** operator in perl

irah has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: ** operator in perl by chromatic (Archbishop) on Feb 03, 2009 at 05:21 UTC
According to perlop, the exponentiation operator has a higher precedence than unary minus, so you need parentheses to disambiguate your expected precedence: `print (((-2)**4)+1)` (The extra parentheses denote the entire mathematical operation as the single argument to print.)	[reply] [d/l]
Re^2: ** operator in perl by Random_Walk (Prior) on Feb 03, 2009 at 12:55 UTC
I was puzzled by this one too ... `[me@ptmr]/home/me$ perl -le 'print -2 4 ' # binds before - -16 [me@ptmr]/home/me$ perl -le 'print (-2) 4 ' # this puzzled me -2 [me@ptmr]/home/me$ perl -le 'print ((-2) 4)' # parenthesis force wh +at I want 16` [download] but I guess print is handling the second case as a list followed by some irrelevance, do I have that about right ? Update: Thanks Fletch now I have been enlightened! I last read perlfunc a long time ago and had not really absorbed it all at the time. Now I see the explanation `print (something) something_else` gives the warning `print (...) interpreted as function` because it looks like a function. Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply] [d/l] [select]
Re^3: ** operator in perl by Fletch (Bishop) on Feb 03, 2009 at 13:16 UTC
print is a list operator, so the second parses as `(print(-2))4` and will produce a warning if you run with `-w`/warnings (`Useless use of exponentiation () in void context at -e line 1.`). Update: I mean it's not like perlfunc uses `print` as the example of precedence issues that can come up with list operators right in the second paragraph or anything . . . The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^2: ** operator in perl by deprecated (Priest) on Feb 03, 2009 at 20:58 UTC
I was taught to use a plus to disambiguate: `print +( ... )` is that no longer accepted? -- Tilly is my hero.	[reply] [d/l]
Re^3: ** operator in perl by TimToady (Parson) on Feb 03, 2009 at 23:30 UTC
I was taught to use a plus to disambiguate: `print +( ... )` is that no longer accepted? That's still fine in Perl 5, but Perl 6 disambiguates it on the basis of the whitespace alone, which is what most newcomers seem to expect. Also, unary `+` actually means something in Perl 6, and wouldn't work anymore for the purpose of doing nothing. So the Perl 5 usage is deprecated in that sense. But only if you believe in Perl 6... `:-)` Actually, what most people expect is for Perl to read their minds, but we've been having some trouble implementing that part, so we may put it off till Perl 7.	[reply]
Re^4: ** operator in perl by shmem (Chancellor) on Feb 09, 2009 at 01:25 UTC
Re^4: ** operator in perl by deprecated (Priest) on Feb 03, 2009 at 23:32 UTC
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Re^4: ** operator in perl by Lawliet (Curate) on Feb 04, 2009 at 00:08 UTC
Re: ** operator in perl by moritz (Cardinal) on Feb 03, 2009 at 08:26 UTC
Even in mathematics the exponentiation binds tighter than a sign. I write thinks like -e^{i k x} all the time, and it always means that I apply the exponentiation first.	[reply]
Re: ** operator in perl by ww (Archbishop) on Feb 03, 2009 at 10:49 UTC
To date, irah, all your questions could have been readily answered using the documentation (see `$ perldoc perldoc` at your local command prompt for a summary; the documentation referenced in perlman:perlfaq; or a decent book (say, for example, the Llama. You may want to try those.	[reply] [d/l]
Re: ** operator in perl by lakshmananindia (Chaplain) on Feb 03, 2009 at 05:46 UTC
I think it is a repetition. refer Confusion on **operator	[reply]
Re: ** operator in perl by Anonymous Monk on Feb 03, 2009 at 08:21 UTC
Hint, check with B::Deparse, even though constants get in the way `D:\>perl -MO=Deparse,-p -e " print -2 4 + 1; " print((-15)); -e syntax OK` [download] just substitute them variables, and you can see `D:\>perl -MO=Deparse,-p -e " print -$two $four + $one; " print(((-($two ** $four)) + $one)); -e syntax OK D:\>` [download]	[reply] [d/l] [select]
Re: ** operator in perl by swampyankee (Parson) on Feb 03, 2009 at 12:59 UTC
I'm going to be more verbose. You seem to be thinking that unary minus binds more tightly than exponentiation under normal arithmetic rules. It doesn't. You're expecting it to parse as (-2)⁴ - 1 this is a misconception I've had to deal with often when tutoring community college students. By the normal precedence rules of arithmetic, it will actually be evaluated -(2⁴) - 1. Perl (and most other programming languages) try to replicate the normal arithmetic precedence rules as closely as possible. Remember, under normal arithmetic rules, exponentiation (the ** operator in Perl) binds more closely than multiplication (there's an implied multiplication in -2; it's shorthand for (-1) * 2), which binds more tightly than addition. Information about American English usage here and here. Floating point issues? Please read this before posting. — emc	[reply]
Re: ** operator in perl by poolpi (Hermit) on Feb 03, 2009 at 09:20 UTC
This core module may help you : Math::BigInt `#!/usr/bin/perl use strict; use warnings; use Math::BigInt lib => 'GMP'; # (((-2)**4)+1) my $x = Math::BigInt->new('-2'); print $x->bpow(4)->badd(1)->bstr(); __END__ Output: 17` [download] hth, PooLpi 'Ebry haffa hoe hab im tik a bush'. Jamaican proverb	[reply] [d/l]
Re: ** operator in perl by repellent (Priest) on Feb 05, 2009 at 00:42 UTC
Dominus's Precedence Problems provides a great explanation about the importance of operator precedence in Perl 5, and why it's laid out as-is.	[reply]


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