This sample code is Solenoid Door Lock Using Keypad Tutorial.

keypad_solenoid.ino

/*
  Project: CUnlock the Soenoid Door Lock Using A Keypad and Maker Nano #ArduinoNano
  Board: Arduino Nano (Maker Nano)

  Connections:
  Nano | Relay
   GND – GND
    5V – VCC
    A0 – SIG
  Nano | Keypad
    11 – pin 1 (most left)
    10 – pin 2
     9 – pin 3
     7 – pin 4
     6 – pin 5
     5 – pin 6
     4 – pin 7
     3 – pin 8
  Nano | I2C LCD
   GND – GND
    5V – VCC
   SDL – SDL
   SCA - SCA

  External libraries:
  – Keypad by Mark Stanley V3.1.1 (Manager)
*/
#include <Keypad.h>
#define Password_Lenght 5 // Give enough room for 4 chars + NULL char
#define PIEZO    8
#define RELAY    A0
#define NOTE_G5  784
#define NOTE_C6  1047

#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 20, 2);


int pos = 0;    // variable to store the servo position

char Data[Password_Lenght]; 
char Master[Password_Lenght] = "5698";
byte data_count = 0, master_count = 0;
char customKey;

const byte ROWS = 4;
const byte COLS = 4;

char keys [ROWS] [COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};
byte rowPins[ROWS] = {6, 5, 4, 3};
byte colPins[COLS] = {11, 10, 9, 7};

Keypad myKeypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS );

int OpenMelody[] = {NOTE_G5, NOTE_C6};
int OpenNoteDurations[] = {12, 8};

int CloseMelody[] = {NOTE_C6, NOTE_G5};
int CloseNoteDurations[] = {12, 8};

#define playOpenMelody() playMelody(OpenMelody, OpenNoteDurations, 2)
#define playCloseMelody() playMelody(CloseMelody, CloseNoteDurations, 2)

char inChar;
String inString;

void setup() {
  pinMode(PIEZO, OUTPUT);
  pinMode(RELAY, OUTPUT);
  digitalWrite(RELAY, HIGH);
  lcd.init();
  lcd.backlight();
  lcd.clear();
  lcd.print(" ENTER PASSWORD");
}

void loop() {
  customKey = myKeypad.getKey();
  if (customKey)                  // makes sure a key is actually pressed, equal to (customKey != NO_KEY)
  {
    tone(8, NOTE_G5, 100);
    Data[data_count] = customKey; // store char into data array
    data_count++;                 // increment data array by 1 to store new char, also keep track of the number of chars entered
  }


  if (data_count == Password_Lenght - 1) // if the array index is equal to the number of expected chars, compare data to master
  {
    if (!strcmp(Data, Master))           // equal to (strcmp(Data, Master) == 0)
    {
      playOpenMelody();
      digitalWrite(RELAY, LOW);
      lcd.clear();
      lcd.print("    DOOR OPEN");
      delay(5000);                       //time for open the door
      lcd.clear();
      lcd.print("    DOOR LOCK");
      digitalWrite(RELAY, HIGH);
      playCloseMelody();
      delay(1000);
      lcd.clear();
      lcd.print(" ENTER PASSWORD");
    }
    else
    {
      lcd.clear();
      lcd.print(" WRONG PASSWORD");
      delay (1000);
      lcd.clear();
      lcd.print(" ENTER PASSWORD");
      playCloseMelody();
    }
    clearData();
  }
}

void clearData()
{
  while (data_count != 0)
  { // This can be used for any array size,
    Data[data_count--] = 0; //clear array for new data
  }
  return;
}

void playMelody(int *melody, int *noteDurations, int notesLength)
{
  pinMode(PIEZO, OUTPUT);

  for (int thisNote = 0; thisNote < notesLength; thisNote++) {
    int noteDuration = 1000 / noteDurations[thisNote];
    tone(PIEZO, melody[thisNote], noteDuration);
    int pauseBetweenNotes = noteDuration * 1.30;
    delay(pauseBetweenNotes);
    noTone(PIEZO);
  }
}