Learning Perl Objects, References amp;amp; Modules [Electronic resources] نسخه متنی

5.2 Viewing Complex Data with Data::Dumper

Another way to visualize a complex data
structure rapidly is to dump it. A particularly
nice dumping package is included in the Perl core distribution,
called Data::Dumper. Let's
replace the last half of the byte-counting program with a simple call
to Data::Dumper:

use Data::Dumper;
my %total_bytes;
while (<>) {
my ($source, $destination, $bytes) = split;
$total_bytes{$source}{$destination} += $bytes;
}
print Dumper(\%total_bytes);

The Data::Dumper
module defines the Dumper subroutine. This
subroutine is similar to the x command in the
debugger. You can give Dumper one or more values,
and Dumper turns those values into a printable
string. The difference between the debugger's
x command and Dumper, however,
is that the string generated by Dumper is Perl
code:

myhost% perl bytecounts2 <bytecounts-in
$VAR1 = {
'thurston.howell.hut' => {
'lovey.howell.hut' => 1250
},
'ginger.girl.hut' => {
'maryann.girl.hut' => 199,
'professor.hut' => 1218
},
'professor.hut' => {
'gilligan.crew.hut' => 1250,
'lovey.howell.hut' => 1360
}
};
myhost%

The Perl code is fairly
understandable; it shows that you have a reference to a hash of three
elements, with each value of the hash being a reference to a nested
hash. You can evaluate this code and get a hash
that's equivalent to the original hash. However, if
you're thinking about doing this in order to have a
complex data structure persist from one program invocation to the
next, please keep reading.

Data::Dumper, like the debugger's
x command, handles shared data properly. For
example, go back to that "leaking"
data from Chapter 4:

use Data::Dumper;
$Data::Dumper::Purity = 1; # declare possibly self-referencing structures
my @data1 = qw(one won);
my @data2 = qw(two too to);
push @data2, \@data1;
push @data1, \@data2;
print Dumper(\@data1, \@data2);

Here's the output from this program:

$VAR1 = [
'one',
'won',
[
'two',
'too',
'to',
[  ]
]
];
$VAR1->[2][3] = $VAR1;
$VAR2 = $VAR1->[2];

Notice how you've created two different variables
now, since there are two parameters to Dumper. The
element $VAR1 corresponds to a reference to
@data1, while $VAR2 corresponds
to a reference to @data2. The debugger shows the
values similarly:

DB<1> x \@data1, \@data2
0  ARRAY(0xf914)
0  'one'
1  'won'
2  ARRAY(0x3122a8)
0  'two'
1  'too'
2  'to'
3  ARRAY(0xf914)
-> REUSED_ADDRESS
1  ARRAY(0x3122a8)
-> REUSED_ADDRESS

Note that the phrase
REUSED_ADDRESS indicates that some parts of the
data are actually references you've already seen.