Reverse polish notation

#include <iostream>
#include <string>
#include <algorithm>
#include <unordered_map>
#include <stack>

using namespace std;

bool isOperator(char c)
{
    return c == '+' || c == '-' || c == '*' || c == '/' || c == '(' || c == ')';
}

string convert(string expr)
{
    stack<char> temp;
    string result = "";

    expr.erase(remove_if(expr.begin(), expr.end(), isspace), expr.end());

    for (char ch : expr)
    {
        switch (ch)
        {
        case '+':
        case '-':
        {
            while (!temp.empty() && temp.top() != '(')
            {
                result.push_back(temp.top());
                temp.pop();
            }

            temp.push(ch);
            break;
        }
        case '/':
        case '*':
        {
            while (!temp.empty() && temp.top() != '(' && temp.top() != '+' && temp.top() != '-') {
                result.push_back(temp.top());
                temp.pop();
            }

            temp.push(ch);
            break;
        }
        case '(': {
            temp.push('(');
            break;
        }
        case ')':
        {
            while (!temp.empty())
            {
                char t = temp.top();
                temp.pop();

                if (t != '(')
                {
                    result.push_back(t);
                }
                else break;
            }

            break;
        }
        default:
            result += ch;
        }
    }

    while (!temp.empty())
    {
        result.push_back(temp.top());
        temp.pop();
    }

    return result;
}

double eval(string expression, unordered_map<char, double> vars)
{
    stack<double> values;

    for (char c : expression)
    {
        if (!isOperator(c))
        {
            values.push(vars[c]);
        }
        else
        {
            double v1 = values.top();
            values.pop();
            double v2 = values.top();
            values.pop();

            switch (c)
            {
            case '+': values.push(v2 + v1); break;
            case '-': values.push(v2 - v1); break;
            case '*': values.push(v2 * v1); break;
            case '/': values.push(v2 / v1); break;
            default:
                cerr << "Error!";
            };
        }
    }

    return values.top();
}

int main()
{
    unordered_map<char, double> vars;
    string input; // test = "( 8 + 2 * 5 ) / ( 1 + 3 * 2 - 4 )";

    cout << "Enter string: " << endl;
    cin >> input;

    for (char c = 'a'; c <= 'e'; c++)
    {
        cout << c << "=";
        cin >> vars[c];
    }

    string rpn = convert(input);

    std::cout << rpn << endl;
    std::cout << eval(rpn, vars);
}