Debug Assertion Failed in for - c++

Question

I got an error on second if
Making the exactly same if outside of for seems to work just fine, it returns the value from priority function, but when I try to do it in for it said that I'm trying to pass an invalid parameter to a function which considers it to be fatal.
#include <iostream>
#include <stack>
#include <string>
using namespace std;
int priority(char c);
int main()
{
stack<char>Operator;
string input;
string output;
getline(cin, input);
for (int i = 0; i < input.size(); i++)
{
if (input[i] == '(' || input[i] == ')' || input[i] == '+' || input[i] == '-' || input[i] == '*' || input[i] == '/')
Operator.push(input[i]);
else
output.push_back(input[i]);
if (priority(Operator.top()) == 3)
{
int aux = Operator.top();
Operator.pop();
while (priority(Operator.top() == 4))
{
output.push_back((char)Operator.top());
Operator.pop();
}
Operator.push(aux);
}
if (priority(Operator.top() == 2))
{
Operator.pop();
while (priority(Operator.top()) != 1)
{
output.push_back(Operator.top());
Operator.pop();
}
Operator.pop();
}
}
cout << output;
cin.ignore();
cin.get();
return 0;
}
int priority(char c)
{
if (c == '(')
return 1;
if (c == ')')
return 2;
if (c == '+' || c == '-')
return 3;
if (c == '*' || c == '/')
return 4;
}

Answer 1

The statement Operator.top() is being called on every symbol in the input string. That would probably work in case your string starts with a '(' symbol or with a unary plus, but what would happen in case of "2*2"?
As a perfect solution you should redesign your code to make it a finit automaton. As a simple ad hoc fix you should check if the current symbol is an operation or digit and don't call the Operator.top() in the latter case:
if (isdigit(input[i])) {
output.push_back(input[i]);
}
else {
Operator.push(input[i]);
if (priority(Operator.top()) == 3) {
// ...
}
// ...
}