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Re^2: Getting loopy while appending HoH

by jedikaiti (Friar)
on Mar 30, 2010 at 15:59 UTC ( #831888=note: print w/ replies, xml ) Need Help??


in reply to Re: Getting loopy while appending HoH
in thread Getting loopy while appending HoH

When I googled for how to append a perl hash, that's what came up. So I am curious, why is it a "horrible" hash concatenation trick?

Kaiti
Swiss Army Nerd


Comment on Re^2: Getting loopy while appending HoH
Re^3: Getting loopy while appending HoH
by almut (Canon) on Mar 30, 2010 at 17:19 UTC
    why is it a "horrible" hash concatenation trick?

    It's just that it does way too much unnecessary operations.  To "visualize" what's going on, you could tie the hash and put debugging prints into the routines like STORE, FETCH, etc.  For example:

    #!/usr/bin/perl -l package MyHash; use Tie::Hash; our @ISA = 'Tie::StdHash'; sub TIEHASH { my $class = shift; return bless {}, $class; } sub STORE { my $self = shift; print "storing $_[0] => $_[1]"; $self->SUPER::STORE(@_); } sub FETCH { my $self = shift; print "fetching @_"; $self->SUPER::FETCH(@_); } sub CLEAR { my $self = shift; print "clear"; $self->SUPER::CLEAR(); } package main; use strict; use warnings; use Data::Dumper; tie my %myhash, "MyHash"; print "Variant 1 (bad):"; %myhash = ( %myhash, "key_$_" => { foo => 42 } ) for 1..5; print "\nDumping..."; print Dumper \%myhash; %myhash = (); print "\nVariant 2 (good):"; $myhash{"key_$_"} = { foo => 42 } for 1..5; print "\nDumping..."; print Dumper \%myhash; __END__ Variant 1 (bad): clear storing key_1 => HASH(0x65ecb0) fetching key_1 clear storing key_1 => HASH(0x65ecb0) storing key_2 => HASH(0x6c2050) fetching key_1 fetching key_2 clear storing key_1 => HASH(0x65ecb0) storing key_2 => HASH(0x6c2050) storing key_3 => HASH(0x74a420) fetching key_3 fetching key_1 fetching key_2 clear storing key_3 => HASH(0x74a420) storing key_1 => HASH(0x65ecb0) storing key_2 => HASH(0x6c2050) storing key_4 => HASH(0x74a3f0) fetching key_3 fetching key_1 fetching key_4 fetching key_2 clear storing key_3 => HASH(0x74a420) storing key_1 => HASH(0x65ecb0) storing key_4 => HASH(0x74a3f0) storing key_2 => HASH(0x6c2050) storing key_5 => HASH(0x74aaf0) Dumping... fetching key_3 fetching key_1 fetching key_5 fetching key_2 fetching key_4 $VAR1 = { 'key_3' => { 'foo' => 42 }, 'key_1' => { 'foo' => 42 }, 'key_5' => { 'foo' => 42 }, 'key_2' => { 'foo' => 42 }, 'key_4' => { 'foo' => 42 } }; clear Variant 2 (good): storing key_1 => HASH(0x74ac80) storing key_2 => HASH(0x737c40) storing key_3 => HASH(0x6c2050) storing key_4 => HASH(0x74ad00) storing key_5 => HASH(0x74ac00) Dumping... fetching key_3 fetching key_1 fetching key_5 fetching key_4 fetching key_2 $VAR1 = { 'key_3' => { 'foo' => 42 }, 'key_1' => { 'foo' => 42 }, 'key_5' => { 'foo' => 42 }, 'key_4' => { 'foo' => 42 }, 'key_2' => { 'foo' => 42 } };
Re^3: Getting loopy while appending HoH
by GrandFather (Cardinal) on Mar 30, 2010 at 20:05 UTC

    Almut has provided an excellent answer to why that technique is bad and illustrated why the internet is wonderful. The answer you got through googling illustrates the dark side of the internet. Now, if you'd have asked on PerlMonks ...


    True laziness is hard work

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