//
// is_balanced returns true if the string expression contains
// balanced parentheses and false otherwise.
//
// It demonstrates one way to use a stack, in this case, to keep
// track of left parentheses that have been seen in order to match
// them to right parentheses.
//
bool is_balanced(const string& expression)
{
	const char LEFT_PARENTHESIS = '(';
	const char RIGHT_PARENTHESIS = ')';

	stack<char> store;
	string::size_type i;
	char next;
	bool failed = false;

	for (i = 0; !failed && (i < expression.length()); i++) {
		next = expression[i];
		if (next == LEFT_PARENTHESIS) 
			store.push(next);
		else if ((next == RIGHT_PARENTHESIS) && (!store.empty()))
			store.pop();
		else if ((next == RIGHT_PARENTHESIS) && (store.empty()))
			failed = true;
	}

	return (store.empty() && !failed);
}