public:
/*
Evaluate expression
Note: Expression is expected to be in infix form.
*/
double calculate (const std::string& expr) {
ShuntingYard shunting(expr);
RPNExpression rpn = shunting.to_rpn ();
flush ();
while (! rpn.empty ()) {
TokenBase * token = rpn.pop ();
token->evaluate (this);
delete token;
}
return result ();
}
/* Expose the consume() methods to the Tokens */
template< class T > friend class Token;
};
template< class T >
void Token< T >::evaluate (Calculator * c) { c->consume (token_); }
int main () {
Calculator c;
std::cout << c.calculate ("(20+10)*3/2-3") << std::endl;
return 0;
}
```