2.5 Printing Complex Data Structures

Sometimes you need to look inside your complex data structures to see what the settings are. One of the most useful ways to examine a data structure is by means of the Data::Dumper module. This module comes standard with all recent versions of Perl.

Here is the summary and part of the synopsis and description as output from the perldoc Data::Dumper command:

 NAME        Data::Dumper - stringified perl data structures, suitable        for both printing and "eval" SYNOPSIS            use Data::Dumper;            # simple procedural interface            print Dumper($foo, $bar); (...) DESCRIPTION        Given a list of scalars or reference variables, writes out        their contents in perl syntax. The references can also be        objects.  The contents of each variable is output in a        single Perl statement.  Handles self-referential strucTures correctly.        The return value can be "eval"ed to get back an identical        copy of the original reference structure. (...) 

This output of a two-dimensional array illustrates its use:

 use Data::Dumper; $array = [  ]; # Initialize the array for($i=0; $i < 4 ; ++$i) {   for($j=0; $j < 4 ; ++$j) {       $array->[$i][$j] = $i * $j;   } } # Print the array "by hand" for($i=0; $i < 4 ; ++$i) {   for($j=0; $j < 4 ; ++$j) {       printf("%3d ", $array->[$i][$j]);   }   print "\n"; } # Print the array using Data::Dumper print Dumper($array); 

This produces the output:

 0   0   0   0    0   1   2   3    0   2   4   6    0   3   6   9  $VAR1 = [           [             0,             0,             0,             0           ],           [             0,             1,             2,             3           ],           [             0,             2,             4,             6           ],           [             0,             3,             6,             9           ]         ]; 

You can make a nicer display by knowing exactly what the data is and in what form to write it out. Data::Dumper can also display the data in a fairly readable format (and there are several options as to how the data is displayed). In addition, Data::Dumper allows you to dump a data structure out to a file and then read it in to another program. See the perldoc Data::Dumper manpage for more details.

You can also print out an array of arrays @array by printing each row one at a time. Remember that each row is an anonymous array, so each entry of the @array array is a reference to an anonymous array:

 @array = (   [0, 0, 0, 0],    [0, 1, 2, 3],    [0, 2, 4, 6],    [0, 3, 99, 9]  ); for $anon (@array) {   print "@$anon\n"; } 

See the Perl perllol reference page for more information on initializing and printing arrays of arrays.