C++实现简单计算器

---

利用编译原理中递归下降的 语法分析 和 语义分析 ，使用 C++ 实现简单计算器。

文法如下：
EXP -> TERM {AlphaOP TERM}
TERM -> FACTOR {BetaOP FACTOR}
FACTOR -> (EXP) | number
AlphaOP -> + | -
BetaOP -> * | \

---

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

/*
 *  EXP     ->  TERM {AlphaOP TERM}  
 *  TERM    ->  FACTOR {BetaOP FACTOR}
 *  FACTOR  ->  (EXP) | number
 *  AlphaOP ->  + | -
 *  BetaOP  ->  * | \
 */

string inputExp;
string::iterator ite;

void handleError();
void matchChar(char token);
double matchDigit();
double factor();
double term();
double expression();

//错误处理
void handleError() {
	cerr << "Error expression !  " + inputExp <<endl;
	exit(-1);
}

//匹配字符
void matchChar(char token) {
	if (*ite == token) {
		ite++;
	}
	else {
		handleError();
	}
}

//匹配数字
double matchDigit() {
	string tempString = "";
	while (isdigit(*ite)) {
		tempString += *ite;
		ite++;
	}
	istringstream iss(tempString);
	double temp;
	iss >> temp;
	return temp;
}

double factor() {
	double temp;
	if (isdigit(*ite)) {
		temp = matchDigit();
	}
	else if (*ite == '(') {
		matchChar('(');
		//处理负数
		if(*ite == '-'){
			ite++;
			temp = matchDigit() * (-1);
		}
		else{
			temp = expression();	
		}
		matchChar(')');
	}
	else {
		handleError();
	}

	return temp;
}

double term() {
	double temp = factor();
	while (*ite == '*' || *ite == '/') {
		if (*ite == '*') {
			matchChar('*');
			temp = temp * factor();
		}
		else {
			matchChar('/');
			temp = temp / factor();
		}
	}
	return temp;
}

double expression() {
	double temp = term();
	while (*ite == '+' || *ite == '-') {
		if (*ite == '+') {
			matchChar('+');
			temp = temp + factor();
		}
		else {
			matchChar('-');
			temp = temp - factor();
		}
	}
	return temp;
}

int main() {
	double result = 0;
	cout << "请输入表达式：";
	cin >> inputExp;

	//加上空格防止迭代器溢出
	inputExp += " ";
	ite = inputExp.begin();

	while (*ite != inputExp.back()) {
		result = expression();
	}

	cout << result << endl;

	system("pause");
	return 0;
}