How to add write to file function with input over RFID?

Question

I am trying to write the ID found over RFID to a file - but when I send the bytes to the file, it gives the wrong ID.

I am adding the write file function to this example - where the ID comes from an RFID tag, and when I open the file, the format is completely different to what I have output to my terminal.

I am working on raspberry pi with 125KHz RFID sheild.

Here is the example code with my add-ons:

   /*  
     *  RFID 125 kHz Module
     *  
     *  Copyright (C) Libelium Comunicaciones Distribuidas S.L. 
     *  http://www.libelium.com 
     *  
     *  This program is free software: you can redistribute it and/or modify 
     *  it under the terms of the GNU General Public License as published by 
     *  the Free Software Foundation, either version 3 of the License, or 
     *  (at your option) any later version. 
     *  a
     *  This program is distributed in the hope that it will be useful, 
     *  but WITHOUT ANY WARRANTY; without even the implied warranty of 
     *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the 
     *  GNU General Public License for more details.
     *  
     *  You should have received a copy of the GNU General Public License 
     *  along with this program.  If not, see http://www.gnu.org/licenses/. 
     *  
     *  Version:           2.0
     *  Design:            David Gascón 
     *  Implementation:    Marcos Yarza & Luis Martin
     */

    //Include ArduPi library
    #include "arduPi.h"


    int led = 13;
    byte data_1 = 0x00;
    byte data_2 = 0x00;
    byte data_3 = 0x00;
    byte data_4 = 0x00;
    byte data_5 = 0x00;
    int val = 0;

    void setup(){
        // Start serial port 19200 bps
        Serial.begin(19200);
        pinMode(led, OUTPUT);

        delay(500);

        // Setting Auto Read Mode - EM4102 Decoded Mode - No password
        // command: FF 01 09 87 01 03 02 00 10 20 30 40 37
        Serial.print(0xFF,BYTE);
        Serial.print(0x01,BYTE);
        Serial.print(0x09,BYTE);
        Serial.print(0x87,BYTE);
        Serial.print(0x01,BYTE);
        Serial.print(0x03,BYTE);
        Serial.print(0x02,BYTE);
        Serial.print(0x00,BYTE);
        Serial.print(0x10,BYTE);
        Serial.print(0x20,BYTE);
        Serial.print(0x30,BYTE);
        Serial.print(0x40,BYTE);
        Serial.print(0x37,BYTE);

        delay(500);
        Serial.flush();

        printf("\n");

        printf("RFID module started in Auto Read Mode\n");

    }

    void loop(){

        printf("Waiting card...\n");
        val = Serial.read();
        while (val != 0xff){
            val = Serial.read();
            delay(1000);
        }

        // Serial.read();    // we read ff
        Serial.read();    // we read 01
        Serial.read();    // we read 06
        Serial.read();    // we read 10
        data_1 = Serial.read();    // we read data 1
        data_2 = Serial.read();    // we read data 2
        data_3 = Serial.read();    // we read data 3
        data_4 = Serial.read();    // we read data 4
        data_5 = Serial.read();    // we read data 5
        Serial.read();    // we read checksum


        // LED blink
        for(int i = 0;i < 4;i++){
            digitalWrite(led,HIGH);
            delay(500);
            digitalWrite(led,LOW);
            delay(500);
        }

        // Printing the code of the card
        printf("\n");
        printf("EM4100 card found - Code: ");
        printf("%x",data_1);
        printf("%x",data_2);
        printf("%x",data_3);    
        printf("%x",data_4);
        printf("%x",data_5);

        printf("\n\n");
    }

int writeFile(){
    ofstream myfile;
    osstringstream mystream;
    myfile.open(example);
    mystream << data_1;
    mystream << data_2;
    mystream << data_3;
    mystream << data_4;
    mystream << data_5;
    myfile << mystream.str();
    myfile.close();
    return 0;
}

int main (){
    setup();
    while(1){
        loop();
        writefile();
    }
    return (0);
}

It compiles and runs, but when I check my example file, it gives a load of rubbish. I have tried a few other tweaks on the write file, but nothing seems to work. I am a bit lost so any help would be amazing!