working example

#include <fstream>
#include <iostream>
#include <string>
#include <thread>


int main()
{
    // Loop for ever, so user can enter pins to search for them in a file
    while(true)
    {
        // User Input
        std::cout << ("Please enter your PIN: ");
        std::string pin;
        std::cin >> pin;

        // Validate user input
        // if this fails, the loop will continue to the start!!! *IMPORTANT*
        if(pin.length() != 4) {
            std::cout << "\033[2J\033[1;1H";
            std::cout << "Your pin is to long/short/incorrect. Please try again\n";
            std::this_thread::sleep_for(std::chrono::seconds(3));
            continue;
        }

        // If we got here, the above verifcation was successful!! *IMPORANT*
        // Open the file to begin a search for the pin entered above **IMPORATNT**
        std::ifstream file_in("file.txt");

        // Search file for pin **STARTING AT BEGINNING**
        while (file_in)
        {

            // Read next line from file
            std::string line;
            std::getline(file_in, line);

            // Check if the above getline failed if we have read the entire file
            // When we have completed reading the whole file, the eof() END_OF_FILE
            // flag is set. So we cant read any more data. EXIT THE LOOP - BREAK.
            if(file_in.eof()) { break; }


            // if we got here, the above getline DIDNT fail. so there is data in the
            // line string. Print it.

            // Print the line
            std::cout << line << "\n";
        }

        // Close file after search is concluded!!!! **IMPORATNT**
        file_in.close();
    }


    return 0;
}