http://qs321.pair.com?node_id=1192590


in reply to Re^2: Sort undef
in thread Sort undef

> actually it works undef is not cast to "" and hence less than empty string

No, it works because sort is stable and you declared the lucky case to be the input. Try again with 

use List::Util qw{ shuffle };
my @array = shuffle((undef) x 3, qw/c b a/, ("") x 3);

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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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Re^4: Sort undef
by LanX (Saint) on Jun 12, 2017 at 14:18 UTC
    Ah, thanks! :)

    > you declared the lucky case to be the input

    In my defence:

    To be sure I also tried

     my @array =  ( ("")x3, (undef)x 3, qw/c b a/, ("")x3, (undef) x3 );

    which still gives for cmp_undef1 

    [
  "",
  "",
  "",
  "",
  "",
  "",
  "a",
  "b",
  "c",
  undef,
  undef,
  undef,
  undef,
  undef,
  undef,
]

    [download]

    which seems to be another "lucky case".

    I don't know how "stable" is defined but it turns out to be not that easy to construct a counterexample without shuffle.

    my @array =  ( undef, "", qw/c b a/, "", undef  );

    ->

    ["", "a", "b", "c", undef, "", undef]

    Cheers Rolf
    (addicted to the Perl Programming Language and ☆☆☆☆ :)
    Je suis Charlie!

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