Function to Evaluate Postfix Resulting in a Segfault - c++

Question

I just had a small question about my program. So, I have a function that evaluates a postfix expression and returns the integer or float calculated. Here are the following functions involved:
#include <iostream>
#include <sstream>
#include <cstring>
#include <stack>
#include <limits>
float postfixUtility::evaluatePostfix(string pexp)
{
stack<int> S;
int pexpLength = pexp.length();
cout << pexpLength << endl;
for (int i = 0; i < pexpLength; i++)
{
if(pexp[i] == ' ' || pexp[i] == ',')
{
continue;
}
else if(isOperator(pexp[i]))
{
int operand2 = S.top(); S.pop();
int operand1 = S.top(); S.pop();
int result = isOperate(pexp[i], operand1, operand2);
S.push(result);
}
else if(isDigit(pexp[i]))
{
int operand = 0;
while(i<pexp.length() && isDigit(pexp[i]))
{
operand = (operand*10) + (pexp[i] - '0');
i++;
}
i--;
S.push(operand);
}
}
return S.top();
}
bool postfixUtility::isDigit(char C)
{
if(C >= '0' && C <= '9')
{
return true;
}
return false;
}
bool postfixUtility::isOperator(char C)
{
if(C == '+' || C == '-' || C == '*' || C == '/')
{
return true;
}
return false;
}
int postfixUtility::isOperate(char operation, int operand1, int operand2)
{
if(operation == '+')
{
return operand1+operand2;
}
if(operation == '-')
{
return operand1-operand2;
}
if(operation == '*')
{
return operand1*operand2;
}
if(operation == '/')
{
return operand1/operand2;
}
}
These functions work together to solve the postfix expression inputted. The expression inputted is not empty and actually holds a postfix expression. However, every time I run the code, it results in a segfault. I am quite baffled actually because it seems to me that my code should work.
Thank you!
EDIT #1: So, the original input to my function is: "(4+3* 12)/ ( 12+ 3/ 2+ 46 /4)"
Then, I put that through another function to convert it to postfix. This is that function:
int postfixUtility::priority(char a)
{
int temp;
if (a == '^')
temp = 1;
else if (a == '*' || a == '/')
temp = 2;
else if (a == '+' || a == '-')
temp = 3;
return temp;
}
string postfixUtility::getPostfix(string nexp)
{
stack<char> operator_stack;
stringstream output;
for (unsigned i = 0; i < nexp.length(); i++) {
if (nexp[i] == '+' || nexp[i] == '-' || nexp[i] == '*' || nexp[i] == '/' || nexp[i] == '^') {
while (!operator_stack.empty() && priority(operator_stack.top()) <= priority(nexp[i])) {
output << operator_stack.top();
operator_stack.pop();
}
operator_stack.push(nexp[i]);
} else if (nexp[i] == '(') {
operator_stack.push(nexp[i]);
} else if (nexp[i] == ')') {
while (operator_stack.top() != '(') {
output << operator_stack.top();
operator_stack.pop();
}
operator_stack.pop();
} else {
output << nexp[i];
}
}
while (!operator_stack.empty()) {
output << operator_stack.top();
operator_stack.pop();
}
//cin.ignore(numeric_limits<streamsize>::max(), '\n');
return output.str();
}
Which converts it to: "43 12*+ 12 3 2/+ 46 4/+/", is that wrong? Would that be the reason why I am getting a segfault?
EDIT #2: So, I have commented out 2 lines in my code and I am no longer getting a segfault.
float postfixUtility::evaluatePostfix(string pexp)
{
stack<int> S;
int pexpLength = pexp.length();
for (int i = 0; i < pexpLength; i++)
{
if(pexp[i] == ' ' || pexp[i] == ',')
{
continue;
}
else if(isOperator(pexp[i]))
{
float operand2 = S.top();
//S.pop();
float operand1 = S.top();
//S.pop();
float result = isOperate(pexp[i], operand1, operand2);
S.push(result);
}
else if(isDigit(pexp[i]))
{
int operand = 0;
while(i<pexp.length() && isDigit(pexp[i]))
{
operand = (operand*10) + (pexp[i] - '0');
i++;
}
i--;
S.push(operand);
}
}
return S.top();
}
However, the answer is supposed to be 1.6, but I am getting 1. Any reason why that is happening?
EDIT #3: I changed the isOperate function to the following:
float postfixUtility::isOperate(char operation, float operand1, float operand2)
{
if(operation == '+')
{
return operand1+operand2;
}
else if(operation == '-')
{
return operand1-operand2;
}
else if(operation == '*')
{
return operand1*operand2;
}
else if(operation == '/')
{
return operand1/operand2;
}
}
However, I still get 1 as a result.

Answer 1

Those pops where important:
float operand2 = S.top();
//S.pop();
float operand1 = S.top();
//S.pop();
Without popping the last element, operand1 will always be equal to operand2 and dividing a number by itself usually results in 1.
The reason you where getting a segfault in the first place is that your postfix converter is giving you (very) wrong results (which should be clear from the fact that its output contains numbers not present in its input). Let's have look at the first part of the output you posted: 43 12*+. Your parser will identify 43 and 12 as numbers and push them on the stack. It will identify * as an operator, take the two numbers from the stack, multiply them and push the result on the stack. Then it will encounter + and try to take two operands from the stack. However, there's only one element on the stack, namely the result of the multiplication. Calling top() on an empty stack is causing your segfault.
Edit: It is generally a good idea to do a sanity check before performing a operation that could result in undefined behavior for incorrect input.This allows you to either do something to solve the problem or to produce an error message containing diagnostic information (e.g. in your case the operator, the position in the string, the string itself, etc.). This will help you identify such problems more easily.
tl;dr: Fix your postfix converter.