Perl Refernces

1. Perl Refernces

2. Kinds of references: • hard: a scalar variable that points to data • symbolic: a variable that names another reference • typeglob: a kind of symbolic reference $language = “english”; # a hard reference $english = “Brooklyn”; # another hard reference print “you speak with a ${$language} accent\n”; eval “ \$$language = ‘British’;”; # another symbolic ref Print “Now you speak with a $english accent\n”;

3. Symbolic references are tricky: • You can disallow symbolic references using the pragma use strict “refs”;

4. Life revolves around hard references: • A hard reference is a pointer; an address. • You can point to • a scalar • a list or array • an associative array or hash • a subroutine • Purpose is to allow you to create things like • array of arrays (n-dimensional arrays) • array of hashes • hash of hashes (a file hierarchy where directory == hash)

5. The \ Operator: • Similar to & in C++. • $p is a hard reference to the “a scalar” string; • Like & in C++, you de-reference a field by using . $x = “a scalar”; $p = \$x; # a pointer to $x

6. Examples: $scalarref = \$foo; $arrayref = \@ARGV; $hashref = \%ENV; $coderef = \&handler; $globref = \*STDOUT; $reftoref = \$scalarref;

7. De-referencing a pointer: • The de-reference operator is the character that precedes the variable name and indicates the data type of the variable. $num = 5; $p = \$num; Printf ‘The address assigned to $p is ‘ . $p . “\n”; Printf “The value stored there is $$p\n”; #output The address assigned to $p is SCALAR(0xb075c) The value stored there is 5

8. De-referencing another pointer: @toys = qw/slinky yo-yo jack-in-the-box/; $num = @toys; $ref1 = \$num; $ref2 = \@toys; printf “There are $$ref1 toys\n”; printf “They are: @$ref2\n”; printf “My favourite toy is $ref2->[0]\n”; # could also write this as $$ref2[0]

9. Anonymous variables: • Variables with no names are called anonymous. • Anonymous arrays are created with [ ] @array = (3, 4 ,5); $arrayref = [ 3, 4, 5]; @matrix = ( [1, 2, 3], [4, 5, 6], [7, 8, 9]); printf “@matrix\n”; # prints a list of 3 addresses printf “matrix[0,0] = $matrix[0][0]\n’; # prints matrix[0][0] = 1 # can also be written $matrix[0][0] # print out the matrix using a loop for ($i = 0; $i < 3; $i++) { printf “@{$matrix[0]}\n”; }

10. Purpose of  • The  operator, called the arrow or infix operator, is used to de-reference an anonymous array or hash. • $matrix[0] is a reference to an array •  de-references to get first the actual array • and then [0] gets the first element in it. $matrix[0][0]

11. Anonymous hashes: • Anonymous hashes are created with { } $hashref = { Name => ”Woody”, Type => “Cowboy”}; printf $hashref{Name} . “\n”; printf keys(%$hashref) . “\n”; printf values(%$hashref) . “\n”; #output Woody NameType WoodyCowboy