double evaluate_expression(istream& in, bool& okay)// Process each line for infix-to-postfix conversion and evaluation {	const char DECIMAL = '.';	const char LEFT_PARENTHESIS = '(';	const char RIGHT_PARENTHESIS = ')';		stack<double> numbers;	stack<char> operations;	double number;	char symbol;		while(in)	{				if(in.peek() == LEFT_PARENTHESIS)		{			in >> symbol;			operations.push(symbol);		}				if(isdigit(in.peek()) || (in.peek() == DECIMAL))		{			in >> number;				numbers.push(number);			cout<< "First Number " << numbers.top() << endl;		}		else if(strchr("+-*/", in.peek()) != NULL)		{			in >> symbol;					operations.push(symbol);			cout<< "First Symbol " << symbol << endl;					}						else if(in.peek() == RIGHT_PARENTHESIS)		{			in.ignore();			do_operation(symbol, numbers, okay);		}			else			in.ignore();	}	return numbers.top();}

// CP2001 - 2007// Assignment 1// Write a simple calculator program that takes as input an infix expression, // and outputs the equivalent postfix expression and the evaluation of the infix expression. // Assume that the input may contain (floating) numbers, arithmetic operations +,-,*, and /, // as well as parentheses. However, the expression need not b e fully parenthesized, // and when parentheses are missing, the usual C++ precedence rules are used to determine the order of evaluation. // When an invalid infix expression is entered, then an error message will be printed and the user will // be re-prompted to enter a new infix expression. The program will continue until the user type ENTER ('\n') without expressions. #include <cctype>     // Provides isdigit#include <cstdlib>    // Provides EXIT_SUCCESS#include <cstring>    // Provides strchr#include <iostream>   // Provides cout, cin, peek, ignore#include <stack>      // Provides the stack template class#include <string>using namespace std;// PROTOTYPES for functions used by this demonstration program:void do_operation(char operation, stack<double>& numbers, bool& okay);double evaluate_expression(string &postfix, bool& okay);void convert(string& infix, string& postfix);bool is_operand(char ch);bool takes_precedence(char operatorA, char operatorB);int main( ){	double answer;	bool okay;	do{		string infix;		string postfix;		cout << "================================================================" << endl;		cout << "Type an infix arithmetic expression (Just type ENTER to quit!!):" << endl;		cout << "Infix: ";		cin >> infix;		convert(infix, postfix);				cout << "Postfix: " << postfix << endl;		cout<< "you have gone past postfix" << endl;		answer = evaluate_expression(postfix, okay);		if(okay)			cout << "Evaluation: " << answer << "." << endl;		else			cout << endl << "Invalid infix notation!!" << endl;	}while(true);	system("pause");    return EXIT_SUCCESS;}double evaluate_expression(string& postfix, bool& okay)// Process each line for infix-to-postfix conversion and evaluation {	const char DECIMAL = '.';	const char LEFT_PARENTHESIS = '(';	const char RIGHT_PARENTHESIS = ')';		stack<double> numbers;	stack<char> operations;	double number;	char symbol;	char hold;	cout << "inside" << endl;	for(int i = 0; i < postfix.size(); i++)	{		hold = postfix;				if(isdigit(postfix) || (postfix == DECIMAL))		{			number = hold;				numbers.push(number);			cout<< "First Number " << numbers.top() << endl;		}		else if(strchr("+-*/", postfix))		{			symbol = hold;					operations.push(symbol);			cout<< "First Symbol " << symbol << endl;		}						else if(postfix == '\n')		{			cin.ignore();			do_operation(symbol, numbers, okay);		}				else			okay = !okay;	}	return numbers.top();}void do_operation(char operation, stack<double>& numbers, bool& okay)// Pops two numbers from s, applies the operation, and pushes the answer// back on the stack s. Sets okay to false if any errors are found,// but leaves okay alone if there are no errors.{	double operand1;	double operand2;		operand2 = numbers.top();	numbers.pop();	operand1 = numbers.top();	numbers.pop();	switch(operation)	{		case '+': 	numbers.push(operand1 + operand2);				break;		case '-': 	numbers.push(operand1 - operand2);				break;		case '*': 	numbers.push(operand1 * operand2);				break;		case '/': 	numbers.push(operand1 / operand2);				break;		default:	okay = !okay;	}}bool is_operand(char ch){	if(((ch >= 'a') && (ch <= 'z')) || ((ch >= 'A') && (ch <= 'Z')) || ((ch >= '0') && (ch <= '9')))		return true;	else		return false;}bool takes_precedence(char operatorA, char operatorB){	if(operatorA == '(')		return false;	else if(operatorB == '(')		return false;	else if(operatorB == ')')		return true;	else if((operatorA == '^') && (operatorB == '^'))		return false;	else if(operatorA == '^')		return true;	else if(operatorB =='^')		return false;	else if((operatorA == '*') || (operatorA == '/'))		return true;	else if((operatorB == '*') || (operatorB == '/'))		return false;	else		return true;}void convert(string& infix, string& postfix){	stack<char> hold_symbol;	char top_symbol;	char symbol;		for(int i = 0; i < infix.size(); ++i)	{		symbol = infix;		if(is_operand(symbol))			postfix = postfix + symbol;		else		{			while((!hold_symbol.empty()) && (takes_precedence(hold_symbol.top(), symbol)))			{				top_symbol = hold_symbol.top();				hold_symbol.pop();				postfix = postfix + top_symbol;			}			if((!hold_symbol.empty()) && (symbol == ')'))				hold_symbol.pop();			else				hold_symbol.push(symbol);		}	}	while(!hold_symbol.empty())	{		top_symbol = hold_symbol.top();		hold_symbol.pop();		postfix = postfix + top_symbol;	}}