I cant read from files to confirm username - c++

Question

I'm trying to make a program where you have to login with username (to make it simple i'm first using username, i will add password some time later)
To make it work i want to write the username to a file (which i will encrypt later) and when you want to login, it will check through the file if the username is in fact correct. But when i type in a faulty username, it automatically logs in and dosen't ask to make a new account. Same thing if i do login with the correct username. What am i doing wrong?
(i'm pretty new and this is my first half-decent program so please don't be too harsh if i'm doing something obviously wrong.)
Soo. I did some of those things i understood. This is what i got now:
BUT, now it won't write the new username to the Usernames_and_passwords file.
i am incredibly confused...
#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <string>
#include <fstream>
using namespace std;
string user_input;
string birth_year;
string user_age;
string current_user_profile, current_user_password;
string username, password;
string newprofile;
string makenewusername, makenewpassword;
void TalkToAi() { //VOID TALKTOAI
while (true) {
cout << "write something: ";
cin >> user_input;
transform(user_input.begin(), user_input.end(), user_input.begin(), ::tolower); //TRANSLATES ALL UPPERCASE LETTERS TO LOWER CASE SO THE SYSTEM CAN UNDERSTAND!
cout << user_input << "\n"; //###FOR TESTING PORPOSES!!!###
//IF LIBRARY!
if (user_input == "what's my age?" || user_input == "count my age" || user_input == "whats my age?") {
//###CONTINUE HERE!!!###
}
}
}
void StartUp() { //VOID UPONSTARTUP (WHAT TO DO, ALSO READS DIFFRENT PROFILES AND PASSWORDS FOR LOGIN)
cout << "what profile should i load?" << "\n" << "profile name: ";
cin >> current_user_profile;
fstream Myfile;
Myfile.open("Usernames_and_passwords.txt");
if (Myfile.is_open()) {
while (getline (Myfile, username) ) {
if (username == current_user_profile) {
cout << "\n" << "Hello, " << username << "\n";
break;
}
}
if (username != current_user_profile) {
cout << "wrong username or username unfortunately not found.\n";
cout << "shall i create a new profile? Yes/No: ";
cin >> newprofile;
if (newprofile == "Yes" || newprofile == "yes") {
cout << "new profile username: ";
cin >> makenewusername;
Myfile << makenewusername << endl;
}
}
}
}

Answer 1

It won't write to the file because once you finish reading the file with getline(), the eof flag will be set. You need to open the file using:
Myfile.open("Usernames_and_passwords.txt",fstream::in | fstream::out | fstream::app);
This tells the program to open the file for reading and writing. fstream::app tells the program to append text to the end of the file.
Then, in order to reset once an eof has been hit you can do
Myfile.clear();
Myfile.seekg(0, ios::beg);
This will clear the eof flag, and move the pointer back to the beginning of the file.
After this you can write to the file.
A few other remarks:
Your loop is broken: it won't work if the file is empty and it will write duplicate usernames if the entered username is on the second line instead of the first.
Here is a modified version of your function:
void StartUp() { //VOID UPONSTARTUP (WHAT TO DO, ALSO READS DIFFRENT PROFILES AND PASSWORDS FOR LOGIN)
cout << "what profile should i load?" << "\n" << "profile name: ";
cin >> current_user_profile;
fstream Myfile;
Myfile.open("Usernames_and_passwords.txt",fstream::in | fstream::out | fstream::app);
if (Myfile.is_open()) {
while (getline (Myfile, username) ) {
if (username == current_user_profile) {
cout << "\n" << "Hello, " << username << "\n";
return;
}
}
cout << "wrong username or username unfortunately not found.\n";
cout << "shall i create a new profile? Yes/No: ";
cin >> newprofile;
if (newprofile == "Yes") {
cout << "new profile username: ";
cin >> makenewusername;
Myfile.clear();
Myfile.seekg(0, ios::beg);
Myfile << makenewusername << endl;
}
}
}
I would also suggest to let StartUp return a boolean value on whether or not the operation succeeded, so you can decide to terminate the program. E.g. if the user entered "No".