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Advanced Perl Programming

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Previous: A.8 HList - Hierarchical List Appendix B Next: B.2 Nested Data Structures

B. Syntax Summary

Thus spake the Master Ninjei:"To the intelligent man, one word, to the fleet horse, one flick of the whip, to the well-written program, a single command."

- The Zen of Programming

This appendix provides a distillation of all syntax used in this book.

B.1 References

  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
    $rl  = [1,2,3];          # reference to anon. scalar
    push (@$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