in reply to Re^2: Boolean counter?
in thread Boolean counter?

The behaviour of that is actually not defined. You may end up with $Counter being -1, or with purple daemons coming out of your USB port, ready to chew off your fingers.

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Re^4: Boolean counter?
by Anonymous Monk on Dec 07, 2009 at 16:15 UTC
    $i appears only once, the behaviour is defined
    $ perl -MO=Deparse,-p -le"print - --$i" BEGIN { $/ = "\n"; $\ = "\n"; } print((-(--$i))); -e syntax OK
    You were thinking of
    print(-( ++$i - --$i ))

      While the operand evaluation order for the ops you've used in your example isn't defined, it doesn't matter (unless there's an overflow) for that example. It's just a clarity issue.

      $i appears only once, the behaviour is defined

      Except it ($Counter) appeared twice.

Re^4: Boolean counter?
by ikegami (Pope) on Dec 07, 2009 at 16:09 UTC
    What about it is not defined?

      duh, you're referring to the fact that he's assigning to the variable he just modified. I was still asleep.

      While such constructs should be avoided over clarity concerns, it's wrong to say it's undefined. The precedence and evaluation order for that statement is actually clearly defined in perlop.

        Quoting perlop:
        Note that just as in C, Perl doesn’t define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying +a variable twice in the same statement will lead to undefined behaviour. Avoid statements like: $i = $i ++; print ++ $i + $i ++; Perl will not guarantee what the result of the above statements is.
        To me, that seems that perlop quite explicitly says the behaviour is undefined.

        Perhaps you have an other reading of perlop which defines the order?