How to stop a while loop

Question

This while loop never ends. For example, when i enter a wrong password it will keep on going to the \"incorrect password\" part over and over again.

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Answer 1

First, read the correct usernames and passwords from the file and save them in memory. That way, if the username/password validation fails the first time, you don't have to reopen or seek-to-beginning-in the file before checking on the next username/password typed by the user....

std::map usernames_passwords; std::string username, password;

if (ifstream in("UsernamePassword.txt"))
    while (in >> username >> password)
        usernames_passwords[username] = password;
else
{
    std::cerr << "Unable to open username/password file\n";
    exit(EXIT_FAILURE);
}

Then prompt for login details until they're valid:

bool logged_in = false;
while (!logged_in &&
       std::cout << "\n\n\n    Please enter username: " &&
       std::cin >> username &&
       std::cout << "    Please enter password: " &&
       std::cin >> password)
{
    // look for a match in the earlier-read login details...
    auto it = usernames_passwords.find(username);
    logged_in = it != std::end(usernames_passwords) && it->second == password;
}

// the while loop could also exit because "cin >>" failed, indicating EOF
// that could be because the user typed e.g. ^D (UNIX) or ^Z (Windows), or
// input was from a redirected file or pipe or over a network connection...

if (!logged_in)
{
    std::cerr << "error reading login details from stdin\n";
    exit(EXIT_FAILURE);
}

...ok - we know the username/password are good - do whatever else...