infixtopostfix.h

postfixComputer, Calculate the postfix expression, such as 45+,which means 4+5,and the result is 9.

// InfixToPostfix.cpp : Defines the entry point for the console application.

#include "stdafx.h"

#include<iostream>
#include<string>
#include<stack>
#include<cctype>
#include<vector>

using namespace std;

int Priority(char optr)//设定运算符的优先级
{
	switch (optr)
	{
		//	case '(': 
		//		return 0;
	case '+':
	case '-':
		return 1;
	case '*': 
	case '/':
		return 2;
	} 
}

int InfixToPostfix()
{
	stack<char> s;
	vector<char> vecPostfix;
	string operatorSymbol = "+-*/()";
	enum state{OPERAND, OPERATOR};

	state prospectiveElement = OPERAND;

	char ch,topElement;
	int operandNum = 0, operatorNum = 0;
	int errorFlag = 0;

	cout<<"Please enter an arithmetic expression and end it by #:" << endl;
	while(cin.get(ch)&&ch!='#')
	{
		if(isdigit(ch)||isalpha(ch))
		{
			operandNum++;
			if (prospectiveElement != OPERAND)
			{
				cout << "Invalid Input!" << endl; //lack of operator
				errorFlag = 1;
			}
			vecPostfix.push_back(ch);
			prospectiveElement = OPERATOR;
		}
		else 
		{
			int index =  operatorSymbol.find_first_of(ch);
			if (index == -1)
			{
				cout << "Invalid Input!" << endl; //illegal type of operator (legal type:\"+-*/()\")
				errorFlag = 1;
			}

			if (ch == '(')
				s.push(ch);
			else if (ch == ')')
			{
				if (prospectiveElement != OPERATOR)
				{
					cout << "Invalid Input!" << endl;//lack of operand
					errorFlag = 1;
				}
				while (!s.empty() && s.top() != '(')
				{

					topElement = s.top();
					s.pop();
					vecPostfix.push_back(topElement);

				}
				if (s.empty())
				{
					cout << "Invalid Input" << endl;//lack of '('
					errorFlag = 1;
				}
				else
					s.pop();
			}
			else
			{
				operatorNum++;
				if (prospectiveElement != OPERATOR)
				{
					cout << "Invalid Input!" << endl; //lack of operator
					errorFlag = 1;
				}
				prospectiveElement = OPERAND;
				if (s.empty())
					s.push(ch);
				else
				{
					topElement = s.top();
					while ((!s.empty()) && (topElement != '(') && (Priority(topElement) >= Priority(ch)))
					{
						s.pop();
						vecPostfix.push_back(topElement);
						if (!s.empty())
							topElement = s.top();
					}
					s.push(ch);
				}
			}
		}
	}


	if (operandNum - operatorNum != 1)
	{
		cout << "Invalid Input!" << endl;//lack of operand
		errorFlag = 1;
	}

	while (!s.empty())
	{
		topElement=s.top();
		if (topElement == '(')
		{
			cout << "Invalid Input!" << endl;  //lack of ')'
			errorFlag = 1;
		}
		s.pop();
		vecPostfix.push_back(topElement);
	}

	if (errorFlag == 1)
		return 1;

	for (int i = 0; i < vecPostfix.size(); i++)
	{
		if (i != vecPostfix.size() - 1)
			cout << vecPostfix[i] << ", ";
		else
			cout << vecPostfix[i];
	}

	cout << endl;
	system ("PAUSE");
	return 0;
}