Hash of hashes

[p5pRT]

Migrated from rt.perl.org#1264 (status was 'resolved')

Searchable as RT1264$

[p5pRT]

From kenc@research.canon.com.au

Perl Version​: 5.004_04, 5.005_03
OS​: RedHat Linux 6.0

*****************************************************************
%hash = ();

$hash{10}{5} = 1;

while ($i < 1000000)
{
  $i++;
  $j = $i % 6;
  print "Yeah! $i\n" if (defined $hash{$i}{$j});
}
*****************************************************************

Regards
Ken Carroll

--

---

Ken Carroll, Senior Software Engineeer
Canon Information Systems Research Australia P/L
Phone​: +61 2 9805 2031 Fax​: +61 2 9805 2929
mailto​:kenc@​research.canon.com.au
http​://www.research.canon.com.au/

[p5pRT]

From [Unknown Contact. See original ticket]

On Wed, 25 Aug 1999 at 03​:59​:11 +1000, Ken Carroll wrote​:

Hi.
Run this program and watch the size of your PERL process grow
uncontrollably. Is this a bug? The variables $i and $j are chosen for
illustration purposes only. There is no logic to this code.

Arguable as to whether it's a bug. Referring to $hash{$i}{$j} - even
with 'defined' - means that a reference to $hash{$i} springs into
existence, so that perl can see if it contains the desired element.

This doesn't leak memory​:

  print "Yeah! $i\n" if (defined $hash{$i} && defined $hash{$i}{$j});

[I've a sneaking feeling that a related issue was discussed recently,
but your program still leaks on 5.005_61]

BTW if you don't want to iterate over %hash (i.e. if you never use
'keys', 'each' or slices on %hash or its elements) then you may find
that using 'multidimensional hash emulation' (see $SUBSEP in perlvar(1))
is better​:
  $hash{ 10, 5 } = 1;

Ian
--
  I am confident this explanation will dispell any feelings
  of certainty that may have been troubling you.
  - BWHOLMES@​SJSUVM1.sjsu.edu (Cabbage) in <9601221753.AA27669@​spock>

[p5pRT]

From [Unknown Contact. See original ticket]

Ian Phillipps <ian@​dial.pipex.com> writes​:

This doesn't leak memory​:

print "Yeah\! $i\\n" if \(defined $hash\{$i\} && defined $hash\{$i\}\{$j\}\);

[I've a sneaking feeling that a related issue was discussed recently,
but your program still leaks on 5.005_61]

The related issue was that _60 deprecated 'defined $hash{$i}' so
that the workround above solicited a warning.

--
Nick Ing-Simmons

[p5pRT]

From [Unknown Contact. See original ticket]

Nick Ing-Simmons <nick@​ing-simmons.net> wrote

The related issue was that _60 deprecated 'defined $hash{$i}' so
that the workround above solicited a warning.

I hope not! defined() is a reasonable operation on any scalar
expression.

I thought the discussion related to defined(@​array) and defined(%hash),
which is a separate subject from this thread.

Mike Guy

[p5pRT]

From [Unknown Contact. See original ticket]

On Tue, 24 Aug 1999 at 14​:52​:16 +0000, Nick Ing-Simmons wrote​:

Ian Phillipps <ian@​dial.pipex.com> writes​:

This doesn't leak memory​:
print "Yeah! $i\n" if (defined $hash{$i} && defined $hash{$i}{$j});

[I've a sneaking feeling that a related issue was discussed recently,
but your program still leaks on 5.005_61]

The related issue was that _60 deprecated 'defined $hash{$i}' so
that the workround above solicited a warning.

I don't think that's right - $hash{$i} is a perfectly ordinary scalar,
which can be defined, undefined or nonexistent; in this example it's
probably going to be a ref(HASH) or nonexistent. Anyhow, my code
doesn't give a warning under 5.005_61. About 3.73 iotas of efficiency
could be got by saying​:
  print "Yeah! $i\n" if (exists $hash{$i} && defined $hash{$i}{$j});

Now you remind me, wasn't it defined(%hash) that was deprecated, for
local hashes? Quick test - yes​:

% perl5.00561 -we 'my %hash; defined %hash'
defined(%hash) is deprecated at -e line 1.
(Maybe you should just omit the defined()?)
Useless use of defined operator in void context at -e line 1.

Ian

[p5pRT]

From [Unknown Contact. See original ticket]

Nick Ing-Simmons <nick@​ing-simmons.net> writes​:

Ian Phillipps <ian@​dial.pipex.com> writes​:

This doesn't leak memory​:

print "Yeah\! $i\\n" if \(defined $hash\{$i\} && defined $hash\{$i\}\{$j\}\);

[I've a sneaking feeling that a related issue was discussed recently,
but your program still leaks on 5.005_61]

The related issue was that _60 deprecated 'defined $hash{$i}' so
that the workround above solicited a warning.

Ignore me - I meant defined(%hash) was deprecated...

--
Nick Ing-Simmons