Turing machine that goes left on first symbol

Question

I have a turing machine with transitions given by the following table

I'm inputting the string aaaa. So if I look at the first symbol "a" in state A, it says to replace it with an X, go into state B, and move Left. This is where I'm confused. How can I move left if I'm looking at the first input symbol? Do I just go to a blank symbol?

Thanks!

Answer 1

It dependes on the variant of the Turing machine you are considering. If you consider the "standard" Turing machine, the tape is infinite for both sides and by moving the header left on the input, you get a blank symbol.

However, if your tape is infinite just for the right side, your algorithm must not make movements to the left after reaching the first symbol. For the sake of simplicity, you can tell that by moving left on the beginning of the tape, you stay on the first input symbol.