References
Arrays and hashes and lists and the manipulation thereof are very useful, but sometimes you want something a bit less flat; a little more multidimensional. Perl does indeed allow for multidimensional arrays (of a sort), but the syntax is a little odd. Let us enter the world of the reference.
References are like signposts to other data. To create a multidimensional array, what you actually need to do is create a normal 1-dimensional array whose elements are these signposts to other arrays. In this way, we can fake a multidimensional array. So, how do we create a reference? There are two ways. The first is to use the \ operator, which creates references to bits of data you have named and defined previously:
$scalar = "hello";
$scalar_ref = \$scalar;
@array = ( 'q', 'w', 'e', 'r', 't', 'y' );
$array_ref = \@array;
%hash = ( key => "value", lemon => "curd" );
$hash_ref = \%hash;
Because references are just a single signpost saying “the thing I’m referring to is just over there“, they are $ingular data, hence they are always $calars. Creating arrayrefs and hashrefs is such a common thing to do, there is a shorthand, which allows you to create them directly, and without naming them:
$array_ref = [ "q", "w", "e", "r", "t", "y" ];
$array_ref2 = [ qw{ you can use qw to save time } ];
$hash_ref = { key => "value", lemon => "curd" };
These create references to anonymous arrays and anonymous hashes. In the first example, we created references to named arrays and hashes, like @array and %hash. In this example, there is no named array or named hash, only the [ ] brackets or { } braces, which are never named.
You can also create references to anonymous subroutines:
$code_ref = sub { return 2**$_[0] };
Which is very, very useful, as we shall see.
Dereferencing
Creating references is easy then: you just need the right mix of \, and { } [ ] braces and brackets. How do we get back to the contents? This is dereferencing. To see how this works, let’s take an example of the multidimensional array we wanted in the first place:
@unidimensional = (
( "this is just a one dimensional array", "even though"),
( "we have nested", "parentheses" )
);
# parentheses don't nest in perl:
# everything gets flattened to a single list
@multidimensional = (
[ "but this one", "really is multidimensional" ],
[ "two by", "two" ]
);
# brackets mean create references to anonymous arrays
# and put them in @multidimensional
The multidimensional array is really a one-dimensional array that contains pointers (references) to the location of other arrays. So Perl doesn’t really have multidimensional arrays, but you can fake them using references. Getting at the data is a little complex:
my @multidimensional = (
[ "but this one", "really is multidimensional" ],
[ "two by", "two" ]
);
my $element_1_1 = $multidimensional[1]->[1]; # gets 'two'
print $element_1_1;
two
The $multidimensional[1] bit is obvious: we’re just using the usual (counting from zero) syntax for pulling out the 2nd element of the array. If we actually captured this:
my @multidimensional = (
[ "but this one", "really is multidimensional" ],
[ "two by", "two" ]
);
$element_1 = $multidimensional[1];
print ref $element_1;
print $element_1;
the scalar $element_1 would contain the reference itself. We can use the ref operator to find out what the scalar points to:
print ref $element_1;
ARRAY
We can also find out what perl calls the reference:
print $element_1;
ARRAY(0x183f1dc)
or similar, which isn’t very informative! The 0x... is a hexadecimal code to the array’s location in memory, basically the ‘map reference’ where the array has been stored. So although:
$multidimensional[1]
returns a reference to an array ('0x183f1dc'):
->[1]
is the bit that actually ‘dereferences’ the reference, chasing the pointer to the array at 0x183f1dc. You could think of the -> as ‘follow this signpost to the real data’. The -> dereferencing arrow can also be used to pull out bits of multidimensional hashes:
my %multi_hash = (
name => {
fore => "Steve",
sur => "Cook",
},
age => 26 # fucking hell, this tutorial is 10 years old
);
my $forename = $multi_hash{name}->{'fore'};
my $age = $multi_hash{'age'};
print "name $forename, age $age\n";
name Steve, age 26
HoH, AoA and other Thingies
You can see here that a multidimensional structure needn’t be a boring rectangular matrix (an array of arrays, AoA), exactly 2 by 2 or 4 by 4 by 3. Not does it need to be a hash of hashes (HoH). It can be an unbalanced and weird Thingy: you can have anything you like:
my @complex = ( # is an array at the top-level, containing...
"a scalar",
{ and => "an internal hashref" },
"another scalar",
[ # element 3 of the @complex is this big arrayref
[
"a two deep arrayref",
"with two elements"
],
{ # element 1 of this arrayref is a little hashref
lemon => "curd" # key lemon of this hashref is curd
}
]
);
print my $wanted = $complex[3]->[1]->{'lemon'};
Hopefully the syntax should be OK now: if you wanted the words “curd”, you need:
my $wanted = $complex[3]->[1]->{'lemon'};
i.e. take the third element of @complex (don’t forget we count from 0!), that is the big arrayref at the end, dereference it to get the first element (the little hashref inside the arrayref), then dereference the hashref with the key lemon.
When we do objects later, we’ll find that they are usually anonymous hashrefs:
$anonymous_hashref = { name => "Cornelia", species => "Elaphe guttata" };
to get at the name this time, we need the syntax:
$name = $anonymous_hashref->{'name'};
Note that because we are starting with a scalar (not an array as in the last example), the very first thing we need to do is dereference it with an arrow: the scalar $anonymous_hashref points to an (anonymous) hash, so you can’t just look at its keys, because a scalar doesn’t have keys. If this doesn’t make sense, compare these:
@array = ( qw/ some elements/ );
$zeroeth_element_of_array = $array[0];
$arrayref = [ qw/some elements/ ];
$zeroeth_element_of_arrayref = $arrayref->[0];
There is a certain shortcut you can take when dealing with multidimensional arrays and hashes (and mixtures, like arrays of arrayrefs of hashrefs). Since an array or hash can only contain a single 1 dimensional list, we know that:
@array = ( [ "two", "by" ], [ "two", "elements" ] );
$element_0_1 = $element[0][1];
can only mean:
$element_0_1 = $array[0]->[1];
So, in general, if you’re dealing with something that is a real array or hash at the highest level, you can miss off the -> arrows completely unambiguously, so our earlier example:
$wanted = $complex[3]->[1]->{'lemon'};
could be written with less line noise as:
$wanted = $complex[3][1]{'lemon'};
However, you must be careful with this syntax if you’re dealing with something that is a scalar at the highest level (i.e. an anonymous hashref or arrayref):
$arrayref = [ [ "two", "by" ], [ "two", "elements" ] ];
# note this is scalar (an arrayref), not an array
$element_0_1 = $arrayref->[0]->[1];
$element_0_1 = $arrayref->[0][1];
The last two are equivalent, but note you can’t dispense with the first -> , which is actually obvious, as:
$element[0][1];
means the first element of the zeroeth arrayref contained in @element, which doesn’t exist (or if it does, you’ll get the value from that, which isn’t what you’re after).
You’ve now seen that constructing references can be done in many ways. TIMTOWTDI. Some examples:
$arrayref = [ 0, 1, 2, 3 ];
# create a scalar that is an anonymous arrayref
@array = ( 0, 1, 2, 3 );
$arrayref = \@array;
# create a reference to a real named array
@multi = ( \@one, \@two, \@three );
# multidimensional array constructed from references to named arrays
@multi2 = ( [ 1, 2 ], [ 3, 4 ], [ 5, 6 ] );
# multidimensional array constructed from anonymous arrayrefs
$arrayref2 = [ [ 1, 2 ], [ 3, 4 ], [ 5, 6 ] ];
# multidimensional: arrayref constructed from anonymous arrayrefs
Slicing thingies
Often you won’t want to play with just a single element of a thingy (which is the not very descriptive term for a Perl data structure composed of some gungy mass of references). You’ll want a slice of a thingy, or to iterate over the whole array pointed to by an arrayref. How do we do this? Well, both need the same sort of dereferencing syntax:
$arrayref = [ 0, 1, 2, 3, 4 ];
@slice = @{ $arrayref }[ 0, 2 ];
@anonymous_array_pointed_to_by_arrayref = @{ $arrayref };
The way to read @{ $arrayref } is ‘the array referred to by $arrayref‘. If you want a hash, you use a similar syntax:
$hashref = {
lemon => "curd",
strawberry => "jam",
orange => "marmalade",
};
@hashslice = @{ $hashref }{ lemon, orange };
# don't forget slices are plur@l data
%anonymous_hash_pointed_to_by_hashref = %{ $hashref };
This (of course?) produces yet another way of accessing the individual elements of a reference:
$hashref = {
lemon => "curd",
strawberry => "jam",
orange => "marmalade",
};
$hashelement = ${ $hashref }{ "lemon" };
# the pairs inside the hash referred to by $hashref are $calar data
$arrayref = [ 0, 1, 2, 3, 4 ];
$arrayelement = ${ $arrayref }[2];
#the elements inside the array referred to by $arrayref are $calar too
So that if we have a dereferenced arrayref:
@{ $arrayref }
we access its elements with the usual Perl syntax:
${ $arrayref }[ INDEX ]
In simple cases, the { } braces can be omitted, although I personally never miss them off, as I get easily confused:
@array = @{ $arrayref };
@array = @$arrayref;
are equivalent nonetheless.
Creating reference structures at run-time
All the examples so far have relied on you knowing the structure of the thingy at compile time. Although there will be times when you will create huge perl thingies in your code, more often than not, you will be building them at run time from data entered by users or from other files. In fact, a great deal of programming parsing is to do with converting data from one format to another. Data stored in flat files is called ‘serialised’ data. Data stored within a script will usually be more tree-like. Conversion from serialised input (such as XML files, text files or user input) to serialised output (such as an HTML file, or an email) via an internal parse tree of some sort is often a job for modules from CPAN, but you’ll frequently need to create a quick and dirty converter for a simple format such as the one below. Here, we use simple array and hash manipulation operators (like push, for, and hash assignment) to create a Thingy on the fly. Since references are just pointers to hash and array structures, building and manipulating them is a simple matter of getting up close and personal to these functions.
#!/usr/bin/perl
use strict;
use warnings;
my $record;
my @people;
while ( <DATA> ) {
chomp;
if ( /\*\*\*/ ) { # start of record marker
$record = {}; # create an empty hashref
}
elsif ( my ( $field, $data ) = / (\w+) \s* = \s* (.*) /x ) {
if ( $field eq "pets" ) {
$data = [ split /\s*,\s*/, $data ];
# create an anonymous arrayref:
# you can wrap things that return lists in []
# and create arrayrefs as simply as this. Good, isn't it?
}
$record->{ $field } = $data;
# add key/value pair to the anonymous hashref $record
}
elsif ( /---/ ) { # end of record marker
push @people, $record; # add the hashref to the @people array
}
else {
next;
}
}
# we have now created a tree in memory, looking something like:
# @people = (
# { name=>'alice', age=>37, pets=>[] },
# { name=>'bob', age=>23, pets=>[ 'dog' ] },
# { name=>'eve', age=>26, pets=>[ 'millipede', 'snake' ] },
# );
for my $person ( @people ) {
print ucfirst "$person->{'name'} is $person->{'age'} years old ";
# ucfirst capitalises the first letter of a string
if ( my @pets = @{ $person->{'pets'} } ) {
# assignment is neatest here, as we use @pets in a minutes
local $" = ', ';
# the $" is a special perl variable, containing
# the thing used to separate array elements in quoted strings:
# usually it's a space, but we make it a comma and space for
# our output here
print "and has the following pets: @pets.\n";
}
else {
print "and has no pets.\n";
}
}
# everything after a __DATA__ token is available to a perl script and
# can be read automagically via the DATA filehandle, which is opened
# on running your script
__DATA__
***
name = alice
age = 37
pets =
---
***
name = bob
age = 23
pets = dog
---
***
name = eve
age = 26
pets = millipede, snake
---
Alice is 37 years old and has no pets.
Bob is 23 years old and has the following pets: dog.
Eve is 26 years old and has the following pets: millipede, snake.
So we take one serial format (our __DATA__ section), convert it into an internal thingy, then dump the thingy in our own chosen format.
Passing by value and passing by reference
What other practical uses are there for references? Well, if you’ve tried to return arrays from subroutines, you’ll find that it doesn’t work how you expected:
my ( @one, @two ) = fruits();
print "one @one\ntwo @two\n";
sub fruits {
my @first = qw/ lemon orange lime/;
my @second = qw/ apple pear medlar/;
return @first, @second;
}
one lemon orange lime apple pear medlar
two
The problem is that arrays in list context (such that return gives its arguments) interpolate their members. That is, return flattens @first and @second into one big list. The second problem is that arrays are greedy, so when we return this flattened list from the subroutine, @one slurps up all the return values, and @two gets nothing. There is a kludge to get around this:
my ( @one, @two );
( @one[ 0 .. 2 ], @two[ 0 .. 2 ] ) = thingy();
but this is obviously dependent on knowing how many elements are returned: this will break if the two arrays have variable lengths. What we really need to do is pass the arrays by reference:
my ( $one, $two ) = fruits();
print "one @{$one}\ntwo @{$two}\n";
sub fruits {
my @first = qw/ lemon orange lime/;
my @second = qw/ apple pear medlar/;
return \@first, \@second;
}
The subroutine returns a list of arrayrefs (effectively an array of arrays), which the body of the program then plays with using the @{ $ref } dereferencing syntax. You can be explicit if you want, and dereference the arrayrefs back to real arrays if you’d rather:
my ( $one, $two ) = fruits();
my @one = @{ $one };
my @two = @{ $two };
There’s something rather important to note about passing things by reference instead of by value, as we have done here:
my @spices = qw( ginger kratchai galangal );
by_value( @spices );
print "@spices\n";
sub by_value {
my @spices = @_;
push @spices, "turmeric";
}
ginger kratchai galangal
When you pass things by value, you are passing copies of the data, so manipulating these data in the sub does nothing interesting to the original data. To do that, you’d have to return the altered array, and capture it in @spices to change the original data. However, when you pass by reference, you are passing pointers to the original data, so if you manipulate what the references point to, you are modifying the original data:
my @spices = qw( ginger kratchai galangal );
by_reference( \@spices );
print "@spices\n";
sub by_reference {
my $spices = shift;
push @{ $spices }, "turmeric";
}
ginger kratchai galangal turmeric
Another source of subtle bugs is directly messing with the elements of @_:
my @spices = qw( ginger kratchai galangal );
by_value( @spices );
print "@spices\n";
sub by_value {
$_[0] = 'cardamom';
}
cardamom kratchai galangal
Like $_ in a foreach loop, the items in @_ are aliased to the items in the list of arguments passed to the subroutine. Modifing them will modify the data in the body of the program, which is generally something you don’t want to do. It’s always best to be explicit if you want the subroutine to modify its arguments. If you don’t want it to, pass by value, and immediately copy the contents of @_ into some nice lexically scoped variables.
Data::Dumper
References can get quite complicated, which is where the module Data::Dumper comes in very handy. Modules can be used, like you have seen with strict: they imports some extra functionality to your program, in this case, a function called Dumper().
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;
my @complex = (
"a scalar",
{ and => "a internal hashref" },
"another scalar",
[
[
"a two deep arrayref",
"with two elements"
],
{
lemon => "curd"
}
]
);
print Dumper( \@complex );
$VAR1 =
[
'an array',
{
'containing' => 'a hashref'
},
'a scalar',
[
[
'a two deep arrayref',
'with two elements'
],
{
'lemon' => 'curd'
}
]
];
Data::Dumper takes a reference to any perl data structure and spits out the contents in a) a pretty printed format, and b) in a format that you can actually print to a file:
open my $FILE, $file;
print $FILE Dumper( \@complex );
to save it for later (serialisation). You can then recover the data with:
do $file;
Although don’t do this if you do not trust the data!
Anonymous functions and closures
You may remember a while ago that I said you can create anonymous subroutines:
$coderef = sub { return 2*$_[0] };
To use these, you can dereference them like normal, using () parentheses, as you might have guessed:
print $coderef->(3);
6
This is very powerful: it means you can actually return bits of code from subroutines:
#!/usr/bin/perl
use strict;
use warnings;
my $multiply = <STDIN>;
my $coderef = construct( $multiply );
print $coderef->( 2 );
sub construct {
my $number = shift;
my $coderef = sub { return $number * $_[0] };
return $coderef;
}
This creates coderefs on the fly which will multiply by whatever number you decide via STDIN. The technical term for such things is closures: the weird thing about them is that the $number in sub construct is a lexically scoped my variable, and ought to disappear forever (go out of scope) when you leave the subroutine. However, when you use:
$coderef->( 2 ); # or equivalently
&{ $coderef }( 2 );
# & is the sigil for subroutines, like arrays get @ and hashes get %
the value of $number you entered when you constructed the coderef is still there, deeply bound into the coderef, even though it ‘shouldn’t’ really exist outside the scope of sub construct. Magic.
So far we have only talked about so called hard references. There are also such things as soft references, which if you try to use when strict is on will barf. In the interests of hygiene, I’ll not tell you about them, they’re not much used, for good reason, and you can always find out about them yourself. If you ever think you need one, it’s almost certain what you actually need is a hash.
Next up…bits and bobs.
![Vorticella micrograph [CC-BY-SA-3.0 Steve Cook]](http://www.polypompholyx.com/wp-content/uploads/2012/10/vorticella_micrograph.jpg "Vorticella micrograph [CC-BY-SA-3.0 Steve Cook]")
![Stentor diagram [CC-BY-SA-3.0 Steve Cook]](http://www.polypompholyx.com/wp-content/uploads/2012/10/stentor_diagram.png "Stentor diagram [CC-BY-SA-3.0 Steve Cook]")