Arithmetic Evaluation using Stacks and Postfix Notation - c++

Question

I'm a C++ newcomer.
For the next project in class, the professor asked us to code a program that calculates a mathematical expression, that we input an infix notation, convert it to postfix and calculates it. I think I have pretty much finished the code, but there are some lines that I'm probably doing wrong and I don't really know what is it. Can you please take a look at it and tell me why am I wrong.
#include <iostream>;
#include <stack>;
#include <string>;
using namespace std;
bool operatorCheck(char c)
{
if (c == '+' || c == '-' || c == '*' || c == '/')
{
return true;
}
return false;
}
int priority(char c)
{
if (c == '*' || c == '/')
{
return 2;
}
else if (c == '+' || c == '-')
{
return 1;
}
return 0;
}
string InfixToPostfix(string expression)
{
stack<char> operators;
string postfix;
for (int i = 0;i< expression.length();i++)
{
if (expression[i] == ' ')
{
continue;
}
else if (expression[i] >= '0' && expression[i] <= '9')
{
string temp;
temp += expression[i];
int temp2 = i + 1;
while (true)
{
if (expression[temp2] >= '0' && expression[temp2] <= '9')
{
temp += expression[temp2];
temp2 += 1;
}
else
{
break;
}
}
postfix= postfix + temp + " ";
}
else if (operators.empty())
{
operators.push(expression[i]);
}
else if (operatorCheck(expression[i]))
{
if (priority(operators.top()) < priority(expression[i]))
{
operators.push(expression[i]);
}
else
{
while (!operators.empty() && priority(operators.top()) < priority(expression[i]))
{
postfix += operators.top();
operators.pop();
}
operators.push(expression[i]);
}
}
else if (expression[i] == '(')
{
operators.push(expression[i]);
}
else if (expression[i] == ')')
{
while (!operators.empty() && operators.top() != '(')
{
postfix += operators.top();
operators.pop();
}
operators.pop();
}
}
while (!operators.empty())
{
postfix += operators.top();
operators.pop();
}
return postfix;
}
//Main program
int main()
{
string expression;
stack<int> calc;
cout << "Welcome to my program!" << endl;
cout << "This program will help you to calculate arithmetic expression." << endl;
cout << "Please enter an arithmetic expression (Remember to enter space between numbers and operators):" << endl;
getline(cin, expression);
string postfix = InfixToPostfix(expression);
cout << postfix << endl;
for (int i = 0;i < postfix.length();i++)
{
if (postfix[i] == ' ')
{
continue;
}
else if (postfix[i] >= '0' && postfix[i] <= '9')
{
string temp;
int temp2 = i + 1;
temp += postfix[i];
while (true)
{
if (postfix[temp2] >= '0' && postfix[temp2] <= '9')
{
temp += expression[temp2];
temp2 += 1;
}
else
{
break;
}
}
int n = atoi(temp.c_str());
calc.push(n);
}
else if (postfix[i] == '+')
{
int temp1 = calc.top();
calc.pop();
int temp2 = calc.top();
calc.pop();
int result = temp2 + temp1;
calc.push(result);
}
else if (postfix[i] == '-')
{
int temp1 = calc.top();
calc.pop();
int temp2 = calc.top();
calc.pop();
int result = temp2 - temp1;
calc.push(result);
}
else if (postfix[i] == '*')
{
int temp1 = calc.top();
calc.pop();
int temp2 = calc.top();
calc.pop();
int result = temp2 * temp1;
calc.push(result);
}
else if (postfix[i] == '/')
{
int temp1 = calc.top();
calc.pop();
int temp2 = calc.top();
calc.pop();
int result = temp2 / temp1;
calc.push(result);
}
}
cout << calc.top() << endl;
}
For example, with this expression ( ( 7 + 6 ) * ( 16 - 2 ) ) / 2,
It returns 7 6 +16 6 2 -*2 / ( there is an extra 6) for the postfix. Why is this happened?