Have I achieved a null- terminated C-string through the code?

Projects Programming
39 
#include <Wire.h>
#include <hd44780.h>                       // main hd44780 header
#include <hd44780ioClass/hd44780_I2Cexp.h> // i2c expander i/o class header

hd44780_I2Cexp lcd; // declare lcd object: auto locate & config exapander chip

// LCD geometry
const int LCD_COLS = 16;
const int LCD_ROWS = 2;

#include <Keypad.h>

const byte ROWS = 4; //four rows
const byte COLS = 4; //three columns
char keys[ROWS][COLS] = 
{
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};
byte rowPins[ROWS] = {6, 7, 8, 9}; //connect to the row pinouts of the keypad
byte colPins[COLS] = {5, 4, 12, 11}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );
bool a = true;

void setup()
{
  lcd.begin(LCD_COLS, LCD_ROWS);
  lcd.setCursor(3, 0);
  lcd.print("  PRIYA");
  lcd.setCursor(3, 1);
  lcd.print("ELECTRONICS");
  delay(100);
  for (int positionCounter = 0; positionCounter < 40; positionCounter++)
  {
    lcd.scrollDisplayLeft();
    delay(120);
  }
  delay(800);
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Set Count:");
}

//unsigned long total;
void loop()
{
  const byte entryMaxSize = 5;
  static char digits[entryMaxSize + 1];
  static byte x; // index

  char key = keypad.getKey();

  if (key != NO_KEY) // Check for a valid key.
  {
    switch (key)
    {
      case 'A':
        lcd.clear();
        lcd.setCursor(0, 0);
        lcd.print("Set Count:");
        x = 0; // reset the counter
        a = true;
        break;

      case 'D':
        if (a) 
        {
          if (x == 0)
          {
            invalidCount();
          }
          else 
          {
            lcd.clear();
            lcd.print("Count Set:");
            for (byte i = 0; i < x; i++) 
            {
              lcd.print(digits[i]);
              unsigned long count = atol(digits);
              Serial.print(count);
              a = false;
            }
          }
        }
        break;

      default:
        if (a)
        {
          // if not 5 characters yet
          if (x < entryMaxSize)
          {
            // add key to userinput array and increment counter
            if ( key >= '0' && key <= '9' ) // key is of type char and has a value between 0 and 9 so do something with it.
            {
              digits[x++] = key;
              digits[x] = '\0';
              lcd.print( key ) ;  // new print statement <<<<<<<<<<<<<<<<
              //total = ((count[ 0 ] - '0') * 10000UL  + (count[ 1 ] - '0') * 1000 + (count[ 2 ] - '0') * 100 + (count[3] - '0') * 10 + (count[4] - '0'));
            }
          }
          else 
          {
            countWarning();
          }
        }
        break;
    }
  }
}

void invalidCount()
{
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Invalid Count!!");
  lcd.setCursor(0, 1);
  lcd.print("Press A"); // suggesting the user to enter the count again
  a = false;
}

void countWarning()
{
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("5 Digits Only!!"); // warning for the user if more than permitted digits are entered
  lcd.setCursor(0, 1);
  lcd.print("Press A"); // suggesting the user to enter the count again
  a = false;
}

count doesn't get printed on Serial monitor.