Re: Operator Precedence (unary nagation and exponentiation)

[Arien]

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
$

Update: B::Deparse helps sometimes:

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

The following does work as intended:

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

— Arien

[tommyw]

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.

[Abigail-II]

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

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.

Abigail

[Arien]

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

[BrowserUk]

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.

[tommyw]

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.