Infix to Postfix to Answer Program (thought process right?) - c++

Question

I'm aware that there's already stuff here regarding this but I'm just posting to check if my logic lines up with my code (if I'm thinking about this whole infix Postfix thing the way I should be).
The code I've seen regarding this topic on this site look a little different from mine. I'm kind of a novice at C++. Anyway, my code so far looks like it is just fine and should work just fine but it's not working the way it should. I've included my thought process as comments in the code below. Please let me know if what I'm thinking is okay.
Here's the code:
#include <iostream>
#include <stack>
#include <sstream>
#include <string>
using namespace std;
int priority (string item)
{
int prior = 0;
if (item == "(")
{
prior = 1;
}
if ((item == "+") || (item == "-"))
{
prior = 2;
}
else if ((item == "*") || (item == "/"))
{
prior = 3;
}
else
{
prior = 4;
}
return prior;
}
int main()
{
stack<string> st;
string output;
cout << "Welcome. This program will calculate any arithmetic expression you enter" << endl;
cout << "Please enter an arithmetic expression below: " << endl;
char line[256];
cin.getline(line, 256);
string exp;
exp = line;
cout << "This is the expression you entered: ";
cout << exp << endl;
string item;
istringstream iss(exp);
iss >> item;
// While there are still items in the expression...
while ( iss )
{
// If the item is a number
if (isdigit('item') == true)
{
output = output + " " + item;
}
// If the stack is empty
else if (st.size() == 0)
{
st.push(item);
}
// If the item is not a number
else if (isdigit('item') == false)
{
// convert that item in the expression to a string
atoi(item.c_str());
int prior1 = 0;
int prior2 = 0;
// If that string is a LEFT PARENTHESIS
if (item == "(")
{
st.push(item);
}
// If that string is a RIGHT PARENTHESIS
else if (item == ")")
{
while ((st.empty() == false) && (st.top() != "("))
{
output = st.top() + " ";
st.pop();
}
st.pop();
}
else
{
// store what is returned from "priority" in "prior1"
prior1 = priority(item);
// pass item on top of the stack through "priority"
// store that in "prior2;
prior2 = priority(st.top());
// while loop here, while item has a higher priority...
// store numbers in a variable
while (prior1 > prior2)
{
output = output + " " + st.top();
st.pop();
prior2 = priority(st.top());
}
}
}
}
while (st.empty() == false)
{
output = st.top() + " ";
st.pop();
}
cout << "Here's the postfix: " << output << endl;
}
This is for evaluating postfix to an actual answer:
// EVALUATE POSTFIX
stack<int> st2;
string output2;
istringstream iss2(exp);
iss2 >> item;
while (iss)
{
// If the item is a number
if (isdigit('item') == true)
{
st.push(item);
}
if (isdigit('item') == false)
{
// store item in a variable
// pop another item off the stack
// store that in a variable.
// apply operator to both.
// push answer to the top of the stack.
int m = 0;
int n = 0;
int total = 0;
m = st2.top();
st2.pop();
n = st2.top();
st2.pop();
if (item == "+")
{
total = m + n;
st2.push(total);
}
else if (item == "-")
{
total = m - n;
st2.push(total);
// add the thing you popped of the stack with the first thing on the stack
}
else if (item == "*")
{
total = m * n;
st2.push(total);
}
else if (item == "/")
{
total = m / n;
st2.push(total);
}
}
}
cout << "Here's your answer: " << st2.top() << endl;
}
Am I thinking about this whole infix-postfix thing the way I should be? Am I on track to solving this problem? My apologies if this question seems simple. I'm kind of a novice at this. Thank you very much.