Areas to improve/change? - c++

Question

I'm not too sure if questions of this sort is allowed on stackoverflow but here it goes. I finished up this assignment to calculate reverse polish expressions with a small set of operators. All the tests passed and I have already submitted the assignment. I was wondering if you guys had any suggestions where I could improve on the code that could be applied to my future assignments/projects.
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
class TheStack
{
public:
void push(double d1);
double pop();
int getNodeTracker();
private:
struct Node
{
double data;
Node* next;
};
Node* bottom = NULL;
Node* top = NULL;
Node* newP = NULL;
int nodeTracker = 0;
};
void TheStack::push(double d1)
{
newP = new Node;
newP->data = d1;
newP->next = NULL;
if (bottom == NULL)
{
bottom = top = newP;
top->next = NULL;
}
else
{
newP->next = top;
top = newP;
}
nodeTracker += 1;
}
double TheStack::pop()
{
Node* tempP;
double tempD;
if (top == NULL)
{
cout << "The stack is empty." << endl;
}
else
{
tempP = top;
tempD = tempP->data;
top = top->next;
delete(tempP);
nodeTracker -= 1;
return(tempD);
}
}
int TheStack::getNodeTracker()
{
return nodeTracker;
}
bool isOperand(string s1);
bool isOperator(string s1);
double operate(string s1, double d1, double d2);
int processor(string str);
bool validString(string str);
int main()
{
// getting user input
string str;
getline(cin, str);
// input loop
while (str != "0")
{
processor(str);
cout << endl;
getline(cin, str);
}
// end program
return 0;
}
bool isOperand(string s1)
{
try
{
stod(s1);
}
catch (...)
{
return false;
}
return true;
}
bool isOperator(string s1)
{
string validOps[4] = { "+", "-", "*", "/" };
for (int i = 0; i < 4; i++)
{
if (s1 == validOps[i])
{
return true;
}
}
return false;
}
double operate(string s1, double d1, double d2)
{
char op = s1[0];
switch (op)
{
case '+':
return d1 + d2;
case '-':
return d1 - d2;
case '*':
return d1 * d2;
case '/':
return d1 / d2;
}
}
bool validString(string str)
{
for (int i = str.size()-1; i >= 0; i--)
{
if (str[i] == '=')
{
return true;
}
}
return false;
}
int processor(string str)
{
TheStack stacklist;
istringstream iss(str);
string streamVar;
iss >> streamVar;
// checks if expression has = sign
if (!validString(str))
{
cout << "Error: Please input valid expression." << endl;
return -1;
}
// builds and operates upon stack
while (streamVar != "=")
{
if (isOperand(streamVar))
{
stacklist.push(stod(streamVar));
}
else if (isOperator(streamVar))
{
if (stacklist.getNodeTracker() > 1)
{
double d2 = stacklist.pop();
double d1 = stacklist.pop();
if (streamVar == "/" && d2 == 0)
{
cout << "Error: Division by 0." << endl;
return -1;
}
double result = operate(streamVar, d1, d2);
stacklist.push(result);
}
else
{
cout << "Error: Too many operators." << endl;
return -1;
}
}
iss >> streamVar;
}
// operand error case
if (stacklist.getNodeTracker() > 1)
{
cout << "Error: Too many operands." << endl;
return -1;
}
// output
cout << stacklist.pop() << endl;
return 0;
}