Hash of Hashes

ewhitt has asked for the wisdom of the Perl Monks concerning the following question:

I have been trying to figure out hash of hashes. I have found a few URLs, but the examples aren't clicking. Here is the code I am playing with and output.


use Net::Amazon;

my $ua = Net::Amazon->new(token => 'xxxxxxxxxxxxxxxxxxxx', locale => '
+us');
my $response = $ua->search(browsenode=>"3839", mode=>"books", keywords
+=>"perl");

if ($response->is_success()) {
     for my $prop ($response->properties) {
          $books{$prop->publication_date()}{'title'}        = $prop->t
+itle();
      $books{$prop->publication_date()}{'authors'}      = join(", ",$p
+rop->authors());
      $books{$prop->publication_date()}{'isbn'}         = $prop->isbn(
+);
      $books{$prop->publication_date()}{'pages'}        = $prop->numpa
+ges();
      $books{$prop->publication_date()}{'publisher'}    = $prop->publi
+sher();
      $books{$prop->publication_date()}{'image_small'}  = $prop->Image
+UrlSmall();
      $books{$prop->publication_date()}{'image_medium'} = $prop->Image
+UrlMedium();
     }    
     print Dumper %books;
}
[download]

Data Dumper outputs

$VAR1 = '2005-07-12';
$VAR2 = {
          'image_medium' => 'http://ecx.images-amazon.com/images/I/21m
+kODqYmdL.jpg',
          'authors' => 'Damian Conway',
          'title' => 'Perl Best Practices',
          'isbn' => '0596001738',
          'publisher' => 'O\'Reilly Media, Inc.',
          'image_small' => 'http://ecx.images-amazon.com/images/I/11Ol
+nA6slCL.jpg',
          'pages' => '542'
        };
[download]

I expected $VAR2's content to be under $VAR1? On a side note, does anyone know why there is a "\" in "O'Reilly" in the publisher key?

Comment on Hash of Hashes Select or Download Code

Replies are listed 'Best First'.
Re: Hash of Hashes by FunkyMonk (Bishop) on Mar 09, 2008 at 09:52 UTC
$VAR2 is "under" $VAR1, it's your Data::Dumper call that's wrong. You need to backslash the hash (to create a hashref): `print Dumper \%books` [download] Apostrophes need to be quoted inside a single-quoted string, otherwise it will be interpreted as being the end of the string. Update: must `use Spelling` before writing "apostrophes"	[reply] [d/l] [select]
Re: Hash of Hashes by Corion (Patriarch) on Mar 09, 2008 at 09:53 UTC
The `Dumper` routine of Data::Dumper takes a reference, so you'll see the "real" data structure by `print Dumper \%books;` [download] The backslash in `'O\'Reilly Media, Inc.'` is in the output to quote the single-quoted string.	[reply] [d/l] [select]
Re^2: Hash of Hashes by ewhitt (Scribe) on Mar 09, 2008 at 10:23 UTC
Wow, the whole time I thought it was the hash statements. Thank you all very much for the insight. In regards to the backslash, are there other options than single quotes? As you can see in my code, it is being returned from the Amazon query.	[reply]
Re^3: Hash of Hashes by olus (Curate) on Mar 09, 2008 at 11:22 UTC
There is another way for Data::Dumper to display the results and that is by simply turning on the Useqq flag like this `$Data::Dumper::Useqq = 1;`, though it is not recommended. This way you'll get double quotes instead of single quotes. As a side note, I don't know what you are trying to do with your code, but you may get into trouble if you try to get two books published on the same date. If that happens, from your code, the first book retrieved will get overwritten and you won't know it was ever there. My advice to overcome that situation is to make, for each date, an array of hash references with the other book properties. Something like `if ($response->is_success()) { for my $prop ($response->properties) { my %info; $info{'title'} = $prop->title(); $info{'authors'} = join(", ",$prop->authors()); $info{'isbn'} = $prop->isbn(); $info{'pages'} = $prop->numpages(); $info{'publisher'} = $prop->publisher(); $info{'image_small'} = $prop->ImageUrlSmall(); $info{'image_medium'} = $prop->ImageUrlMedium(); push $books{$prop->publication_date()}, \%info; } print Dumper \%books; }` [download]	[reply] [d/l] [select]
Re^4: Hash of Hashes by Cody Pendant (Prior) on Mar 10, 2008 at 02:01 UTC
Re: Hash of Hashes by bingos (Vicar) on Mar 09, 2008 at 09:54 UTC
Try something like `print Dumper \%books;` [download] Your hash was becoming a list. There is a '\' in front of "'", because Dumper is escaping that character.	[reply] [d/l]