mycalc.cpp

一个基于递归下降的四则运算计算器

/*
    program:
	END			   // END is end-of-input
	expr_list END

    expr_list:
	expression PRINT	   // PRINT is semicolon
	expression PRINT expr_list

    expression:
	expression + term
	expression - term
	term

    term:
	term / primary
	term * primary
	primary

    primary:
	NUMBER
	NAME
	NAME = expression
	- primary
	( expression )
*/

#include <string>
#include<iostream>

using namespace std;

double error(const char* s)
{
    cerr << "error: " << s << '\n';
    exit(1);
}

enum Token_value {
	NUMBER,		END,
	PLUS='+',	MINUS='-',	MUL='*',	DIV='/',
	PRINT=';',	ASSIGN='=',	LP='(',		RP=')'
};

Token_value curr_tok = PRINT;
double number_value;
string string_value;


Token_value get_token()
{
	char ch = 0;
	cin>>ch;

	switch (ch) {
	case EOF:
		return curr_tok=END;
	case ';':
	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 '.':
		cin.putback(ch);
		cin >> number_value;
		return curr_tok=NUMBER;
	default:
		error("bad token");
	}
}

double expr(bool);

double prim(bool get)
{
	if (get) get_token();

	switch (curr_tok) {
	case NUMBER:
	{	double v = number_value;
		get_token();
		return v;
	}
	case MINUS:		// unary minus
		return -prim(true);
	case LP:
	{	double e = expr(true);
		if (curr_tok != RP) error(") expected");
		get_token();		// eat ')'
		return e;
	}
	default:
		error("primary expected");
	}
}

double term(bool get)
{
	double left = prim(get);

	for (;;)
		switch (curr_tok) {
		case MUL:
			left *= prim(true);
			break;
		case DIV:
			if (double d = prim(true)) {
				left /= d;
				break;
			}
			error("divide by 0");
		default:
			return left;
		}
}

double expr(bool get)
{
	double left = term(get);

	for (;;)
		switch (curr_tok) {
		case PLUS:
			left += term(true);
			break;
		case MINUS:
			left -= term(true);
			break;
		default:
			return left;
		}
}

int main()
{
	while (cin) {
		get_token();
		if (curr_tok == END) break;
		if (curr_tok == PRINT) continue;
		cout << expr(false) << '\n';
	}
	return 0;	
}