#include <LiquidCrystal_I2C.h> 
#include <Keypad.h>
LiquidCrystal_I2C lcd(0x27, 20, 4); 

const byte ROWS = 4; //four rows
const byte COLS = 4; //three columns
int count = 0, column = 0, row = 0;
int gap = 0;
char keys[ROWS][COLS] = {
  {'1','2','3', '+'},
  {'4','5','6', '-'},
  {'7','8','9', '*'},
  {'E','0','=', '/'}
};
char temp[6];
char output;
byte rowPins[ROWS] = {5, 4, 3, 2}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {6, 7, 8, 9}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );
char keypadText(char input){    
  
  switch(input){
      case '1':
      if(count==1){
        return '1';
      }
      else if(count==2){
        return 'A';
      }
      else if(count==3){
        return 'B';
      }
      else if(count==4){
        return 'C';
      }
      break;

      case '2':
      if(count==1){
        return '2';
      }
      else if(count==2){
        return 'D';
      }
      else if(count==3){
        return 'E';
      }
      else if(count==4){
        return 'F';
      }
      break;

      case '3':
      if(count==1){
        return '3';
      }
      else if(count==2){
        return 'G';
      }
      else if(count==3){
        return 'H';
      }
      else if(count==4){
        return 'I';
      }
      break;

      case '4':
      if(count==1){
        return '4';
      }
      else if(count==2){
        return 'J';
      }
      else if(count==3){
        return 'K';
      }
      else if(count==4){
        return 'L';
      }
      break;

      case '5':
      if(count==1){
        return '5';
      }
      else if(count==2){
        return 'M';
      }
      else if(count==3){
        return 'N';
      }
      else if(count==4){
        return 'O';
      }
      break;

      case '6':
      if(count==1){
        return '6';
      }
      else if(count==2){
        return 'P';
      }
      else if(count==3){
        return 'Q';
      }
      else if(count==4){
        return 'R';
      }
      break;

      case '7':
      if(count==1){
        return '7';
      }
      else if(count==2){
        return 'S';
      }
      else if(count==3){
        return 'T';
      }
      else if(count==4){
        return 'U';
      }
      break;

      case '8':
      if(count==1){
        return '8';
      }
      else if(count==2){
        return 'V';
      }
      else if(count==3){
        return 'W';
      }
      else if(count==4){
        return 'X';
      }
      break;

      case '9':
      if(count==1){
        return '9';
      }
      else if(count==2){
        return 'Y';
      }
      else if(count==3){
        return 'Z';
      }
      break;

      case '0':
      if(count==1){
        return '0';
      }
      else if(count==2){
        return ' ';
      }
      else{
        return '0';
      }
      break;
      case 'E':
      if(count!=0){
        return 'E';
      }
      break;
      case '=':
      if(count!=0){
        return '=';
      }
      break;
      case '+':
      if(count!=0){
        return '+';
      }
      break;
      case '-':
      if(count!=0){
        return '-';
      }
      break;
      case '*':
      if(count!=0){
        return '*';
      }
      break;
      case '/':
      if(count!=0){
        return '/';
      }
      break;

    }  
}

void setup() { 
lcd.init(); 
lcd.backlight(); 
lcd.setCursor(1, 0); 

} 

void loop(){
  char key = keypad.getKey();
  
  char nextKey;  
  
  if (key != NO_KEY){
    temp[count] = key;
    count++;  
    gap = 0;
    for(int i = count; i > 0; i--){
      if(temp[i-1] == key){
        output = keypadText(key);
      }
    }
    
  }    
  else if(key == NO_KEY){
    gap++;
  }
  
  if(gap >= 8 && count != 0){
    count = 0;
    lcd.print(output); 
    column++;
    if(column>16){
      row++;
      column = 0;
    }      
    lcd.setCursor(column, row);
  }  

  delay(75);
}