#include <Keypad.h> //This includes the Keypad library, which simplifies working with matrix keypads.

const uint8_t ROWS = 4;
const uint8_t COLS = 4; //Defines a 4x4 keypad — 4 rows and 4 columns.

/*uint8_t stands for:
u = unsigned
int = integer
8 = 8 bits
_t = type (a naming convention)

uint8_t x = 100;
unsigned char x = 100;
*/

char keys[ROWS][COLS] = {
  { '1', '2', '3', 'A' },
  { '4', '5', '6', 'B' },
  { '7', '8', '9', 'C' },
  { '*', '0', '#', 'D' }
};

uint8_t colPins[COLS] = { 5, 4, 3, 2 }; 
uint8_t rowPins[ROWS] = { 9, 8, 7, 6 }; 

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS); //This line initializes the keypad object


void setup() {
   Serial.begin(9600); 
}

void loop() {
  char key = keypad.getKey();  // Get key pressed

  if (key != NO_KEY) {
    Serial.println(key);  
  }

}