#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

char buf[3];
void setup() {
  lcd.init();
  lcd.backlight();
  for ( int i = 0; i < 40; i++ ) {
    sprintf( buf, "%2d", 2 * i );
    lcd.print(buf);
  }
}

int  n = 0; // scroll counter
bool leftScroll = true;
void loop() {
  if ( leftScroll ) {
    lcd.scrollDisplayLeft(); lcd.scrollDisplayLeft();
    if (n++ > 20) {
      leftScroll = false; n = 20;
    }
  } else {
    lcd.scrollDisplayRight(); lcd.scrollDisplayRight();
    if (n-- < 0) {
      leftScroll = true;  n = 0;
    }
  }
  delay(1000);
}