This is the eighth set of slightly updated slides from a Perl programming course that I held some years ago. I want to share it with everyone looking for intransitive Perl-knowledge. A table of content for all presentations can be found at i-can.eu. The source code for the examples and the presentations in ODP format are on https://github.com/kberov/PerlProgrammingCourse

1. Perl Programming
Course
References and nested
data structures
Krassimir Berov
I-can.eu

2. Contents
1. What are references?
2. Reference types
3. Creating references
4. Using references
5. Reference interpolation

3. What are references?
• A piece of data that tells us the location
of another piece of data
• A reference is a scalar value that refers to
an array or a hash
(or to just about anything else).
• References are the way to implement
complicated data structures in Perl
• References are like pointers that know
what they point to

4. Reference types
• Builtin types include:
SCALAR
ARRAY
HASH
CODE
REF
GLOB
LVALUE
FORMAT
IO
VSTRING
Regexp

5. Reference types
• Most used:
SCALAR – reference to scalar
ARRAY – reference to array
HASH – reference to hash
CODE – reference to anonymous sub

6. Creating references
• Just two ways to make a reference
1. By putting a in front of a variable
$aref = @array; # $aref - reference to @array
$href = %hash; # $href - reference to %hash
$sref = $scalar; # $sref - reference to $scalar
2. [ITEMS] makes a new, anonymous array,
{ITEMS} makes a new, anonymous hash
$aref = [ 1, "foo", undef, 13 ];
# $aref now holds a reference to an array
$href = { APR => 4, AUG => 8 };
# $href now holds a reference to a hash

7. Using references
• Just two ways to use references
1. Use curly braces preceded with the sigil
of the variable you refer to
@{$aref}; # An array
reverse @{$aref}; # Reverse the array
${$aref}[3]; # An element of the array
${$aref}[3] = 17; # Assigning an element
%{$href}; # A hash
keys %{$href}; # Get the keys from the hash
${$href}{'red'} # An element of the hash
${$href}{'red'} = 17; # Assigning an element

8. Using references
(2)
• Just two ways to use references
2. Use the arrow operator to access
elements
● In between two subscripts, the arrow is optional.
@$aref; # An array
reverse @$aref; # Reverse the array
$aref->[3]; # An element of the array
$aref->[3] = 17; # Assigning an element
$aref->[0]->[2] can be written as $aref->[0][2]
%$href; # A hash
keys %$href; # Get the keys from the hash
$href->{'red'} # An element of the hash
$href->{'red'} = 17; # Assigning an element
$href->{'green'}->{leaf}
can be written as $href->{'green'}{leaf}

9. Using references
• Example:
my $self = {
name =>'Krassi',
family_name => 'Berov',
children =>[qw|Maria Pavel Viktoria|],
favorite_things =>{
}
};
print "Hello! I am $self->{name} $self->{family_name}"'
print 'I have '. scalar @{$self->{children}}
.' children.';
print 'They are named:'
. map {$_ . "n"} @{$self->{children}};
#see using.pl for more

10. Using references
• Example:
my $self = {
name =>'Krassi',
family_name => 'Berov',
children =>[qw|Maria Pavel Viktoria|],
favorite_things =>{
}
};
print "Hello! I am $self->{name} $self->{family_name}"'
print 'I have '. scalar @{$self->{children}}
.' children.';
print 'They are named:'
. map {$_ . "n"} @{$self->{children}};
#see using.pl for more

11. Reference interpolation
• Example:
my @pets = qw|Goldy Amelia Jako|;
my $self = {
name =>'Krassi',
family_name => 'Berov',
can => sub { return shift },
pets => @pets,
children =>[qw|Maria Pavel Victoria|],
};
print <<TXT;
Hello!
I am $self->{name} $self->{family_name}.
I can ${$self->{can}('talk')}.
My first child is $self->{children}[0].
My last child is $self->{children}[-1].
I do not have a pet named $self->{pets}[1].
TXT

12. References
and nested data structures
• Resources
• perlreftut
• perllol
• perldsc
• perlref
• Beginning Perl (Chapter 7 – References)

13. References
and nested data structures
Questions?