Don't get me wrong guys...

Vorm — Tue, 12 Dec 2000 04:56:05 +0000

I'm not a total hash/Perl novice, I just sometimes miss obvious stuff.

And the Perl manpages, while a good source of data, I feel that they are VERY confusing and hard to understand. That's why I prefer to ask the people hear about stuff.

Mark Hensler — Fri, 01 Dec 2000 05:10:55 +0000

thanks japhy, I never knew about those functions... very usefull.

defined() vs. exists()

japhy — Sun, 26 Nov 2000 04:38:33 +0000

Technically, you should be using exists() to see if a key-value pair is found in a hash:

%hash = ( foo => undef );
if ($hash{foo}) { print 1 }          # not shown
if (defined $hash{foo}) { print 2 }  # still not shown
if (exists $hash{foo}) { print 3 }   # shown
if ($hash{bar}) { print 4 }          # not shown
if (defined $hash{bar}) { print 5 }  # still not shown
if (exists $hash{bar}) { print 6 }   # still not shown

Rob Pengelly — Sun, 26 Nov 2000 04:19:13 +0000

I wrote a few examples that will hopefully help you better understand hashes.

<strong>example 1</strong>
my %person_vidcard;

$person_vidcard{Frank} = 'Nvidia TNT 2';
$person_vidcard{Alex} = 'Nvidia TNT';
$person_vidcard{Xena} = 'Nvidia Geforce 2';
$person_vidcard{Guy} = 'Voodoo 3';

#Voodoo 3's suck, so we will delete Guy from the list
delete $person_vidcard{Guy};
foreach(sort keys %person_vidcard)
{
	print "$_ has a $person_vidcard{$_}\n";
} 

output:
Alex has a Nvidia TNT
Frank has a Nvidia TNT 2
Xena has a Nvidia Geforce 2

<strong>example 2</strong>
my %name_age = (
	Donna => 45,
	Sammy => 32,
	Mike  => 22
);
#It is Mike's birthday.  If he is found in the hash
#1 more year will be added to his age
if(defined $name_age{Mike})
{
	print "Mike is ".++$name_age{Mike};	
}	

output:
Mike is 23

On quoting hash keys...

japhy — Sat, 25 Nov 2000 15:54:15 +0000

If you read the 'perldata' documentation, you'll see:

an identifier within such curlies is forced to be a string,
as is any single identifier within a hash subscript. Our
earlier example, $days{'Feb'} can be written as $days{Feb}
and the quotes will be assumed automatically. But anything
more complicated in the subscript will be interpreted as an
expression.

[...]

It is often more readable to use the => operator between
key/value pairs. The => operator is mostly just a more
visually distinctive synonym for a comma, but it also
arranges for its left-hand operand to be interpreted as a
string--if it's a bareword that would be a legal identifier.
This makes it nice for initializing hashes:

  %map = (
    red   => 0x00f,
    blue  => 0x0f0,
    green => 0xf00,
  );

Mark Hensler — Sat, 25 Nov 2000 08:16:27 +0000

why?
you don't access them by numbers (usually, I guess you could give them # names...), so how would perl know what to name the next 'element'?

Vorm — Sat, 25 Nov 2000 06:18:55 +0000

This is sorta off topic, but don't you have to '' those hash keys Japhy? Or am I wrong again?

Your answer does make sense, I guess I just didn't think out of the box. I mean, it does seem logical that hashes would have a push, doesn't it?

Hashes are not ordered!

japhy — Sat, 25 Nov 2000 05:06:04 +0000

You need to read the 'perldata' documentation. A good place to start is http://www.perldoc.com/.

There is no "beginning" or "end" of a hash -- at least, not one that you are thinking of, or have control over. Hashes return their data in an order that is determined by Perl's source code, not you.

That being said, there is no push() equivalent for hashes. Just create a new key-value pair:

my %hash = (
  EFnet => 'irc.core.com',
  DALnet => 'liberty.nj.us.dal.net',
);
$hash{EFnet} = 'irc.prison.net';  # changing existing value
$hash{Undernet} = 'us.undernet.org';  # adding new pair