Perl_Part6

Within the code sample above, the only
unfamiliar symbol should be the square
brackets, these are used to denote
anonymous lists. Anonymous lists are
arrays without a name. A clue to this is
the square brackets ‘[]‘, usually seen
when accessing elements of an array:
print "$some_array[4]n";
So it’s not really counter intuitive that
square brackets be used elsewhere for
arrays. Using this philosophy can you
guess what sigils we use to create an
anonymous hash? We use ‘{}‘ curly
braces, as we use curly braces to retrieve
a value from a hash:
print "$some_hash{four}n";
Or if you prefer:
print $some_hash{'four'} . "n";
Returning to our list of users favourite
commands again, when we need
to reference the data inside our
nested structures, we need a means of
specifying the element inside the parent
data structure we want. We can access
‘%commands’, in the normal fashion:
#returns a string akin to U
'ARRAY(0x1ab54d0)'
print "$commands{'Billy'}n";
But this returns a string that looks like
“ARRAY(0x1ab54d0)” which actually
tells us a lot but not what we wanted.
The uppercase ‘ARRAY’ tells us that the
returned value is of an array reference
type and the characters with in the
parenthesis tell us where Perl stores the
reference. To access the data from the list
within the hash, we use the arrow
operator ‘->‘, this enables us to access
the list within the hash:
$commands{Billy}->[0];
This will return the first item from Billy’s
list of shell commands. For the purposes
of this exercise, we’ll say that the list
places favourite items first.
We can now make a hash of lists using
anonymous lists, we can make an array
of hashes too. you may by now be asking
yourself if there any other things that can
be made anonymously.
Anonymous Subroutines
It’s the Perl way, if you’ve got
anonymous hashes and anonymous lists,
then what about the functions and the
scalars? Perl provides them too. An
anonymous function seems like an odd
thing to have, until you get sufficiently
lazy, then you find yourself using them.
my %func = (
stdout => sub { print @_ },
log => sub { print LOG @_ },
stderr => sub U
{ print STDERR @_ },
not_stderr => sub {
print @_;
print LOG @_;
},
not_stdout => sub {
print STDERR @_;
print LOG @_;
},
all => sub {
print STDERR @_;
print LOG @_;
print @_;
}
);
# Print to all bar stdout:
&$func{not_stdout}( 'hello', U
66 November 2002 www.linux-magazine.com
W
hile the aspects of Perl that
have been covered in this
series so far are enough to
start you upon your way to becoming yet
another Perl hacker they’ve been the
basics of the language and offer nothing
other languages do not, albeit in a lot
less lines of code.
Nested Data structures
Perl facilitates complex datastructures in
several ways, by far the most readily
understood are the “hash of hashes” and
“list of lists”; these are nested data-
structures. It is also possible to have
“lists of hashes” and “hashes of lists”.
What we mean when we say a “list of
hashes” is that the data structure is a list
that contains hashes as its elements. One
example might be a list of people and for
each person a list of their top five
favourite shell commands:
Terry: rm -rf*, chmod 777, U
kill -9, ln -s, reboot
Billy: vim, df -h, ls -lah, U
ps -eaf, mutt
We could write that very quickly in Perl,
using nested data structures. In this
instance a hash of lists appears to be the
most sensible as the user’s names will be
unique identifiers and the top five
commands have no other significance
but the order in which they occur. Using
a hash structure for the names and a list
for the commands for each user we can
access the information in an intuitive
fashion and pull out details such as the
favourite command used by Billy. We’ll
show ways of obtaining this data later.
First we’d better store the data. One way
of writing this in Perl would be:
my %commands = (
Terry => [ 'rm -rf*', 'chmod U
777', 'kill -9', 'ln -s', U
'reboot' ],
Billy => [ 'vim', 'df -h', U
'ls -lah', 'ps -eaf', 'mutt' ]
);
This month we introduce some of the more powerful idioms and features of
Perl and show why it’s still one of the hackers languages of choice.
BY DEAN WILSON AND FRANK BOOTH
Perl: Part 6
Thinking in Line Noise
Perl Tutorial: Part 6PROGRAMMING

'world' );
# Print lots of times to each:
&$func{$_} for keys %func;
The last command seems completely nonsensical as it’s a
hash data structure, there is no telling what order the
elements will emerge when using the keys command,
which could cause problems. If you require order, use an
array. The following example will produce a set of error
levels, increasing in urgency.
my @warn = (
sub { print STDERR @_ },
sub { print LOG @_ },
sub { print @_ },
sub { die "I can't function under these U
conditions: ", @_,"n" }
);
sub notify {
my $error_level = shift || -1;
&$func{$_}(@_) for ( 0..$error_level );
-1
}
This example will report messages back according to the
error level. If the error level was 1, it would write to
‘STDERR’ and the log file. At level 3, it would write to the
‘STDERR’, ‘LOG’, ‘STDOUT’ and the finally stop the
program with a final message. It does this by looping
through the array of anonymous subroutines. There are a
few things that happen implicitly, that we’ll examine now:
my $error_level = shift || -1;
When variables are passed into a function they’re passed
in as an array ( @_ ). The ‘shift’ operator removes the first
item from an array, its default is ‘@_’, so if no array is
specified, it defaults to using @_. If the ‘@_’ array is false
( it has no contents ) the value -1 is placed in the variable
instead, why will become clear soon.
&$func{$_}(@_) for 0..$error_level;
This line uses Perl’s ‘for’ looping construct to iterate over
the first part of the line. In this instance it will repeat for
every number from 0 to $error_level, the value for the cur-
rent iteration will be put into the default variable ‘$_’.
Since the ‘for’ loop occurs at the end of the line the loop
condition doesn’t need braces. It is worth noting that the
range operator ( ‘..’ ) will not work backwards, it won’t
count from 10 to 1 using 10..1, it will merely skip the
entire loop as having failed on the first attempt.
The first part of the line calls the function from the array
‘@warn’, the element it references is the value of ‘$_’,
and the parameters the function is passed are the
remainder of the parameters passed to the notify function.
The ampersand ‘&‘ denotes that the thing in the hash is a
function. It is necessary to explain that the contents are a
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$_ = ref( $reference);
/SCALAR/ and return U
$$reference;
/ARRAY/ and return join U
(', ',@$reference );
/HASH/ and return join U
(', ', keys %$reference );
$_
}
This program uses the ‘ref’ function to
determine the data type of a reference.
‘ref’ returns one of a number of possible
values including the more common:
SCALAR, ARRAY, HASH or ‘’. The last
value indicates that the parameter sent
was in fact not a reference at all.
What the code does is define the
response taken when passed different
data types: Here is a list of the input type
and result output. ‘ref’ is extremely useful
when using generic datastructures that
can nest any type of data and have
no defined limit of depth to which it
is nested as it allows fully automatic
determination of the references type. You
may want to create a reference to an
existing structure, to enable access from a
function, or to link to a dynamically
structured list. We use the ‘‘ backslash
operator to dereferrence a value:
# Makes a reference to a scalar
my $foo_ref = $foo;
# Makes a reference to @foo U
called $foo_arrref
my $foo_arrref = @foo;
# Make an array of references.
my @list_of_arrays = U
( @foo, @bar, @baz );
# this can also be written:
@list_of_arrays = U
( @foo, @bar, @baz );
Here be Dragons
Closures are one of the more complex
features of Perl in that they build upon
previous knowledge and require a grasp
of a number of the language basics such
as scope and pass by reference before
they become readily comprehensible.
However like most magic, you don’t
need to understand it to wield it.
A closure is a function that exploits
both the lexical scope it is declared in
and Perl’s garbage collection algorithm
to preserve a variable beyond its ex-
pected lifetime.
We’ve not yet discussed Perl’s garbage
disposal routine in any depth as it is
unobtrusive and rarely falls to the
programmer to know or care what it
does and how it works. It tidies up after
us and ensures that the memory no
longer used in our programs is released.
The garbage collector in Perl works on
a very simple (in theory) principle
known as reference counting. Whenever
a new variable is created it starts off with
a reference count of 1 and each time a
reference to that variable is taken the
count increases by one.
Each time a reference to the variable
falls out of scope the reference count
decreases by one and when no more
references point to it (IE the reference
count is zero) the variable is ‘reaped’
by Perl’s garbage collection and the
memory it used is released automaticly,
no explicit ‘malloc’ and ‘free’ for us! To
clarify how closures work let’s look at
what we know. We know that a variable
declared in the scope of a block only
exists for that block…
{
my $count;
print "$countn";
}
# this line fails compilation
# as $count is not visible
print "$countn";
We also know that a function is global
regardless of where it is defined:
{
sub phrase {
return U
' I can be called anywhere ';
}
}
print phrase(); # this works.
So what happens when we mix the two?
{
my $count = 0;
sub set($){ $count = shift }
sub incr(){ $count++ }
sub getcnt() { $count }
}
set(5); # sets the count to 5
incr; # adds 1 to count.
#this prints 6
print getcnt(), "n";
function, otherwise Perl would expect a
normal scalar value and would interpret
the function as such.
References
References are scalar variables used to
point to anonymous data types and
functions. In all the above examples
we’ve relied on the containing data
structure to ensure we look at the data
we meant to or call the function we
intended. We can just as easily use a
scalar variable to do the same task.
my $array_reference = U
[ 1, 2, 3, 4, 5 ];
my $hash_reference = {
beef => 'corned',
cabbage => 'over-cooked'
};
We refer to the elements within the refer-
ence using the arrow operator ‘->‘:
$array_reference->[0];
@$array_reference[0];
$hash_reference->{beef};
%$hash_reference{beef};
We can refer to anonymous functions:
$func = sub { print "foon" };
&$func;
We can refer to scalar values too:
$func = '3.14';
print $$func;
Here we’ve prefixed the variable we’re
applying with a data type constraint.
Putting the wrong type in a data type
constraint will result in the program
concluding rather sooner than you’d
hoped, if you don’t know what type of
data to expect try something like this:
sub handleref ($) {
my $reference = U
shift or return;
Type Action
SCALAR Return thevalue.
HASH Returnajoinedlist ofkeys.
ARRAY Returnajoinedlist ofvalues.
NOTAREF Return thevalueitself.
DATA TYPES

We get a variable named ‘$count’ that
exists only for the functions ‘set’, ‘incr’
and ‘getcnt’ any other attempt to
reference the variable will fail. This gives
us a “tamed” global variable that has
limited ways of being altered while also
providing some data encapsulation; A
global variable we can manage.
There are instances when global
variables need to be used and there are
instances when you can use a closure
instead to make the code a little safer
and avoid another global. If you think
this looks a little like very primative
Object Orientation (OO) then you may
not be surprised to know that these
principles will hold you in good stead
when we get to Perl’s OO facilities.
While the above is a useful application
of a closure, it is not the most common
use of closures. In the example below we
use an anonymous subroutine to create a
bespoke function. This is probably the
most popular and often seen use of
closures within Perl.
sub hello($) {
my $message = shift;
return sub U
{ print "Hello $messagen";
}
This is a customisable function. A
closure can be created by calling the
function like so:
my $std = hello('world');
my $song = hello('dolly');
my $phrase = hello('nurse!');
We can call all the separate closures
using the ampersand symbol to signify
its a function and the variable that holds
the reference to the anonymous
subroutine. So:
&$std #will print: Hello world.
&$song #will print: Hello dolly.
&$phrase # will print: U
Hello nurse!
These rather trite examples serve only to
illustrate the basics of how closures
work but hopefully they will whet your
appetite for the advanced potential uses
they provide once you have made it past
the initial hurdle and understand how
they work.
Data::Dumper
Once you’ve started to use more
complex references you’ll inevitably
want to view the contents of a complex
data structure.
While your first instinct may be
to ‘unroll’ the structure with a number
of loops, a better approach would be to
use a module from the Perl core (it’s
installed by default) called ‘Data::U
Dumper’. We’ll show uses of Data::U
Dumper here without explaining all the
details behind using modules as a gentle
introduction.
A full explanation will be covered in
a future column. ‘Data::Dumper’ is a
module that is capable of serializing Perl
data structures so they can be printed to
screen or even written to a file while
remaining valid Perl code.
The last point is an important one that
warrants a deeper explanation, the
stringified version of the data structure is
still valid Perl code, this allows it to be
used in an ‘eval’ to recreate the
structures in the current application and
even to be read in from a file and used as
a simple persistence layer.
The example ‘simple_dump.pl’ below
shows a rudimentary use of ‘Data::U
Dumper’ to print a hash containing hash
references. Although the example may
look slightly contrived the principles can
still be applied to larger code such
as a function passing back a complex
hash ref of configuration settings such
as for example an ‘ini’ file style con-
figuration.
#Example: simple_dump.pl
use Data::Dumper;
my (%config, $config_ref);
%config = (
email => {
workdir => U
'/home/dwilson/work',
logdir => '/var/log/U
perlapps/examples/email'
},
news => {
workdir => U
'/home/dwilson/work',
logdir => '/var/log/U
perlapps/examples/news'
}
);
$config_ref = %config;
print Dumper($config_ref);
This example shows a simple use of
Data::Dumper’s procedural interface to
print the representation to the console.
The first line imports the ‘Data::Dumper’
module and allows any of its exported
functions to be called. We then create
both a hash and a scalar and immedi-
ately put some sample data in the hash.
It’s useful to note how the hash
of hashes is built up manually as
the ‘Data::Dumper’ representation is
remarkable similar.
The row following should now
be familiar as we take a reference to
the hash. Finally we make use
of Data::Dumper with the exported
‘Dumper’ function. If you run the code
you’ll see how closely the output
resembles the original code.
The ‘Data::Dumper’ module itself can
be used in either a procedural or object
orientated (OO) fashion allowing it to fit
inconspicuously in to the surrounding
code as all good third party modules
should. The example below uses the OO
interfaces and requires only minimal
changes:
#Example: simple_dump_oo.pl
#above here we would create U
the hash
my $dumper = Data::Dumper->U
new([$config_ref]);
print $dumper->U
Dump($config_ref);
We start the ‘simple_dump_oo.pl’
example with the same set up code used
in the ‘simple_dump.pl’ example. The
code changes begin in the last few lines
as we create an instance of the
Data::Dumper class and pass in the
reference we would like to have it work
on, notice the use of braces to force list
context, Data::Dumper’s constructor
expects its first argument (A second
optional argument is allowed) to be an
array ref.
Once we have a variable holding
the object we then call the ‘Dump’
method and get the same on screen
information we did with the procedural
version.
Now that the basic use of Data::U
Dumper has been shown we move on to
some useful options that can be config-
ured to customize how Data::Dumper
represents its output. These options are
69www.linux-magazine.com November 2002
PROGRAMMINGPerl Tutorial: Part 6

While the default settings are often
enough you may occasionally need
to tweak the settings to suit the use
the module is put to. Two modified
settings are $Data::Dumper::Indent and
$Data::Dumper::Useqq or in OO parlance
$OBJ->Indent and $OBJ->Useqq
The first of these two ‘$Data::U
Dumper::Indent’, controls the general
human readability of the output struc-
ture. From the minimum value of ‘0’
which strips out all but the essential
white space leaving the output as valid
perl code but not easily human readable
through to a maximum value of ‘3’. The
default value is ‘2’ and this causes the
output to have newlines, nicely lines up
entries in hashes and similar and
sensible indentation.
While a value of ‘2’ is often enough if
you are dealing with a large number of
with complex arrays then it is worth at
least considering a value of ‘3’ as its
main benefit is to put out the array index
along with the data allowing quick
visual look-ups at the cost of doubling
the output size. In practical terms it is
often enough to leave the setting at its
default value but if you are using
Data::Dumper to serialize the structures
to disk then you can get away with a
lower level as it only needs to be
machine readable.
The second of the more useful options
is the ‘$Data::Dumper::Useqq’ option
which causes the data to be put out in a
more normalized form which includes
white space represented as meta-
characters ([ntr] instead of literal
white space) characters that are
considered unsafe (Such as the ‘$‘ will
be escaped and non-printable characters
will be printed as quoted octal integers.
#Example: multi_oo_escape.pl
use Data::Dumper;
my %chars;
%chars = (
#one tab and one space
whitespace => ' ',
unsafe => '$',
#literal carriage return
unprintable => '^M'
);
my $dumper = Data::Dumper->U
new([%chars]);
$dumper->Useqq(1);
print $dumper->Dump(%chars);
In the ‘multi_oo_escape.pl’ example
above we have a one of each type of
character used as values in a hash that
we then pass as a reference to the
Data::Dumper constructor. We then set
the ‘Useqq’ to a positive value to turn it
on and then call the Dump getting an
output like this:
$VAR1 = {
"unsafe" => "$",
"unprintable" => "r",
"whitespace" => "t "
};
Notice that the unprintable carriage
return (generated in vi using CTRL-V
and then return) is printed as ‘r’ the tab
is printed as ‘t’ and the single dollar is
escaped to prevent it from having any
special meanings. The downside to the
additional functionality of ‘Useqq’ is that
it will incur a performance penalty due
to the fact that most of Data::Dumper is
implemented in C (Using XSUB) whereas
this function is implemented in pure Perl
which has a performance hit.
Now we have covered the basic and
more useful of the features Data::U
Dumper provides if you want to carry on
experimenting with it you should look at
perldoc Data::Dumper
They think it’s all over…
The use of references is often the
difference between an easy to follow and
maintainable piece of code and a tangled
mess of line noise and remains one of
the more important areas of Perl 5 syntax
to understand. Fortunately the best
documentation on references (although
the examples are quite terse) are
included in the Perl distribution itself:
A good place to start is with
‘perlreftut’, its a lighter read than the
others and has a number of easy to
follow examples. perldoc perlreftut
Once you have the basics down you
can either go for the in-depth details
with perldoc perlref or go for more
example code and explanations in
perldoc perllol which focus’s on arrays of
arrays. More varied examples in the data
structure cookbook in perldoc perldsc
A good final note is the reference page
for Data::Dumper itself, possibly the best
way of viewing or debugging references
perldoc Data::Dumper. ■
set differently depending upon the way
in which you are using the module, for
the moment don’t worry about their
purpose but rather how they are set. For
the procedural version:
$Data::Dumper::Useqq = 1;
$Data::Dumper::Varname = 'foo';
These configuration settings are global
so it is prudent to limit the scope the
changes affect by using them within a
separate often anonymous block, this is
best done using ‘local’:
{ #start anonymous block
local $Data::Dumper::Useqq U
= 1;
local $Data::Dumper::Varname U
= 'foo';
} # changes are lost when the U
code reaches here.
The options are set using methods in the
OO style of use and look like this:
$dumper->Useqq('1');
$dumper->Varname('foo');
When the settings are changed via
methods they do not need require the
jumping through hoops to limit the
scope of the change as any change
applies only to the one object:
my $dumper_cust = Data::DumperU
->new([$config_ref]);
$dumper_cust->Varname('foo');
print $dumper_cust->U
Dump($config_ref);
my $dumper_raw = Data::Dumper->U
new([$config_ref]);
print $dumper_raw->U
Dump($config_ref);
When the second Data::Dumper instance
(‘$dumper_raw’) prints its output it will
use ‘VAR’ instead of ‘foo’. Now we have
covered setting the values it is useful to
know that the methods also act as
accessors and if you call one with no
parameters it returns the current value:
my $prefix = $dumper_cust->U
Varname();
#prints 'default prefix is VAR'
print "default prefix is U
$prefixn";

Perl_Part6

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