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[antimatterworld]

简单整数计算器：
摘录自C++编程宝典Chap06
支持 + - * / ()嵌套
原书有笔误:
rtn = atoi(expr - pos);
正确写法:
rtn = atoi(expr + pos);

////////////////////////
//标准C++编程宝典Chap06
//File Name:pr06025.cpp
////////////////////////


#include <iostream>
#include <cstdlib>
#include <cctype>

//Function prototypes.
int addsubt();
int multdiv();
int number();
void error();

//Global expression buffer.
static char expr[81];
static int pos;
////////////////////////
//The main() function
////////////////////////
int main()
{
	int ans;
	do
	{
		//Initialize the string subscript.
		pos = 0;

		//Read an expression.
		std::cout << "Enter expression (0 to quit):" << std::endl;
		std::cin >> expr;

		//Evaluate the expression.
		ans = addsubt();
		if(expr[pos] != '\0')
			error();
		if(ans != 0)
			std::cout << ans << std::endl;
	}while(ans != 0);

	return 0;
}
////////////////////////////////
//Top of recursive descent:add/subtract.
////////////////////////////////
int addsubt()
{
	int rtn = multdiv();
	while(expr[pos] == '+' || expr[pos] == '-')
	{
		int op = expr[pos++];
		int opr2 = multdiv();
		if(op == '+')
			rtn += opr2;
		else
			rtn -= opr2;
	}

	return rtn;
}

///////////////////////////////
//Highest precedence:multiply/divide.
///////////////////////////////
int multdiv()
{
	int rtn = number();
	while(expr[pos] == '*' || expr[pos] == '/')
	{
		int op = expr[pos++];
		int opr2 = number();

		if(op == '*')
			rtn *= opr2;
		else
			rtn /= opr2;
	}

	return rtn;
}
/////////////////////////////
//Extract a number
/////////////////////////////
int number()
{
	int rtn;

	if(expr[pos] == '(')
	{
		//Parenthetical expression
		pos++;
		rtn = addsubt(); //Back to top.
		if(expr[pos++] != ')')  //Must have ')'
			error();

		return rtn;
	}

	//Extract the number.
	if(!isdigit(expr[pos]))
		error();
	rtn = atoi(expr + pos);
	while(isdigit(expr[pos]))
		pos++;

	return rtn;
}

/////////////////////////////
//Syntax error.
/////////////////////////////
void error()
{
	std::cout << '\r';
	while(pos--)     //Position error pointer.
		std::cout << ' ';

	std::cout << "^ syntax error" << std::endl << '\a';

	exit(-1);
}
/////////////////////////////
/*
Enter expression (0 to quit):
2+3
5
Enter expression (0 to quit):
2*3
6
Enter expression (0 to quit):
(2+3)/3
1
Enter expression (0 to quit):
23+56
79
Enter expression (0 to quit):
((23+56)+80/40)/9-7
2
Enter expression (0 to quit):
89+256*(1024/8+36-100/2)+63-56/7
29328
Enter expression (0 to quit):
89+256*(1024/8+(36-100/2))+(63-56/7)
29328
Enter expression (0 to quit):
*/
//////////////////////////////

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[antimatterworld]

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