How do I print a hash structure in Perl?

Question

Examples:

%hash = (2010 => 21, 2009=> 9);

$hash = {
    a => {
        0 => {test => 1},
        1 => {test => 2},
        2 => {test => 3},
        3 => {test => 4},
    },
};

How do I print the hash?

Answer 1

use keys , values function

@keys = keys %hash ; 

@values = values %hash 

Answer 2

Hi

This should help:

foreach $key (keys %hash)
{
  print "key is : $key, value is : $hash{$key}\n";
}

cheers

Answer 3

printf ("%s = %s\n", $_, $hash {$_}) foreach (keys (%hash));

Answer 4

  while( my( $key, $value ) = each( %hash ) ) {
         ...
  }

Answer 5

instead of

%hash = { 2010=> 21, 2009=> 9 }

you should write

%hash = ( 2010=> 21, 2009=> 9 ); 

with the curly braces you get a REFERENCE to an anonymous hash, which is then stored as the first key of you %hash.

Answer 6

Do you want to print the entire hash, or specific key, value pairs? And what is your desired result? IF it's just for debugging purposes, you can do something like:

use Data::Dumper;
print Dumper %hash; # or \%hash to encapsulate it as a single hashref entity;

You can use the each function if you don't care about ordering:

while ( my($key, $value) = each %hash ) {
    print "$key = $value\n";
}

Or the for / foreach construct if you want to sort it:

for my $key ( sort keys %hash ) {
    print "$key = $hash{$key}\n";
}

Or if you want only certain values, you can use a hash slice, e.g.:

print "@hash{qw{2009 2010}}\n";

etc, etc. There is always more than one way to do it, though it helps to know what you're frying to do first :)

Answer 7

Syntax to access inner cells for your second example is like:

print $hash->{"a"}{0}{"test"}

That will give you 1 in your example.

If you want to iterate on it, you can do it as follows (print lines are for illustration purposes):

my $hash = {"a"=>{ 0=>{"test"=>1}, 1=>{"test"=>2}, 2=>{"test"=>3}, 3=>{"test"=>4} } };
print "Direct access to item : ".$hash->{"a"}{1}{"test"}."\n";

foreach my $k1 (keys(%$hash)) {
    print "keys of level 1: $k1\n";
    foreach my $k2 (keys(%{$hash->{$k1}})) {
        print "keys of level 2: $k2\n";
        print "values: ".$hash->{$k1}{$k2}{"test"}."\n"
     }
}

Notice that things are a bit more tricky than necessary because the external $hash is a scalar reference to an anonymous hash. It would be simpler if it was a hash (i.e., like in my %hash = (1, 2); print $hash{1};).

(TIMTOWTDI: there is obviously more than one way to do it; I believe the above example is the simplest for me, but not the most efficient; using each instead of keys for iterating would avoid one unnecessary hash lookup).