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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :


Summary

Some of the highlights from perlref:

In addition to consulting the obvious documents such as the Perl man pages, look at the Perl source code for standard modules more information. Also the 't/op' directory in the Perl source tree has regression tests that should definitely get you thinking.

Some useful documents and informative posts are available at the Web sites www.perl.com and PerlMonks

Some typical question with answers:

  1. Q: How do I know what type of address a pointer is pointing to?

    A: Function ref provides the type of reference. Also the address printed out with the print statement on a reference has a qualifier word in front of it. For example, a reference to a hash has the word HASH followed by an address value, an array has the word ARRAY, and so on.
     

  2. Q: How are multidimensional arrays possible using Perl?

    A: References in Perl can point to arrays. Elements of arrays can be references to other arrays, hashes, and so on. 
     

  3. Q: What's the best way to pass more than one array into a subroutine?

    A: Pass references to the arrays, using the \@arrayname for each array passed-as in the following call:
    mysub(\@one, \@two);
    Within the subroutine, take each reference off one at a time.
    my ($a, $b) = @_;
    Now use @$a and @$b to get to the arrays passed into the subroutines.
     

  4. Q: Why is *moo more efficient to use than $_main{'moo'}? Is there a difference in usage?

    A: Both *moo and $_main{'moo'} mean the same variable (as long as you aren't using a package). *moo is more efficient because the reference is looked up at compile time, whereas $_main{'moo'} is evaluated at runtime each time the statement with this expression is executed.

 

In Perl, you can pass only one kind of argument to a subroutine: a scalar. To pass any other kind of argument, you need to convert it to a scalar. You do that by passing a reference to it. A reference to anything is a scalar. If you're a C programmer you can think of a reference as a pointer (sort of).

The following table discusses the referencing and de-referencing of variables. Note that in the case of lists and hashes, you reference and dereference the list or hash as a whole, not individual elements (at least not for the purposes of this discussion).
 

Variable Instantiating
the scalar
Instantiating a
reference to it
Referencing it Dereferencing it Accessing an element
$scalar $scalar = "steve";
$ref = \"steve";
$ref = \$scalar $$ref or
${$ref}
N/A
@list @list = ("steve", "fred");
$ref = ["steve", "fred"];
$ref = \@list @{$ref} ${$ref}[3]
$ref->[3]
%hash %hash = ("name" => "steve",
   "job" => "Troubleshooter");
$hash = {"name" => "steve",
   "job" => "Troubleshooter"};
$ref = \%hash %{$ref} ${$ref}{"president"}
$ref->{"president"}
FILE

$ref = \*FILE {$ref} or scalar <$ref>

The two types of references in Perl 5 are hard and symbolic.

You can have references to scalars, arrays, hashes, subroutines, and even other references. References themselves are scalars and have to be de-referenced to the context before being used. Use @$pointer for an array, %$pointer for a hash, &$pointer for a subroutine, and so on for dereferencing.

Multidimensional arrays are possible using references in arrays and hashes.

Parameters are passed into a subroutine through references. The @_ array is really all the passed parameters concatenated in one long array. To send separate arrays, use the references to the individual items.

Tomorrow's lesson covers Perl objects and references to objects. We have deliberately not covered Perl objects in this chapter because it requires some knowledge of references. References are used to create and refer to objects, constructors, and packages.

  1. Scalar references:

    $ra  = \$a;              # reference to scalar
    $$ra = 2;                # dereference scalar-ref
    $ra  = \1.6;             # reference to constant scalar
  2. Array references:

    $rl  = \@l;              # reference to existing
    $rl  = [1,2,3];          # reference to anonymous array
    push (@$rl, "a");        # Dereference
    print $rl->[3]           # 4th element of array pointed to by $rl
  3. Hash references:

    $rh = \%h;               # reference to hash
    $rh = {"laurel" => "hardy", "romeo" => "juliet"}; # ref to anon-hash
    print keys (%$rh);       # Dereference
    $x = $rh->{"laurel"};    # Arrow notation to extract single element
    @slice = @$rh{"laurel","romeo"}; # Hash slice
  4. Code references:

    $rs = \&foo;             # reference to existing subroutine foo
    $rs = sub {print "foo"}; # reference to anonymous subroutine 
                             # (remember the semicolon at the end)
    &$rs();                  # dereference: call the subroutine
  5. Generalized dereferences. Any code inside a block yielding a reference can be dereferenced:

    @a = @{foo()};           # dereference the array reference 
                             # returned by foo()
  6. References gotchas. All the examples below are wrong. Always use -w in developing and testing.

    @foo = [1,3,4];          # Assigning an array-ref to an array
                             # Use parentheses instead.
    
    %foo = {"foo" => "bar"}; # Assigning a hash-ref to a hash.
                             # Use parentheses instead.
    
    $foo = \($a, @b);        # Identical to $foo = (\$a, \@b)
                             # Assiging an enumerated list to a 
                             # scalar yields the last element (so, 
                             # $foo gets \@b). Use [ ] if you need 
                             # an array reference


Etc

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