// The desk calulator// reads from std::cin // uses namespaces, but no exceptions// pp 163-171, sec 8.2, Namespaces// uses += rather than push_back() for string// to work around standard library bug// No guarantees offered. Constructive comments to bs@research.att.com/*	program:		END			   // END is end-of-input		expr_list END	expr_list:		expression PRINT	   // PRINT is ';'		expression PRINT expr_list	expression:		expression + term		expression - term		term	term:		term / primary		term * primary		primary	primary:		NUMBER		NAME		NAME = expression		- primary		( expression )*/#include<iostream>#include<map>#include<string>#include<cctype>// note: no "using namespace std;"namespace Error {	int no_of_errors;	double error(const char* s)	{			std::cerr << "error: " << s << '\n';			no_of_errors++;			return 1;	}}namespace Lexer {	enum Token_value {		NAME,	NUMBER,		END,		PLUS='+',	MINUS='-',	MUL='*',	DIV='/',		PRINT=';',	ASSIGN='=',	LP='(',		RP=')'	};	Token_value curr_tok;	double number_value;	std::string string_value;	Token_value get_token();}Lexer::Token_value Lexer::get_token(){	char ch = 0;	std::cin>>ch;	switch (ch) {	case 0:		return END;	case ';':		return curr_tok=PRINT;	case '*':	case '/':	case '+':	case '-':	case '(':	case ')':	case '=':		return curr_tok=Token_value(ch);	case '0': case '1': case '2': case '3': case '4':	case '5': case '6': case '7': case '8': case '9':	case '.':		std::cin.putback(ch);		std::cin >> number_value;		return curr_tok=NUMBER;	default:			// NAME, NAME =, or error		if (isalpha(ch)) {	// warning isalpha is a macro					// in some implementations			std::cin.putback(ch);			std::cin>>string_value;			return curr_tok=NAME;		}		Error::error("bad token");		return curr_tok=PRINT;	}}std::map<std::string,double> table;	// global; see sec9.???namespace Parser {	double prim(bool get);		// handle primaries	double term(bool get);		// multiply and divide	double expr(bool get);		// add and subtract	using Lexer::get_token;		// commonly used names	using Lexer::curr_tok;	using Error::error;}double Parser::prim(bool get)		// handle primaries{	if (get) get_token();	switch (curr_tok) {	case Lexer::NUMBER:	// floating point constant		get_token();		return Lexer::number_value;	case Lexer::NAME:	{	double& v = table[Lexer::string_value];  		if (get_token() == Lexer::ASSIGN) v = expr(1);  		return v;	}	case Lexer::MINUS:		// unary minus		return -prim(1);	case Lexer::LP:	{	double e = expr(1);		if (curr_tok != Lexer::RP) return error(") expected");		get_token();			  // eat ')'		return e;	}	case Lexer::END:		return 1;	default:		return error("primary expected");	}}double Parser::term(bool get)		// multiply and divide{	double left = prim(get);	for (;;)	switch (curr_tok) {	case Lexer::MUL:		left *= prim(true);		break;	case Lexer::DIV:		if (double d = prim(true)) {		left /= d;			break;		}		return error("divide by 0");	default:		return left;	}}double Parser::expr(bool get)		// add and subtract{	double left = term(get);		for(;;)			// ``forever''	switch (curr_tok) {	case Lexer::PLUS:		left += term(true);		break;	case Lexer::MINUS:		left -= term(true);		break;	default:		return left;		}}int main(int argc, char* argv[]){	// insert pre-defined names:	table["pi"] = 3.1415926535897932385;	table["e"]  = 2.7182818284590452354;	while (std::cin) {		Lexer::get_token();		if (Lexer::curr_tok == Lexer::END) break;		if (Lexer::curr_tok == Lexer::PRINT) continue;		std::cout << Parser::expr(false) << '\n';	}	return Error::no_of_errors;	}