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Old 18th November 2008
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Default Basic Perl arrays question

Hello,

What does this code snippet do?

Code:
my $attributes = ['objectClass'     => ['top', 'posixAccount', 'shadowAccount', 'inetOrgPerson'],
                  'loginShell'      => "/bin/bash"];
In my understanding, $attributes is a reference to an array. I though Perl arrays could not have indexes? What will the => do then?


Thanks!
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Old 18th November 2008
ocicat ocicat is offline
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Quote:
Originally Posted by stukov View Post
In my understanding, $attributes is a reference to an array.
Close. $attributes is a reference to an anonymous hash.
Quote:
What will the => do then?
This is used to establish the (key,value) pairs.
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Old 18th November 2008
J65nko J65nko is offline
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The '=>' separates the hash key from the hash value. It is a more visual appealing replacement then the ',' which also can be used for this purpose
Code:
 %shell = (
         Theo  => '/bin/sh',
         Linus =>  '/bin/bash' ) ;

print "\nTheo's shell : $shell{'Theo'} " ;
print "\nLinus' shell : $shell{'Linus'} " ; 

$ perl hash-shell

Theo's shell : /bin/sh 
Linus' shell : /bin/bash
Another example but now using a comma
Code:
%os = ( 
        'Theo',  'OpenBSD',
        'Linus', 'Linux'
);

print "\nTheo's OS : $os{'Theo'} " ;
print "\nLinus' OS : $os{'Linus'} " ;

$perl hash-os

Theo's OS : OpenBSD 
Linus' OS : Linux
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Old 18th November 2008
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Thanks for the replies. As I wrote a few scripts, I am getting familiar with the "hash" concept and Perl. But this is where it becomes confusing:

For the same hash I pasted in my first post, whenI print $attributes I get:

Code:
ARRAY(0x811a214)
Now, is a HASH an array itself?
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Old 18th November 2008
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Quote:
Originally Posted by stukov View Post
Code:
my $attributes = ['objectClass'     => ['top', 'posixAccount', 'shadowAccount', 'inetOrgPerson'],
                  'loginShell'      => "/bin/bash"];
$attributes is an array reference.

the => operator is effectively a comma operator where the left hand object is taken as a double quoted string. in the given code its probably used to make the association clear (like J65nko said).

i am not sure why they haven't used a hash reference. possibly for a small performance gain?

Last edited by ephemera; 18th November 2008 at 07:12 PM.
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Old 18th November 2008
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Quote:
Originally Posted by stukov View Post
Now, is a HASH an array itself?
No.

hashes can not have duplicate keys.

also, the keys in a hash are not ordered.

For example,
Code:
%hash1 = @array1; @array2 = %hash1 ; %hash2 = @array2;
array2 will likely have a different ordering from array1, hence different. but, hash1 will be the same as hash2.


arrays have a small performance advantage. and neither is particularly space efficient in perl.

Last edited by ephemera; 18th November 2008 at 07:13 PM.
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Old 18th November 2008
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Quote:
Originally Posted by ephemera View Post
i am not sure why they haven't used a hash reference. possibly for a small performance gain?
Maybe I can make the thing clearer: the array/hash I want to build will be given to the add method of the Perl::LDAP:

Code:
$ldapHandle->add($dn, attr => $attributes);
A function example looks like this:
Code:
$result = $ldap->add("uid=john,ou=People,dc=leapster,dc=org", 
                attr => [ 'cn' => 'John Smith',
                          'sn' => 'Smith',
                          'uid' => 'john',
                          'givenName' => 'John',
                          'homePhone' => '555-2020',
                          'mail' => 'john@domain.name',
                          'objectclass' => [ 'person', 'inetOrgPerson']
                        ]
           );
Am I expected to work with an array or an hash?
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Old 18th November 2008
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Quote:
Originally Posted by stukov View Post
Am I expected to work with an array or an hash?
Furhter testing indicates that :
Code:
foreach my $i (@$attributes)
{
    print $i . ENDL;
}
works but
Code:
foreach my $i (%$attributes)
{
    print $i . ENDL;
}
does not.

Now, how am I expected to provide a key-value pair when I need to work with an array?
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Old 18th November 2008
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Quote:
Originally Posted by stukov View Post
Code:
$result = $ldap->add("uid=john,ou=People,dc=leapster,dc=org", 
                attr => [ 'cn' => 'John Smith',
                          'sn' => 'Smith',
                          'uid' => 'john',
                          'givenName' => 'John',
                          'homePhone' => '555-2020',
                          'mail' => 'john@domain.name',
                          'objectclass' => [ 'person', 'inetOrgPerson']
                        ]
           );
Am I expected to work with an array or an hash?
Use just the way it is in your example.

The format is given in the POD: http://search.cpan.org/~gbarr/perl-l...b/Net/LDAP.pod

Last edited by ephemera; 18th November 2008 at 07:40 PM.
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Old 18th November 2008
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Quote:
Originally Posted by stukov View Post
Furhter testing indicates that :
Code:
foreach my $i (@$attributes)
{
    print $i . ENDL;
}
works but
Code:
foreach my $i (%$attributes)
{
    print $i . ENDL;
}
does not.

Now, how am I expected to provide a key-value pair when I need to work with an array?
like i said before, $attributes is an array reference and an array is not equivalent to a hash. you can't use an array reference as a hash reference or the other way around. (internally they have a different DS)

though, assigning an array to a hash and vice-versa is valid (with a caveat) , please see the example i gave earlier (#6).

Last edited by ephemera; 18th November 2008 at 07:34 PM.
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Old 18th November 2008
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Thanks for the explanations ephemera. Understood and script fixed.

Thanks!
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Old 18th November 2008
J65nko J65nko is offline
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Most of these things you can find in the man page perldsc (perl data sctructures cookbook) and perlreftut
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Old 18th November 2008
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Thanks J65nko, I'll take good notes.
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