Преобразование строковой переменной в численную и наоборот

  #include <iostream>
using namespace std;
int main()
{
double val1, val2, answer;
char arg;
cout << "Введите первое, второе значение и значение аргумента: ";
while (!(cin >> val1) || !(cin >> val2))
    {
    cout << "Error...";
    return 0;
}
cin >> arg;
    
if (arg == '+')
    {
    answer = val1 + val2;
    cout << "Сумма " << val1 << " и " << val2 << " = " << answer << ';' << endl;
    return 0;
    }
else if (arg == '-')
    {
    answer = val1 - val2;
    cout << "Разность " << val1 << " и " << val2 << " = " << answer << ';' << endl;
    return 0;
    }
else if (arg == '*')
    {
    answer = val1 * val2;
    cout << "Произведение " << val1 << " и " << val2 << " = " << answer << ';' << endl;
     return 0;
    }
else if (arg == '/')
    {
    answer = val1 / val2;
    cout << "Частное " << val1 << " и " << val2 << " = " << answer;
    return 0;
    }
else {[cout << "Error...";
    return 0;
    }
    
  
 
return 0;
}

 #include <iostream>
#include <vector>
#include <string>
using namespace std;
int main()
{
setlocale(LC_ALL, "Russian");
vector<string> words{"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"};
string word = " ";
cout << "Напишите цифру(слово): ";
cin >> word;
if (word == words[0])
    cout << words[0] << " - " << "0;" << endl;
else if (word == words[1])
    cout << words[1] << " - " << "1;" << endl;
else if (word == words[2])
    cout << words[2] << " - " << "2;" << endl;
else if (word == words[3])
    cout << words[3] << " - " << "3;" << endl;
else if (word == words[4])
    cout << words[4] << " - " << "4;" << endl;
else if (word == words[5])
    cout << words[5] << " - " << "5;" << endl;
else if (word == words[6])
    cout << words[6] << " - " << "6;" << endl;
else if (word == words[7])
    cout << words[7] << " - " << "7;" << endl;
else if (word == words[8])
    cout << words[8] << " - " << "8;" << endl;
else if (word == words[9])
    cout << words[9] << " - " << "9;" << endl;
else { cout << "Wrong input..." << endl;
return 0; }
 
return 0;
}