#include <Keypad.h>
const char rows=4;
const char columns=4;
char row_pins[rows]={9,8,7,6};
char col_pins[columns]={5,4,3,2};
char key_array[rows][columns]={
   { '1', '2', '3', 'A' },
  { '4', '5', '6', 'B' },
  { '7', '8', '9', 'C' },
  { '*', '0', '#', 'D' }
};
Keypad k=Keypad(makeKeymap(key_array),row_pins,col_pins,rows,columns);


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

void loop() {
  char key_pressed=k.getKey();
  if(key_pressed){
    Serial.println(key_pressed);
  }

}