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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :


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Advanced Subroutine Techniques


Subroutine Prototypes

Perl 5.002 introduces the capability to declare limited forms of subroutine prototypes. This capability allows early detection of errors in the number and type of parameters and generation of suitable warnings. This is primarily to allow the declaration of replacement subroutines for built-in commands. To use the stricter parameter checking, however, you must make the subroutine call by using only the subroutine name (without the & prefix). The prototype declaration syntax is concise and not as strict as the named formal parameters mechanism is in languages such as Pascal.

The main use for these prototypes at present is in writing modules for wider use, allowing the modules to specify their parameter types so as to trap errors and print diagnostic messages.

Subroutine Recursion

One the most powerful features of subroutines is their capability to call themselves. Many problems can be solved by repeated application of the same procedure. You must take care to set up a termination condition wherein the recursion stops and the execution can unravel itself. Typical examples of this approach occur in list processing: Process the head item and then process the tail; if the tail is empty, do not recurse. Another neat example is the calculation of a factorial value, as follows:

How to Return Arrays from a Subroutine

Subroutines can also return values, thus acting as functions. The return value is the value of the last statement executed; it can be a scalar or an array value. You can test whether the calling context requires an array or a scalar value by using the wantarray construct, thus returning different values depending on the required context. The following example, as the last line of a subroutine, would return the array (a,b,c) in an array context and the scalar value 0 in a scalar context:

wantarray ? (a, b, c) : 0;

You can return from a subroutine before the last statement by using the return() function. The argument to the return function is the returned value, in this case. The use of return() is illustrated in the following example (which is not a very efficient way to do the test but illustrates the point):

When multiple arrays are returned, the result is flattened into one list so that effectively, only one array is returned.

Imitating named arguments using hashes

Perl subroutines, by default, use "positional arguments." This means that the arguments to the subroutine must provided in pre-defined order to be correctly processed as parameters. For subroutines with a small argument list (three or fewer items), this isn't a problem.

sub pretty_print {
    my ($filename, $text, $text_width) = @_;

    # Format $text to $text_width somehow.

    open my $fh, '>', $filename
        or die "Cannot open '$filename' for writing: $!\n";

    print $fh $text;

    close $fh;

    return;
}

pretty_print( 'filename', $long_text, 80 );

Named Parameters in Perl

However, once everyone starts using your subroutine, it starts expanding what it can do. Argument lists tend to expand, making it harder and harder to remember the order of arguments.

sub pretty_print {
    my (
        $filename, $text, $text_width, $justification, $indent,
        $sentence_lead
    ) = @_;

    # Format $text to $text_width somehow. If $justification is set, justify
    # appropriately. If $indent is set, indent the first line by one tab. If
    # $sentence_lead is set, make sure all sentences start with two spaces.

    open my $fh, '>', $filename
        or die "Cannot open '$filename' for writing: $!\n";

    print $fh $text;

    close $fh;

    return;
}

pretty_print( 'filename', $long_text, 80, 'full', undef, 1 );

Quick question: What does the last parameter equal to one means? If it can't instantly answer this question the number of parameters is two big to pass them positionally and you need to figure a better way.

The most maintainable solution is to use "named arguments." In Perl 5, the best way to implement this is by using a hash reference that will be discussed in datil in the next chapter. Here we just mention that how to pass a hash reference:

sub pretty_print {
    my $ref = shift; # we got a reference into local variable href

    # Format $ref->{text} to $ref->{text_width} somehow.
    # If $ref->{justification} is set, justify appropriately.
    # If $ref->{indent} is set, indent the first line by one tab.
    # If $ref->{sentence_lead} is set, make sure all sentences start with
    # two spaces.

    open my $fh, '>', $ref->{filename}
        or die "Cannot open '$ref->{filename}' for writing: $!\n";

    print $fh $ref->{text};

    close $fh;

    return;
}

pretty_print((filename      => 'filename',
    text          => $long_text,
    text_width    => 80,
    justification => 'full',
    sentence_lead => 1,
));

Now, the reader can immediately see exactly what the call to pretty_print() is doing.

Optional Arguments

By using named arguments, you gain the benefit that some or all of your arguments can be optional without forcing our users to put undef  in all of the positions they don't want to specify.

A more complex example of function

A common Perl task is split a line according to some rules. That usually should be done with every line and subroutine is a natural solution here. For example Unix /etc/passwd file has a structure:

root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:
daemon:x:2:2:daemon:/sbin:
adm:x:3:4:adm:/var/adm:
lp:x:4:7:lp:/var/spool/lpd:
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:
news:x:9:13:news:/var/spool/news:
uucp:x:10:14:uucp:/var/spool/uucp:
operator:x:11:0:operator:/root:
games:x:12:100:games:/usr/games:
gopher:x:13:30:gopher:/usr/lib/gopher-data:
ftp:x:14:50:FTP User:/home/ftp:
nobody:x:99:99:Nobody:/:
postgres:x:100:100:PostgreSQL Server:/var/lib/pgsql:/bin/bash
xfs:x:101:101:X Font Server:/etc/X11/fs:/bin/false
bezroun:x:501:501:Nikolai Bezroukov:/home/bezroun:/bin/bash

If we need to get the first field (login name) and the last field (shell) from all records, we can write the following subroutine:

while(<>) {
  chomp($_);
  $user_shell=getusershell($_);
  print "user $user_shell\n;
}
sub getusershell { 
my @w;  
   @w = split(/:/,$_[1]);
   return ($w[0].': '.$w[-1]);
}

From the example above it's clear that call subroutine one needs to specify its name and the list of parameters, if any:

 $user_shell=getusershell($_);

The last value evaluated in subroutine is the value returns by default

In Perl subroutines, the last value seen by the subroutine becomes the subroutine's return value. In the example above, the return value is provided explicitly in return statement, but it can be rewritten as:

sub getusershell 
{ 
my @w, arg; 
   @w = split(/:/,$_[1]); 
   $arg=$w[0].' '.$w[-1]; # the value of $arg will be returned 
                          # as if statement return $arg was present
}

That's not a good practice to rely on this mechanism and it is better always use explicit return statement.

Recommended Links

Advanced Subroutine Techniques - Perl.com

Subroutines-in-Perl



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