How do I manage flow control with no ELSE statement? (on Turing machine)

Question

I have been given the problem of writing a turing machine with the commands:

if, while, whileNot, read X, write X, goLeft, goRight, HALT

The problem was simply "calculate a two's complement number, from number with \$ on the left and # on the right, ex: $10110100#"

I started by writing this pseudocode to flip the bits:

whileNot read #
    if read 1
        write 0
    if read 0
        write 1
    goRight

But then I realized once 'write 0' executes, 'if read 0' will immediately be true and undo my work. I need a statement that is effectively 'else if read 0', is there any turing trickery that I can employ to do something like this?

Answer 1

The following code should work:

whileNot read #
    while read 1
        write 0
        goRight
    while read 0
        write 1
        goRight

As for implementing IF-THEN-ELSE in general, it depends on the exact semantics of your programming language, which you haven't specified.