What does "! !" do?

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

Hello, I'm working on a DNS decoder and as part of it I need to manipulate bitmasks. I referred to a online source for the bitmasks since that is my weak point and discovered something funky that I don't quite understand. If I use bitmask manipulation like so:

#split apart flags
    my $qr=$flags & 0x8000;
    my $opcode=($flags & 0x001e) >> 11;
    my $aa=$flags & 0x0400);
    my $tc=$flags & 0x0200;
    my $rd=$flags & 0x0100;
    my $ra=$flags & 0x0080;
    my $rcode=$flags & 0x000f;
[download]

The result is wrong:

ID: 2
QR: 1
OPCODE: 0
AA: 0
TC: 0
RD: 256
RA: 128
RCODE 0
QDCOUNT: 1
ANCOUNT: 6
NSCOUNT: 0
ARCOUNT: 0
[download]

(note RD and RA)... Yet if I change the bitmasks like so:

#split apart flags
    my $qr=! ! ($flags & 0x8000);
    my $opcode=($flags & 0x001e) >> 11;
    my $aa=! ! ($flags & 0x0400);
    my $tc=! ! ($flags & 0x0200);
    my $rd=! ! ($flags & 0x0100);
    my $ra=! ! ($flags & 0x0080);
    my $rcode=$flags & 0x000f;
[download]

to include "! !" the results are valid:

ID: 2
QR: 1
OPCODE: 0
AA: 
TC: 
RD: 1
RA: 1
RCODE 0
QDCOUNT: 1
ANCOUNT: 6
NSCOUNT: 0
ARCOUNT: 0
[download]

Why? What does "! !" do?

Comment on What does "! !" do? Select or Download Code

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Re: What does "! !" do? by tobyink (Canon) on Feb 06, 2012 at 14:23 UTC
It's effectively a "cast to boolean" operation. Consider you have a variable `$x` which might contain a string, or might contain a number - you're not quite sure. If you want to force it to be treated as a string, you do something like this: `say ($x . ""); # concatenate an empty string to it` [download] Or if you want to force it to be treated as a number: `say ($x + 0); # add zero to it` [download] `!!` does the same for forcing things to be a "boolean". Perl doesn't have real booleans (though there are CPAN modules for that) so in this case uses the empty string to represent false, and the number 1 to represent true. How does it work? A single `!` is just a "not" operation. Two not operations cancel each other out truth-wise, but still act to cast the value to boolean.	[reply] [d/l] [select]
Re: What does "! !" do? by SuicideJunkie (Vicar) on Feb 06, 2012 at 14:18 UTC
See: Perlop You're looking at a pair of negation operators. By doubling it up, you keep the true values true and the false values false, but convert all true values down to a 1. More concisely written as `!!(expression)`, you can think of it as a boolean-ization operator	[reply] [d/l]
Re: What does "! !" do? by Anonymous Monk on Feb 06, 2012 at 14:16 UTC
Secret Perl Operators: the boolean list squash operator, x!!	[reply]