Re^4: Pick k numbers at random -- hash keys

For small hashes, ordering isn't necessarily "random" from one hash to the next:

c:\@Work\Perl\monks>perl -wMstrict -le
"for (1 .. 8) {
   my @k = keys %{ { map { $_ => 1 } 0 .. 8 } };
   print qq{@k};
   }
"
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
6 3 7 2 8 1 4 0 5
[download]

Same results under ActiveState 5.8.9 and Strawberry 5.14.4.1.

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Comment on Re^4: Pick k numbers at random -- hash keys Select or Download Code

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Re^5: Pick k numbers at random -- hash keys by syphilis (Archbishop) on Nov 13, 2019 at 01:06 UTC
For small hashes, ordering isn't necessarily "random" from one hash to the next I believe it's the version of perl that's important here. IIRC, with perl-5.16 and earlier the order was always repeated, but for later versions of perl the order varies at random (for some meaning of "at random"). For example, with Strawberry Perl 5.18.2.1 your one liner produces for me: `0 4 5 3 2 8 1 7 6 3 5 0 4 6 7 1 8 2 7 6 2 8 1 5 3 0 4 6 7 8 1 2 5 3 0 4 7 6 2 8 1 5 3 4 0 3 5 0 4 6 7 1 8 2 7 6 2 1 8 5 3 0 4 3 5 4 0 7 6 2 1 8` [download] and the output changes again when I re-run it. Interestingly, it's not strictly random - 0, 3, 4 and 5 always appear (in varying orders) either at the beginning or the end of the sequence. (I think it's still possible, via Configure args, to build perl with the pre-5.18 behaviour, though this is rarely done.) Cheers, Rob	[reply] [d/l]
Re^6: Pick k numbers at random -- hash keys (randomness) by Discipulus (Canon) on Nov 13, 2019 at 08:52 UTC
Hello, also hash key randomness can be influenced (I don mean this is a wise thing to do..) by `ENV` vars, specifically `PERL_PERTURB_KEYS` and `PERL_HASH_SEED` perl -v This is perl 5, version 26, subversion 0 (v5.26.0) built for MSWin32-x +64-multi-thread perl -wMstrict -le "for (1 .. 8) {my @k = keys %{ { map { $_ => 1 } 0 +.. 8 } };print qq{@k};}" 2 8 5 3 0 1 4 6 7 2 0 8 5 3 1 6 4 7 2 4 7 6 1 3 8 5 0 1 4 7 6 0 8 5 3 2 2 0 8 5 3 1 7 6 4 4 7 6 1 3 5 8 0 2 3 8 5 0 6 4 7 1 2 0 5 8 3 1 6 4 7 2 set PERL_PERTURB_KEYS=0 set PERL_HASH_SEED=0x0 perl -wMstrict -le "for (1 .. 8) {my @k = keys %{ { map { $_ => 1 } 0 +.. 8 } };print qq{@k};}" 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 6 7 5 2 3 1 0 8 4 set PERL_PERTURB_KEYS= set PERL_HASH_SEED= perl -wMstrict -le "for (1 .. 8) {my @k = keys %{ { map { $_ => 1 } 0 +.. 8 } };print qq{@k};}" 2 8 4 1 6 0 3 5 7 3 5 7 8 2 4 1 6 0 5 3 7 4 1 2 8 0 6 4 1 2 8 0 6 5 3 7 7 3 5 6 0 2 8 4 1 5 3 7 1 4 2 8 0 6 7 5 3 0 6 4 1 8 2 7 3 5 6 0 8 2 1 4 [download] see also PERL_HASH_SEED and PERL_PERTURB_KEYS both on perlrun and Algorithmic-Complexity-Attacks and order of hash and setting PERL_PERTURB_KEYS & PERL_HASH_SEED in a perl file L* There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l] [select]
Re^6: Pick k numbers at random -- hash keys by choroba (Cardinal) on Nov 13, 2019 at 15:55 UTC
> 0, 3, 4 and 5 always appear (in varying orders) either at the beginning or the end of the sequence. My guess: they all fall into the same bucket, so must be grouped together. But why is the bucketting always the same? `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l]