in reply to Pick any key from a very large hash

...each is meant for looping through the entire hash... - you're dead right - the hash is meant to be invariant over each - from each '...If you add or delete elements of a hash while you're iterating over it, you may get entries skipped or duplicated, so don't...'.

In answer to your question, again from each - '..it can be reset by reading all the elements from the hash, or by evaluating keys HASH or values HASH...'.

The moral of the story/posting is ... please RTFM before you post.

A user level that continues to overstate my experience :-))

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Re^2: Pick any key from a very large hash
by FloydATC (Deacon) on Jul 11, 2009 at 22:36 UTC
    Hehe I did RTFM but you know what happens when you get stuck... you start trying creative stuff even if you know it might break things in funny ways :-)

    My theory was that maybe there was some obscure way to manually whack it back to zero. Guess not. Anyway, the problem is solved :-)

    -- Time flies when you don't know what you're doing
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      I realise it's not an issue now, but it seems likely that calling keys in scalar context will reset the each iterator in (very small) constant time. Indeed a small test confirms this:

      use strict;
use warnings;
use Benchmark qw(cmpthese);

my %bigHash   = map {$_ => undef} 1 .. 1e3;
my @resetEach = keys %bigHash;
my ($key)     = each %bigHash;
my $count     = keys %bigHash;
my ($key2)    = each %bigHash;

print "First key '$key', second '$key2'\n";

cmpthese (
    -1,
    {
        scalarKeys => sub {scalar keys %bigHash},
        listKeys   => sub {() = keys %bigHash},
    }
);

      [download]

      Prints:

      First key '559', second '559'
                 Rate   listKeys scalarKeys
listKeys       5184/s         --      -100%
scalarKeys 15969899/s    307964%         --

      [download]
      True laziness is hard work
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        My original assumption was that keys would have to be called in a list context if I was to extract anything useful from it, and that calling it in scalar context would still cause a full traversal in order to count the key/value pairs.

        As usual, it turns out Perl is a bit more clever than expected, so using keys to reset and each to pick a pair may indeed be the Right Thingtm even for a huge hash. Maybe perldoc -f each should point this out more clearly?

        -- Time flies when you don't know what you're doing
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[d/l]
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