Two Stack Algorithm in C++ - c++

Question

novice coder here and I've been asking around how I should go about creating a two stack algorithm for calculating simple expressions (Dijkstra's Two Stack Algorithm) in C++. A quick refresher for anybody that need it:
Two Stack Algorithm:
Value - Push onto value stack
Operator - Push onto operator stack
Left Parenthesis - Ignore
Right Parenthesis - Pop two values from value stack and one value from operator stack and push the result
It appears that using istringstream, which was recommended to me, should allow me to separate the user inputted expression into basic, doubles, and non-doubles. This should allow me to populate my vals and ops stack respectively, however upon debugging, I realized that my vals stack ended up empty at the end (causing a segmentation fault)
I've got no idea what I'm doing wrong, and any help would be appreciated! Keep in mind I am relatively new to coding and my syntax is probably horrible, therefore any type of criticism is welcome.
For reference an input of:
( 1 + ( ( 2 + 3 ) * ( 4 * 5 ) ) )
Should output:
101
Thus far my code looks like this:
stack<string> ops;
stack<double> vals;
string input;
getline(cin, input);
istringstream scanner(input);
while(true){
double num;
scanner >> num;
if(scanner.fail() && scanner.eof()) break;
else if(!scanner.fail()) vals.push(num);
else{
scanner.clear();
string token;
scanner >> token;
if(token == "(") ;
else if(token == "+") ops.push(token);
else if(token == "*") ops.push(token);
/*Add more operations here (Log, sin, cos...)*/
else if(token == ")"){
string op = ops.top();
ops.pop();
if(op == "+"){
double a, b;
a = vals.top();
vals.pop();
b = vals.top();
vals.pop();
vals.push(a+b);
}
else if(op == "*"){
double a, b;
a = vals.top();
vals.pop();
b = vals.top();
vals.pop();
vals.push(a*b);
}
/*Add more operations here*/
}
}
return vals.top();
}
Thank you for your help!

Answer 1

Turns out the problem was with this:
scanner >> num;
if (scanner.fail() && scanner.eof()) break;
else if (!scanner.fail()) vals.push(num);
Changing it to the following fixed the problem:
if (scanner >> num) vals.push(num);
if (scanner.fail() && scanner.eof()) break;
else {
// ...
}
And putting the return statement below the loop also helped.
Live example

Answer 2

Not a straightforward answer to what you asked, but I reworked your example with out the use of stringstreams, which might be confusing to you. You can use the following code as an alternative:
#include <iostream>
#include <string>
#include <stack>
#include <ctype.h>
double parseExpression(std::string const &expr)
{
std::stack<char> ops;
std::stack<double> vals;
std::string num;
for (auto c : expr) {
if (c == '+' || c == '*') {
if(!num.empty()) vals.push(std::stod(num));
num.clear();
ops.push(c);
} else if (c == ')') {
if (!num.empty()) vals.push(std::stod(num));
num.clear();
char op = ops.top();
ops.pop();
switch (op) {
case '+': {
double tmp = vals.top();
vals.pop();
tmp += vals.top();
vals.pop();
vals.push(tmp);
} break;
case '*': {
double tmp = vals.top();
vals.pop();
tmp *= vals.top();
vals.pop();
vals.push(tmp);
} break;
};
num.clear();
} else if(isdigit(c) || c == '.') {
num.push_back(c);
} else if (isspace(c)) {
if (!num.empty()) vals.push(std::stod(num));
num.clear();
} else if(c != '(') {
throw std::runtime_error("Unknown character in expression!");
}
}
return vals.top();
}
int main()
{
std::string expr("( 1.00 + ( ( 2.000000 + 3.00 ) * ( 4.00 * 5.00 ) ) )");
std::cout << expr << " = " << parseExpression(expr) << std::endl;
return 0;
}
HTH