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

Your operands for the subtraction operation are swapped. This because when you process the stack for the + and - operations you're parsing the equation from right-to-left.
With your example, when the - is found in the stack, result is 12. You pop the -, then subtract the 6, leaving 6 in result when it should be -6.
You should use
result = stod(result_stack.top()) - result;
for subtraction.
There is also no error checking in your code for invalid equations.

Answer 2

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");
}