[ACCEPTED]-What does !! (double exclamation point) mean?-negation
It is just two ! boolean not operators sitting 8 next to each other.
The reason to use this 7 idiom is to make sure that you receive a 6 1 or a 0. Actually it returns an empty string 5 which numifys to 0. It's usually only used 4 in numeric, or boolean context though.
You 3 will often see this in Code Golf competitions, because 2 it is shorter than using the ternary ? : operator 1 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
not-not.
It converts the value to a boolean 1 (or as close as Perl gets to such a thing).
Because three other answers claim that the 4 range is "0" or "1", I just thought I'd 3 mention that booleans in Perl (as returned 2 by operators like ==, not, and so on) are undef and 1 1, not 0 and 1.
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