our code from an arduino workshop done by Maxlinear

ArduinoWorkshop.ino

#include <Keypad.h>

const byte ROWS = 4; //four rows
const byte COLS = 4; //four columns
//define the cymbols on the buttons of the keypads
char hexaKeys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};
byte rowPins[ROWS] = {9, 8, 7, 6}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {5, 4, 3, 2}; //connect to the column pinouts of the keypad

// generate a four digit number
int secretRandomNumber[4];

int entry[4];
int digitsEntered = 0;

int bulls = 0;
int cows = 0;

/**
 * 0 - unmatched
 * 1 - bulls
 * 2 - cows
 */
int bullCowChecker[8];

//initialize an instance of class NewKeypad
Keypad customKeypad = Keypad( makeKeymap(hexaKeys), rowPins, colPins, ROWS, COLS); 

void setup(){
  Serial.begin(9600);
  randomSeed(analogRead(A1));
  for (int i=0; i<4; i++) secretRandomNumber[i] = random(0, 9);
  
  // output the answer
  for (int i=0; i<4; i++) Serial.println(secretRandomNumber[i]);

  Serial.println("Guess the four digit number!");
}
  
void loop(){
  char customKey = customKeypad.waitForKey();
  
  if (customKey){
    Serial.print("You pressed: ");
    Serial.println(customKey);
    entry[digitsEntered] = customKey - 48;
    digitsEntered++;
    
    
    if (digitsEntered == 4) {
      // WE DO IT ALL AT ONCE!!!!!
      for (int i=0; i<4; i++) {
        for (int j=0; j<4; j++) {
          if (bullCowChecker[i] != 0) break;
          if (bullCowChecker[4+j] != 0) continue;
 
          if (entry[i] == secretRandomNumber[j]) {
            if (i == j) {
              bullCowChecker[i] = 1;
              bullCowChecker[4+j] = 1;
            }
          }
        }
      }

      for (int i=0; i<4; i++) {
        for (int j=0; j<4; j++) {
          if (bullCowChecker[i] != 0) break;
          if (bullCowChecker[4+j] != 0) continue;
 
          if (entry[i] == secretRandomNumber[j]) {
            bullCowChecker[i] = 2;
            bullCowChecker[4+j] = 2;
          }
        }
      }

      int cows = 0;
      int bulls = 0;
      for (int i=0; i<4; i++) {
        Serial.println(bullCowChecker[i]);
        if (bullCowChecker[i] == 1) bulls++;
        if (bullCowChecker[i] == 2) cows++;
      }

      // print guess, bulls, cows
      Serial.print("Your guess: ");
      for (int i=0; i<4; i++) Serial.print(entry[i]);
      Serial.println();

      Serial.print("Bulls: ");
      Serial.println(bulls);

      Serial.print("Cows: ");
      Serial.println(cows);
      
      digitsEntered = 0;
      for (int i=0; i<4; i++) {
        bullCowChecker[i] = 0;
      }
    }
  }
   
}