in reply to "A meditation on Hashes", or "Why i need more aspirin"

I think one of the potentially more confusing parts for newcomers is understanding how hash references autovivify. For example: 

use strict;
use warnings;

local $\ = "\n";

my %hash;

print '$hash{a} ',     ( $hash{a}            ) ? "true"    : "not true
+";
print '$hash{a} ',     ( defined $hash{a}    ) ? "defined" : "not defi
+ned";
print '$hash{a} ',     ( exists $hash{a}     ) ? "exists"  : "doesn't 
+exist";
print "";

print '$hash{a}{b} ',  ( $hash{a}{b}         ) ? "true"    : "not true
+";
print '$hash{a}{b} ',  ( defined $hash{a}{b} ) ? "defined" : "not defi
+ned";
print '$hash{a}{b} ',  ( exists $hash{a}{b}  ) ? "exists"  : "doesn't 
+exist";
print "";

print '$hash{a} ',     ( $hash{a}            ) ? "true"    : "not true
+";
print '$hash{a} ',     ( defined $hash{a}    ) ? "defined" : "not defi
+ned";
print '$hash{a} ',     ( exists $hash{a}     ) ? "exists"  : "doesn't 
+exist";

[download]

Prints:

$hash{a} not true
$hash{a} not defined
$hash{a} doesn't exist

$hash{a}{b} not true
$hash{a}{b} not defined
$hash{a}{b} doesn't exist

$hash{a} true
$hash{a} defined
$hash{a} exists

[download]

Checking for $hash{a}{b} causes $hash{a} to suddenly contain the reference to an anonymous hash that is checked for the 'b'.

-xdg

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Replies are listed 'Best First'.
Re^2: "A meditation on Hashes", or "Why i need more aspirin"
by jdhedden (Deacon) on Jun 16, 2006 at 00:54 UTC
    Checking for $hash{a}{b} causes $hash{a} to suddenly contain the reference to an anonymous hash that is checked for the 'b'.
    And the exception to this is when using threads::shared:
    Taking references to the elements of shared arrays and hashes does not autovivify the elements, and neither does slicing a shared array/hash over non-existent indices/keys autovivify the elements.
    Thus, the following results in a runtime error: 
    use strict;
use warnings;

use threads;
use threads::shared;

my %hash :shared;

print '$hash{a}{b} ',  ( exists $hash{a}{b} ) ? "exists" : "doesn't ex
+ist";

    [download]
    Remember: There's always one more bug.
[reply]
[d/l]

In Section Meditations