Need some help in understanding some codes - arduino

Question

I'm using RFID RC522 module in Arduino what the code does is, whenever the RFID tag is close to the reader it will read the tag no, with the current timestamp. But i need help in understanding the code line by line. I've understood a few lines which commented in the code but the rest i need help. Thank you
#include <RFID.h>
#include <SPI.h>
#define SS_PIN 10
#define RST_PIN 9
RFID rfid(SS_PIN, RST_PIN);
int serNum[4];
String cardno;
int interval = 15000; // millisec
long now = 0;
long lasttime = millis(); //millis() no.of millisec the sketch was runnning
String readerID = "100"; // This is the reader ID
void setup() {
Serial.begin(9600); //setting data rate in bits per second 9600
SPI.begin();
rfid.init();
}
void loop() {
now = millis();
if (rfid.isCard()) {
if (rfid.readCardSerial()) {
lasttime = now;
cardno = String(rfid.serNum[0]) +
String(rfid.serNum[1]) +
String(rfid.serNum[2]) +
String(rfid.serNum[3]) +
String(rfid.serNum[4]);
Serial.print(readerID);
Serial.print(":");
Serial.println(cardno); //printing the cardno in the serial monitor
}
}
rfid.halt();
delay(1000);
}

Answer 1

There are unused variables in your code. Let's get rid of them so that it is less confusing. I also added comments that explain the if statements.
#include <RFID.h>
#include <SPI.h>
#define SS_PIN 10
#define RST_PIN 9
RFID rfid(SS_PIN, RST_PIN);
String cardno;
String readerID = "100";
void setup() {
Serial.begin(9600);
SPI.begin();
rfid.init();
}
void loop() {
if (rfid.isCard()) { // Look for a card. If found, return true.
if (rfid.readCardSerial()) { // Read the serial number of the card. if successful, return true.
cardno = String(rfid.serNum[0]) +
String(rfid.serNum[1]) +
String(rfid.serNum[2]) +
String(rfid.serNum[3]) +
String(rfid.serNum[4]);
Serial.print(readerID);
Serial.print(":");
Serial.println(cardno);
}
}
rfid.halt();
delay(1000);
}
Guessing from your comment, I think you want to understand how RFID class is implemented. I suggest looking at RFID.h and RFID.cpp.
RFID class has an array called serNum. My guess is that when you call readCardSerial(), an instance of RFID tries to store a card number in this array. If the operation is successful, it returns true.