Hash vs. Hash ref

[Rg]

Saluton,

Please, take a look at the following Perl program:

-----------------------
#!/usr/bin/perl

use strict;

my $hash_ref = {
List => ()
};

$hash_ref->{List}[0]->{Something} = "Boo";
$hash_ref->{List}[1]->{Something_else} = "Not boo";

print "Something is $hash_ref->{List}[0]{Something}\n";
print "Something else is $hash_ref->{List}[1]{Something_else}\n";
-----------------------

The output for this program is

"Something is Boo
Something else is Not boo"

I wonder why. How comes Perl treat $hash_ref->{List}[0] both as a hash
reference and a hash? Am I missing something here? Is there "undefined
behavior" involved?

 

[Xicheng Jia]

Saluton,

Please, take a look at the following Perl program:

-----------------------
#!/usr/bin/perl

use strict;

my $hash_ref = {
List => ()

ITYM:
List => [ ],

};

$hash_ref->{List}[0]->{Something} = "Boo";
$hash_ref->{List}[1]->{Something_else} = "Not boo";

print "Something is $hash_ref->{List}[0]{Something}\n";
print "Something else is $hash_ref->{List}[1]{Something_else}\n";
-----------------------

The output for this program is

"Something is Boo
Something else is Not boo"

I wonder why. How comes Perl treat $hash_ref->{List}[0] both as a hash
reference and a hash? Am I missing something here? Is there "undefined
behavior" involved?

no, they are both references, check what's the difference between:

$hash_ref->{List}[0]{something}
$hash_ref->{List}[0]->{something}

Read "Intermediate Perl" section 4.7

Regards,
Xicheng

 

[Paul Lalli]

Saluton,

Please, take a look at the following Perl program:

-----------------------
#!/usr/bin/perl

use strict;

my $hash_ref = {
List => ()

This doesn't do what you think it does. ALWAYS enable warnings when
writing Perl code!!

};

$hash_ref->{List}[0]->{Something} = "Boo";
$hash_ref->{List}[1]->{Something_else} = "Not boo";

print "Something is $hash_ref->{List}[0]{Something}\n";
print "Something else is $hash_ref->{List}[1]{Something_else}\n";
-----------------------

The output for this program is

"Something is Boo
Something else is Not boo"

I wonder why. How comes Perl treat $hash_ref->{List}[0] both as a hash
reference and a hash?

What gives you the impression anything in the above is indicating
that? The second arrow and lack thereof?

Am I missing something here?

Yes. The "arrow rule". You should read up on how multidimensional
structures are accessed in Perl. Take a look at :
perldoc perlreftut
perldoc perllol
perldoc perldsc.

Is there "undefined behavior" involved?

Not at all. It's perfectly well defined. It's your assumption that
is in error.

Paul Lalli