I've seen videos on youtube about how to emulate a keypad on arduino. Some of them includes a resistor while others do not. What are the difference between them and are there advantages in doing one over another?
Since the Atmega has internall pull-up resistor (50k) on Pins there's no need to use external resistors until you need to meet some special criteria. Just enable pull-up resistor on Pin you are going to use, connect button between this Pin and Gnd and look for signal Low in software.