Searching in Hashes

[Gangreen]

There is a hash in my program.
The problem is I need to search in the hash, but there are some keys that are identical, but I need to get the associated value of all these keys.

when I do something like

$value = $hash{ "theKey" };

obviously I'm only getting the value associated to the first key called "theKey". How can I get the others? Some sort of loop?

thanks

[KevinADC]

I believe you about the unique key part.

Ii'm trying to figure out how I'm gonna bypass that problem. probably by working with arrays, which is easy, instead of hashes, but that will be slow with big files..

Do you know any way of creating a hash system in which I can store the names: like "Harry Potter..." and the productcode of the object. With the knowledge that there will be items with identical names but different productcodes AND that I need to be able to find the productcodes of all items mathing a specified name? (because I need to implement a "search" option based on product names)

Don't be offended, but judging by the code you posted you need to master some basics first. The way you are reading the data in from the file is a bit convoluted and slow, and the way you are building the hash is not correct.

Generic example:

my %hash;
open(FH,'file') or die "$!";
while(<FH>) {
   chomp;
   my @data = split(/\|/);
   push @{$hash{$data[1]}},$data[0];
}
close FH;
 

That will create a hash of arrays from the first two data fields of each line. To access the array that is associated with the hash key (assumes you know the name of the hash key):

foreach my $id (@{$hash{'Harry Potter'}}) {
     print "$id\n";
}
 

which is really the same as a regular array with the exception of some extra symbols @{} to dereference the array that the hash key points to.

[eWish]

If you plan on having your product line growing quickly, then I would suggest that you use a database to handle your data for you. It would simplify things for you greatly.

--Kevin

[minowicz]

Actually, what you are trying to do is not so ill advised as some have tried to make out. It sounds like what you really want to do is search through your database for products that match a given title. A hash is a great way to search for things, but unfortunately, as has been mentioned, the hash keys need to be unique.

The first possible work-around for this would be to simply use the product ID as your key. This gets you something unique, but means that to 'search' you'll have to iterate through all the keys and check for a value that matches the title for which you are searching.

My suspicion is that you want to avoid such a search by instead creating an index with a hash. The problem of course is that you can't simply use the title as the key to your has and associate the product ID with it since each new occurrence of the same title will result in overwriting the previously associated value.

Instead though, what you could do is create what is called a hash of arrays.

Rather than simply storing the value in the hash under a certain key, you instead could have each key associated with a reference to an array. Instead of putting the product ID into the value for a given key, you would add it to the array associated with the key. This gives you a hash that looks like this:

my %search_index = (
  'Title One'  => ['book-123', 'dvd-456'],
);
 
 

Then you'll simply be able to look for the key in the has and find out that book-123 and dvd-456 both match that title. A further lookup into the has that uses the product IDs as its keys will give you the full details for either or both of these matches. This scales out fairly well.

I'd post some example code, but I'm in a bit of a rush. Still, there are plenty of good guides and chapters in books on hashes of arrays and other data structures. One good place to start is

perldoc perldsc
 

[KevinADC]

Instead though, what you could do is create what is called a hash of arrays.

While you explained it in more detail, that is the same suggestion I already made in a previous post. There has been no mention of what he is trying to do to be ill advised that I noticed, it's the way his going about doing it that needs work.

Not that there is anything wrong with repeating what others have already suggested, especially since you took the time to elaborate on it more.

Hope to see you around here more often.

Regards,
Kevin

[Gangreen]

Thanks for the help and explanation, I appreciate it.
I'm gonna try the hash of arrays

thanks

[minowicz]

There has been no mention of what he is trying to do to be ill advised that I noticed

Perhaps that was a poor choice of words on my part. Still, I took the following:

If you plan on having your product line growing quickly, then I would suggest that you use a database to handle your data for you. It would simplify things for you greatly.

To mean that how the original poster intended to solve the problem was somehow deficient and far better served by using a database rather than a hash internal to the perl program. This may well be true, depending on the current and expected sizes of the database and the resources available on the machine that will be running the code.

It is unlikely that the use of a database would actually simplify things for the OP though. Instead it is likely to complicate things, at least in the short term. Selecting a database to use, learning SQL, and introducing a dependency on additional software that may not already be available on the machine where the script will run...

We already know that the current data is kept in a flat file. At a minimum, a hash of arrays or a hash of hashes is quite likely to be an improvement on that.

[Gangreen]

MySQL is no problem, I'm pretty good with it, but i'm simply not allowed to use that in my assignment.

[Gangreen]

You certainly helped me. I got it working now.

I have a new problem, but I'm not gonna sart a new topic, and instead I'll just post it here since it's about searching in hashes as well.

One question: how can i search for keys in a hash using a regular expression?
because if I do something like this:

if (exists $inventoryCodes {qr/.*/})

It doesn't find anything...

[KevinADC]

You certainly helped me. I got it working now.

I have a new problem, but I'm not gonna sart a new topic, and instead I'll just post it here since it's about searching in hashes as well.

One question: how can i search for keys in a hash using a regular expression?
because if I do something like this:

if (exists $inventoryCodes {qr/.*/})

It doesn't find anything...

You have to loop through a hash and check each key to find something with a regexp:

foreach my $key (keys %hash) {
   if ($key =~ /foo/) {
       found it
   }
}
 

You can also use grep on the hash keys but it might not be as efficient unless you wanted to find hash keys that are similar in some way.

Keep in mind that hash keys have to be simple strings, so what you posted just looks to see if there is hash key in the hash table that is literally 'qr/.*/' it does not try and match a hash key using a compiled regexp if thats what you are hoping.

But are you really searching for a hash key or a hash value? If it's a value and you don't know the name of the key you have to loop through the hash and use a regexp to search each value:

foreach my $key (keys %hash) {
   if ($hash{$key} =~ /foo/) {
       found it
   }
}
 

Or use grep to find multiple matches.

[Gangreen]

it's the keys I'm comparing. It's to expand the search I made with the help of this topic by adding functionality like searching for:

'keyword 1' AND 'keyword2'

so both keywords need to be in the string representing the key.

also an 'keyword 1' OR 'keyword2'

and also searching for one word in keys (given the fact the keys are sentences: i.e "Harry Potter and ..." )

I was just hoping there would be a more efficient way than just check each value and compare...becau se that won't give hashes an advantage in speed against arrays.