Using tolower() and isalpha() to on a given string and set their output into another string

Question

I want to pass a string to the compiler for example "Mr. Adam, I am Eve" . I want to lower the case of that phrase and remove all the punctuation marks and spaces by using (isalpha), i.e.- afterwards the string should be: mradamiameve which will be stored in another string called result for example. I need help with this. Any suggestions?

This is what I have done so far, which does not work:

#include <iostream>
#include <string>

using namespace std;

int main(int argc, char* argv[]) 
{
    string str = "Mr. Adam, I am Eve";
    string result;
    for (size_t i = 0; i < str.length(); ++i)
    {
        if (isalpha(str[i]))
        {       
            result[i] = str[i];
        }
    }

    // here str should be = "mradamiameve" YET it is still "Mr. Adam, I am Eve" 
    cout << "after using to tolower() and isalpha() str is:" << str << endl; 
    return 0;
}

Answer 1

If you are on c++11 you could use range based loop:

#include <iostream>
#include <cctype>
#include <string>

int main()
{ 
    std::string result, str = "Mr. Adam, I am Eve";

    for (unsigned char c : str)
        if (std::isalpha(c))
            result += std::tolower(c);

    std::cout << result << std::endl;
}