用C++實作簡易電腦

1. 電腦所接受的語言的文法：

program:
    END:          //END表示輸入結束
    expr_list END
expr_list:
    expression PRINT    //PRINT是分号
    expression PRINT expr_list
expression:
    expression + term
    expression - term
term:
    term / primary
    term * primary
    primary
primary:
    NUMBER
    NAME
    NAME = expression
    -primary
    (expression)
           

2. 實作代碼：

#include<iostream>
#include<string>
#include<map>
#include<cctype>
using namespace std;

map<string,double>table;

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

double number_value;
string string_value;
int no_of_errors;

double error(const string&);
Token_value get_token();
double prim(bool);
double term(bool);
double expr(bool);

double error(const string&s)
{
	no_of_errors++;
	cerr<<"error:"<<s<<'\n';
	return 1;
}

Token_value get_token()
{
	char ch = 0;
	do{
		if(!cin.get(ch))
			return curr_tok = END;
	}while(ch != '\n' && isspace(ch));

	switch(ch){
	case 0:
		return curr_tok = END;
	case';':
	case'\n':
		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'.':
		cin.putback(ch);
		cin>>number_value;
		return curr_tok = NUMBER;
	default:
		if(isalpha(ch)){
			string_value = ch;
			while(cin.get(ch) && isalnum(ch))
				string_value.push_back(ch);
			cin.putback(ch);
			return curr_tok = NAME;
		}
		error("bad token");
		return curr_tok = PRINT;
	}
}

double prim(bool get)
{
	if(get)
		get_token();
	switch(curr_tok){
	case NUMBER:
	{
		double v = number_value;
		get_token();
		return v;
	}
	case NAME:
	{
		double& v = table[string_value];
		if(get_token() == ASSIGN)
			v = expr(true);
		return v;
	}
	case MINUS:
		return -prim(true);
	case LP:
	{
		double e = expr(true);
		if(curr_tok != RP)
			return error(") expected");
		get_token();
		return e;
	}
	}
}

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;
			}
			return 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()
{
	table["pi"] = 3.1415926535897932385;
	table["e"] = 2.7182818284590452354;

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

3. 結果示範：

注：

1. 參考C++程式設計語言（c++之父的經典之作）

2. 退出程式：[enter] Ctrl+Z [enter]