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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?

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  • $\begingroup$ Hm, this is borderline between simulation proof and programming. Community votes, please! $\endgroup$ – Raphael Apr 14 '15 at 15:21
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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.

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  • $\begingroup$ Yeah, the general "else implementation" question is probably beyond the scope of my assignment. Your solution is much more elegant than what I was trying. $\endgroup$ – Daniel Guenther Apr 14 '15 at 15:49

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