Implementation of a function that evaluates simple mathematical expressions. (C++)

Question

I am trying to write a function that evaluates simple mathematical expressions (only four operations). I used stack and vector to do this. But stack operations don't behave as I expect. I couldn't find the cause. I am open to different solutions.

The function should take a string like this:

"5 * 44 + 3 / 2 * 4 - 12"

And return the result as a double.

#include <iostream>
#include <vector>
#include <stack>
#include <string>
#include <cstdlib>

using namespace std;

vector<string> split(const std::string& str, char delim = ' ')
{
    vector<string> elements;
    stringstream ss(str);
    string token;
    while (getline(ss, token, delim)) {
        elements.push_back(token);
    }

    return elements;
}


double evaluate(string operation)
{
    vector<string> values = split(operation, ' ');
    stack<string> result_stack;
    double result = 0;

    for(unsigned int i = 0; i < values.size(); i++){
        if(values[i] == "*"){
            double mini_result = stod(result_stack.top()) * stod(values[i+1]);
            result_stack.pop();
            i++;
            result_stack.push(to_string(mini_result));
        }
        else if(values[i] == "/"){
            double mini_result = stod(result_stack.top()) / stod(values[i+1]);
            result_stack.pop();
            i++;
            result_stack.push(to_string(mini_result));
        }
        else{
            result_stack.push(values[i]);
        }
    }

    for(unsigned int i = 0; i<result_stack.size(); i++){
        if(result_stack.top() == "-"){
            result_stack.pop();
            result = stod(result_stack.top()) - result;
            result_stack.pop();
        }
        else if(result_stack.top() == "+"){
            result_stack.pop();
            result += stod(result_stack.top());
            result_stack.pop();
        }
        else{
            result += stod(result_stack.top());
            result_stack.pop();
        }
    }

    return result;

}




int main()
{

    cout<<evaluate("5 * 44 + 3 / 2 * 4 - 12");
}

Before the second for loop, values in the result_stack should be like this for this example. "12 | - | 6 | + | 220" . And the returning value should be 214.

But before the second for loop, stack contains only "12 | - | 6" values. "+" and "220" values are not there. Some extra pops occur which I don't expect.

stack content should be like this for this example

Answer 1

Answered here: answer of the question

Completed code is here:

#include <iostream>
#include <vector>
#include <sstream>
#include <string>
#include <stack>

using namespace std;

string trim(const string& str)
{
    size_t first = str.find_first_not_of(' ');
    if (string::npos == first)
    {
        return str;
    }
    size_t last = str.find_last_not_of(' ');
    return str.substr(first, (last - first + 1));
}

vector<string> split(const std::string& str, char delim = ' ')
{
    vector<string> elements;
    stringstream ss(str);
    string token;
    while (getline(ss, token, delim)) {
        elements.push_back(token);
    }

    return elements;
}

double evaluate(string operation)
{
    vector<string> values = split(operation, ' ');
    vector<string> result_vector;
    double result = 0;

    for(unsigned int i = 0; i < values.size(); i++){
        if(values[i] == "*"){
            double mini_result = stod(result_vector.back()) * stod(values[i+1]);
            result_vector.pop_back();
            i++;
            result_vector.push_back(to_string(mini_result));
        }
        else if(values[i] == "/"){
            double mini_result = stod(result_vector.back()) / stod(values[i+1]);
            result_vector.pop_back();
            i++;
            result_vector.push_back(to_string(mini_result));
        }
        else{
            result_vector.push_back(values[i]);
        }
    }

    auto iterator = result_vector.begin();
    while(iterator != result_vector.end()){
        if(*iterator == "-"){
            iterator++;
            result -= stod(*iterator);
        }
        else if(*iterator == "+"){
            iterator++;
            result += stod(*iterator);
        }
        else{
            result += stod(*iterator);
        }
        iterator++;
    }
    return result;
}


int main()
{
    cout<<evaluate("5 * 44 + 3 / 2 * 4 - 12");
}