not-not.
It converts the value to a boolean (or as close as Perl gets to such a thing).
David Dorward
2010-01-30 15:48:22
+10
A:
It's also a common idiom in other languages too like Javascript.
mpeters
2010-01-30 16:40:43
@David Dorward Thanks for your answer!
molecules
2010-01-30 17:12:26
+21
A:
It is just two ! boolean not operators sitting next to each other.
The reason to use this idiom is to make sure that you receive a 1 or a 0. Actually it returns an empty string which numifys to 0. It's usually only used in numeric, or boolean context though.
You will often see this in Code Golf competitions, because it is shorter than using the ternary ? : operator with 1 and 0 ($test ? 1 : 0).
!! undef == 0
!! 0 == 0
!! 1 == 1
!! $obj == 1
!! 100 == 1
undef ? 1 : 0 == 0
0 ? 1 : 0 == 0
1 ? 1 : 0 == 1
$obj ? 1 : 0 == 1
100 ? 1 : 0 == 1
Brad Gilbert
2010-01-30 16:17:35
I actually often use `!!!!` in my code, because `!!` confuses vim's syntax highlighting and another `!!` will restore it!
Ether
2010-01-30 16:55:06
It produces 1 or the canonical false value, not 0. This is 0 in numeric context but the empty string in string context. To get 0 or 1, use the `1-!` operator :)
ysth
2010-01-31 07:27:40
@Brad: -1 to get your attention on ysth's comment, will revoke upon fix.
j_random_hacker
2010-01-31 09:28:21
Speaking as the guy that wrote the same code, it's there because I want anything odd in that variable normalised away at compile time. It's just a sort of quality protection mechanism.
Adam Kennedy
2010-02-01 00:14:35
@Brad Thanks for your answer. I just started reading a draft of chromatic's new book and I like how he says that `!!` "forces boelean context".
molecules
2010-05-11 15:58:40
A:
Because three other answers claim that the range is "0" or "1", I just thought I'd mention that booleans in Perl (as returned by operators like ==, not, and so on) are undef and 1, not 0 and 1.
jrockway
2010-01-31 05:20:32
This is not correct. When an operator has to generate a false value, it uses a magic value that evaluates as the empty string in string context or 0 in numeric context. (The exceptions are operators like `and` and `or`, which simply return the operand that caused them to return false.) (See http://perldoc.perl.org/perlsyn.html#Truth-and-Falsehood)
cjm
2010-01-31 07:17:18
undef is also a magic value that evaluates as '' in string context and 0 in numeric context -- the only difference is that when warnings are enabled, using undef for its string or numeric value is apt to produce a warning, while 'the false value' does no such thing ;)
hobbs
2010-01-31 07:55:49
@hobbs: Well, another difference is that `defined(undef)` is false, but `defined(1==2)` is true.
cjm
2010-01-31 10:52:38
+1 You obviously had an influence on the other answers. Brad modified his answer after yours. Also, at least one of those you mentioned ended up deleting his or her answer as well.
molecules
2010-04-12 16:40:09