Converting calculation contained in string to integer - C++

Question

I was wondering if it would be possible to store caluclations contained in a string to integer. For example:

#include <iostream>
#include <string>
using namespace std;
int main()
{
string variable = "-123+432";
cout << stoi(variable) << endl;
}

This returns -123, would it be possible to make it return 309? Also what if there would be more calculations inside the string (eg. "-123+432*2"), and the person writing program would not know how many calculations will there be in a string (eg. string would be entered by user when the programm is runing).
Thanks for all answers

You are going to wat to read up on reverse polish notation and the shunting yard algorithm. That will help you build a parser that can tokenize the expression and allow you to evaluate it. — NathanOliver, Nov 15 '21 at 17:33
The C++ standard library does not include runtime evaluation of mathematical expressions expressed in a string. You would need to find a library that can do this or write your own. — Drew Dormann, Nov 15 '21 at 17:35

score 1 · Accepted Answer · edited Nov 15 '21 at 17:40

It's possible for sure, but it's quite complicated to parse arbitrary strings, work out if they contain a valid mathematical expression and then work out the result.

Unless you are wanting to implement the solution yourself for fun, I would suggest looking up and using a 3rd party library that evaluates string expressions, for example https://github.com/cparse/cparse

This would allow you to do something like (probably not exactly correct, just for rough example):

int main()
{
string variable = "-123+432";
std::cout << calculator::calculate(variable, &vars) << std::endl;
}

If you are wanting to do this yourself for fun, I suggest you look up "Expression evaluation" and start from there. It's quite a large and complicated topic but a lot of fun to write your own evaluator imo.

Converting calculation contained in string to integer - C++

1 Answers1