Use of double negation (!!)

Question

Okay so I came across a code which looks like

@documents_names = sort {
         !!$deleted_documents_names{$a} == !!$deleted_documents_names{$b}
          ? uc($a) cmp uc($b)
          : !!$deleted_documents_names{$a}
          cmp !!$deleted_documents_names{$b}
         } @documents_names;

It's the first time I'm seeing the use of double negation. What's the use of it? When would a person use it?

http://stackoverflow.com/questions/2168406/what-does-double-exclamation-point-mean — FrankB, Sep 05 '12 at 10:09

score 45 · Accepted Answer · edited Sep 05 '12 at 17:52

45

It converts non-boolean types to boolean (dualvar(0,"") or 1).

It is a shortcut way of doing this, instead of trying to cast it explicitly (which may take more characters). The ! operator negates the truthness of its argument. Hence, two of them are used.

Many object types are "truthy", and others are "falsey".

The only false values are 0, undef, "", "0" and some overloaded objects.
Examples of true values are 1, "asdf", and all other values.

edited Sep 05 '12 at 17:52

ikegami

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answered Sep 05 '12 at 10:07

ronalchn

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Your answer + http://stackoverflow.com/a/2168511/257635 = Satisfaction! :P – Chankey Pathak Sep 05 '12 at 10:14
`'null'` would evaluate to true in Perl. Did you mean `undef`? – Zaid Sep 05 '12 at 10:36
this is not my native language... (I mean in terms of programming languages) – ronalchn Sep 05 '12 at 10:37
@amon : What about the empty list `()`? – Zaid Sep 05 '12 at 11:01
@Zaid … I have to read perlsyn more often, thanks – amon Sep 05 '12 at 11:50
BTW, `!!` does not produce the empty string exactly. Perl's canonical false value is an empty string in string context, but 0 in numeric context (without any warning about a non-numeric value, which a normal empty string would generate in numeric context). – cjm Sep 05 '12 at 15:33
1

It's impossible to pass an empty list to a boolean test, so empty lists are neither true nor false. `()` (an operator that returns undef in scalar context) should no more be in the list than `-f "nonexistant"` (an operator that returns undef). Values are true or false, not operators. – ikegami Sep 05 '12 at 17:54
I like using `!!0` as a shorthand for "that special empty string that Perl's standard relational and equality operators produce". As discussed in http://stackoverflow.com/questions/39541833/what-values-should-a-boolean-function-in-perl-return, you cannot refactor, i.e. you cannot get exactly the same behavior, `return $a<$b` into `if($a<$b){return 1} else{return 0}`, but you can if you do `return !!0`. – Krazy Glew Sep 22 '16 at 21:30

Zaid · Answer 2 · 2012-09-05T12:11:55.613

10

That is a lot of funk for a sort block!

It's essentially a two-level sort :

ascii-betical
deleted files first, then undeleted

So one could rewrite it as (untested):

@documents = sort {  exists $deleted_documents_names{$a} # same effect as '!!'
                       <=> 
                     exists $deleted_documents_names{$b}
                  ||
                     uc( $a ) cmp uc( $b )
                  }
             @documents;

edited Sep 05 '12 at 12:11

answered Sep 05 '12 at 11:14

Zaid

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That's not completely right since `<=>` is an operator for numbers. You should use `cmp` instead to work with the alphabetical comparison of strings. Nice sort clarification though. :) – memowe Sep 05 '12 at 12:02
@memowe : Yeah, thanks for that.. like I said I hadn't tested it :) – Zaid Sep 05 '12 at 12:11

Use of double negation (!!)

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