Printing Data Structures

Problem

You want to print out a data structure.

Solution

If the output's legibility and layout are important, write your own custom printing routine.

If you are in the Perl debugger, use the x command:

DB<1> $reference = [ {
 "foo" => "bar" }, 3, sub {
 print "hello, world\n"
}
]; DB<2> x $reference  0 ARRAY(0x1d033c)  0 HASH(0x7b390)  'foo' = 'bar'>  1 3  2 CODE(0x21e3e4)  - & in ???>

From within your own programs, use the Dumper function from the CPAN module Data::Dumper:

use Data::Dumper; print Dumper($reference);

Discussion

Sometimes you'll want to make a dedicated function for your data structure that delivers a particular output format, but often this is overkill. If you're running under the Perl debugger, the x and X commands provide nice pretty-printing. The x command is more useful because it works on both global and lexical variables, whereas X only works on globals. Pass x a reference to the data structure you want to print.

D<1> x \@INC  0 ARRAY(0x807d0a8)  0 '/home/tchrist/perllib'   1 '/usr/lib/perl5/i686-linux/5.00403'  2 '/usr/lib/perl5'   3 '/usr/lib/perl5/site_perl/i686-linux'   4 '/usr/lib/perl5/site_perl'   5 '.'

These commands use the dumpvar.pl library. Here's an example:

{ package main; require "dumpvar.pl"
}
*dumpvar = \&main::dumpvar if __PACKAGE__ ne 'main'; dumpvar("main", "INC"); # show both @INC and %INC

The dumpvar.pl library isn't a module, but we wish it were - so we cajole it into exporting its dumpvar function anyway. The first two lines forcibly import the main::dumpvar function from package main into the current package, assuming it's different. Here's the output of that call:

@INC = (  0 '/home/tchrist/perllib/i686-linux'  1 '/home/tchrist/perllib'  2 '/usr/lib/perl5/i686-linux/5.00404'  3 '/usr/lib/perl5'  4 '/usr/lib/perl5/site_perl/i686-linux'  5 '/usr/lib/perl5/site_perl'  6 '.' ) %INC = (  'dumpvar.pl' = '/usr/lib/perl5/i686-linux/5.00404/dumpvar.pl'  'strict.pm' = '/usr/lib/perl5/i686-linux/5.00404/strict.pm' )

The Data::Dumper module, located on CPAN, has a more flexible solution. It provides a Dumper function that takes a list of references and returns a string with a printable (and evalable) form of those references.

use Data::Dumper; print Dumper(\@INC); $VAR1 = [  '/home/tchrist/perllib',   '/usr/lib/perl5/i686-linux/5.00403',  '/usr/lib/perl5',   '/usr/lib/perl5/site_perl/i686-linux',  '/usr/lib/perl5/site_perl',   '.' ];

Data::Dumper supports a variety of output formats. Check its documentation for details.

See Also

The documentation for the CPAN module Data::Dumper; the section "The Perl Debugger" from of Perl Programming or perldebug (1)

Reading and Writing Hash Records to Text Files		Copying Data Structures