Postfix Notation Calculator (RPN) Problems C++ - c++

Question

Hate coming back for help after getting help on the same program just a few days ago, but I am really struggling to finish this program. In short, I need to create a postfix notation calculator (RPN) with a linked list stack, that allows me to do expressions such as 5 5 5 + + (=15). I've managed to get the main calculation part down now, but I am struggling with handling two of the errors. One of which is "too many operators," and the other, "too many operands." Currently working on "too many operators" and I feel like I am close but can't quite get there.
If the user enters 5 5 + + on the first entry, it catches it and says "too many operands." However, if something is already in the stack from a previous calculation, and they then type the same expression 5 5 + +, it is not saying that the stack is empty, and is instead outputting an answer with the previous number being used. If anyone can see where I am going wrong here, and also point me in a direction for figuring out the other error "too many operators" (ex: 5 5 5 +) that'd be greatly appreciated. Thanks again in advance.
(After messing with it more, it seems the more calculations I do, the more operators actually need to be put in place to be considered empty. I'm guessing I need to popVal somewhere before each expression but not sure where to put it, as I've tried many places and it's not working)
#include<iomanip>
#include<iostream>
#include<string>
#include<sstream>
using namespace std;
class SLLNode
{
double data;
SLLNode *top;
SLLNode *ptr;
public:
SLLNode()
{
top = NULL;
ptr = NULL;
}
bool isEmpty()
{
return top == 0;
}
void pushVal(double val)
{
SLLNode *next = new SLLNode;
next -> data = val;
next -> ptr = top;
top = next;
}
double popVal()
{
if (isEmpty())
{
cout << "Error: Too many operators" << endl;
}
else
{
SLLNode *next = top -> ptr;
double ret = top -> data;
delete top;
top = next;
return ret;
}
}
void print()
{
cout << top -> data << endl;
}
};
bool isOperator(const string& input)
{
string ops[] = {"+", "-", "*", "/"};
for(int i = 0; i < 4; i++)
{
if(input == ops[i])
{
return true;
}
}
return false;
}
void performOp(const string& input, SLLNode& stack)
{
double fVal, sVal;
int errorCheck = 0;
sVal = stack.popVal();
fVal = stack.popVal();
if(input == "+")
{
stack.pushVal(fVal + sVal);
}
else if(input == "-")
{
stack.pushVal(fVal - sVal);
}
else if(input == "*")
{
stack.pushVal(fVal * sVal);
}
else if(input == "/" && sVal != 0)
{
stack.pushVal(fVal / sVal);
}
if(input == "/" && sVal == 0)
{
cout << "Error: Division by zero" << endl;
errorCheck = 1;
}
if(errorCheck == 0)
{
stack.print();
}
}
int main()
{
cout << "::::::::::::::::RPN CALCULATOR:::::::::::::::::" << endl;
cout << "::TYPE IN A POSTFIX EXPRESSION OR 'q' TO QUIT::" << endl;
cout << ":::::::::::::::::::::::::::::::::::::::::::::::" << endl << endl;
string input;
SLLNode stack;
while(true)
{
cin >> input;
double num;
if(istringstream(input) >> num)
{
stack.pushVal(num);
}
else if (isOperator(input))
{
performOp(input, stack);
}
else if (input == "q")
{
return 0;
}
}
}

Answer 1

The basic idea is to:
Read one line (std::getline);
Process this line (std::stringstream);
Output the answer or any errors;
Clean the stack (or destroy it and create a new one on step 2);
Go to 1 and repeat.
What you are missing is the first step. If you get everything directly from stdin, you will treat the newline as a simple whitespace.