in reply to Operator Precedence (unary negation and exponentiation)

Python (python -c 'print -2**2') and Ruby (ruby -e 'print -2**2') do it this way, both commands will print -4.

While we are on the subject, what is going on here??

$ perl -le 'print (-2)**2'
-2
$

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Update: B::Deparse helps sometimes:

$ perl -MO=Deparse -e 'print (-2)**2'
print(-2) ** 2;
-e syntax OK

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The following does work as intended:

$ perl -le 'print((-2)**2)'
4

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— Arien

Replies are listed 'Best First'.
Re: Re: Operator Precedence (unary nagation and exponentiation)
by tommyw (Hermit) on Jan 14, 2003 at 11:00 UTC

    It must be a parsing error, I guess: perl -e '$v=-2; print $v**2' correctly produces 4. So my guess is that the parsing in your example is simply throwing away the **2 component.

    Why is another question...

    --
    Tommy
    Too stupid to live.
    Too stubborn to die.

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[d/l]
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      Use -w! 
          $ perl -wle 'print (-2)**2'
    print (...) interpreted as function at -e line 1.
    Useless use of exponentiation (**) in void context at -e line 1.
    -2

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      Just like any other function, if the function name is followed by a left paren, it's taken as the start of the argument list. print (-2)**2 is parsed as (print (-2)) ** 2. This is even documented. From perldoc print: 

      Also be careful not to follow the
print keyword with a left parenthesis unless you
want the corresponding right parenthesis to termi-
nate the arguments to the print--interpose a "+"
or put parentheses around all the arguments.

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      Abigail

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[d/l]
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        Good catch, I should have used -w... sigh

        Arien mumbles something about how anybody can forget to put in switches now and then. ( -f ;-)

        — Arien

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      Try perl -e '$v = 2; print -$v ** 2;'

      This the use of unary negation that is the subject of the thread.

      Examine what is said, not who speaks.

      The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.

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[d/l]

        But if you look at the problem that Arien posed, the answer doesn't just have the incorrect sign, it's also got the wrong magnitude.

        --
        Tommy
        Too stupid to live.
        Too stubborn to die.

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