想利用DBM存multi-level hashes卻失敗的原因

這裡有個人做了跟我昨晚一模一樣的事
有人有提供解釋
http://bumppo.net/lists/macperl/1999/06/msg00504.html

(為避免以後找不到，原文轉貼)

• To: macperl@macperl.org
• Subject: [MacPerl] dbm and associative arrays of lists
• From: jean-Francois Jobidon <jfjobidon@yahoo.com>
• Date: Wed, 30 Jun 1999 14:32:17 -0700 (PDT)

---

Hi
First, thank for the help with my last question.
Here is another one :

I'd like to store informations in a database using a hash with a list
instead of a single value. And then I would like to change only one
item of a list.

Here is my code for creating the database :

$file = "my_dbm";

dbmopen (%alpha, $file, 0666)  || die "can't create $file\n";

@list1 = qw(item10 item11 item12 item13);
@list2 = qw(item20 item21 item22 item23);
@list3 = qw(item30 item31 item32 item33);

$key1 = "k1";
$key2 = "k2";
$key3 = "k3";

%hash = (
    $key1 => [@list1],
    $key2 => [@list2],
    $key3 => [@list3]
    );

%alpha = %hash;
dbmclose (%alpha);


And now, the code for reading :

$file = "my_dbm";

dbmopen (%beta, $file, 0666) || die "can't open «$file»\n";

@mat = (keys(%beta));
$keyX = pop (@mat);
@list = @{$beta{$keyX}};
$card = $#list;
#$card is the number of items in a list

foreach $item (keys (%beta)) {
    print "the key is $item", "\nthe values are :\n";
    for ($k = 0; $k <= $card; $k++) {
        print "the value of item $k is : $beta{$item}[$k]\n";
        }
    }

dbmclose (%beta);

The problem is $card is set to -1, wich means the list is empty.
What I'm doing wrong ?
Do I must join all the lists into a single value before writing the
database and then split it each time I want to modifie an item ? It
seems long to me, just to change a single value (like "element11"
instead of "item11")

Thanks
Jean-François Jobidon

jfjobidon@yahoo.com

On Wed, Jun 30, 1999 at 02:32:17PM -0700, jean-Francois Jobidon wrote:
> The problem is $card is set to -1, wich means the list is empty.
> What I'm doing wrong ?
> Do I must join all the lists into a single value before writing the
> database and then split it each time I want to modifie an item ? It
> seems long to me, just to change a single value (like "element11"
> instead of "item11")
> 

If you want to store multi-level hashes in a database, you need to use
something like the MLDBM module.  Otherwise, your outer references are
turned into strings, and the inner levels are lost.


http://www.perl.com/CPAN//local/modules/by-module/MLDBM/

Ronald