Array de-referencing problem

[Paul Harwood]

I wrote the following code:



$thread = "TEFD0";
$logfile = "V_DEBUG12121.txt";
$delay = "10232";
@servers = qw (SERVER1 SERVER2 SERVER3 SERVER4);
$server {$thread}->{$logfile}->{$delay} = @servers;

print @$server {$thread}->{$logfile}->{$delay};

My expectation is that it would simply print out the list value of
@servers. Instead I get an error: Can't use an undefined value as a HASH
reference.

If I use: print $server {$thread}->{$logfile}->{$delay};

Then I get the scalar context of the # of values in @servers - 4. So I
know I am missing something obvious. Since the value I am looking for is
the contents of an array, why do I get that error?


--Paul

[Jenda Krynicky]

From: "Paul Harwood" <harwood@nyclimits.org>

> I wrote the following code:
>
> $thread = "TEFD0";
> $logfile = "V_DEBUG12121.txt";
> $delay = "10232";
> @servers = qw (SERVER1 SERVER2 SERVER3 SERVER4);
> $server {$thread}->{$logfile}->{$delay} = @servers;

The first problem is here. You are evaluating the array in scalar
context and storing the resut (the length of the array) into the
hash.

The line above is equivalent to

$length = @servers;
$server {$thread}->{$logfile}->{$delay} = $length;

The values of a hash can only be scalars!
This means that you have to store a reference to the array in the
hash:

$server{$thread}->{$logfile}->{$delay} = \@servers;

> print @$server {$thread}->{$logfile}->{$delay};
>
> My expectation is that it would simply print out the list value of
> @servers. Instead I get an error: Can't use an undefined value as a
> HASH reference.

The problem in this case is that perl thinks that you want to use the
$server as a reference to a hash (or a name of a hash variable in
your case), then take a hash slice, and then use that hash slice as a
reference to hash.

That is it treats the line like this:

print @{$server}{$thread}->{$logfile}->{$delay};
while what you want is
print @{$server{$thread}->{$logfile}->{$delay}};

P.S.: You've accidentaly used "symbolic references". They are a bad
idea usualy even if you do want to use them, but in this case they
just confused the matter. You DO want to
use strict;
use warnings;
no warnings 'uninitialized';
on top of all your scripts.

Jenda
===== [email]Jenda@Krynicky.cz[/email] === [url]http://Jenda.Krynicky.cz[/url] =====
When it comes to wine, women and song, wizards are allowed
to get drunk and croon as much as they like.
-- Terry Pratchett in Sourcery

[Paul Harwood]

Thanks!

I got it to work using:

$server {$thread}->{$logfile}->{$delay} = [@servers];

But I guess it's better to use an array reference?

-----Original Message-----
From: Jenda Krynicky [mailto:Jenda@Krynicky.cz]
Posted At: Friday, November 21, 2003 3:37 AM
Posted To: Perl
Conversation: Array de-referencing problem
Subject: Re: Array de-referencing problem

From: "Paul Harwood" <harwood@nyclimits.org>

> I wrote the following code:
>
> $thread = "TEFD0";
> $logfile = "V_DEBUG12121.txt";
> $delay = "10232";
> @servers = qw (SERVER1 SERVER2 SERVER3 SERVER4);
> $server {$thread}->{$logfile}->{$delay} = @servers;

The first problem is here. You are evaluating the array in scalar
context and storing the resut (the length of the array) into the
hash.

The line above is equivalent to

$length = @servers;
$server {$thread}->{$logfile}->{$delay} = $length;

The values of a hash can only be scalars!
This means that you have to store a reference to the array in the
hash:

$server{$thread}->{$logfile}->{$delay} = \@servers;

> print @$server {$thread}->{$logfile}->{$delay};
>
> My expectation is that it would simply print out the list value of
> @servers. Instead I get an error: Can't use an undefined value as a
> HASH reference.

The problem in this case is that perl thinks that you want to use the
$server as a reference to a hash (or a name of a hash variable in
your case), then take a hash slice, and then use that hash slice as a
reference to hash.

That is it treats the line like this:

print @{$server}{$thread}->{$logfile}->{$delay};
while what you want is
print @{$server{$thread}->{$logfile}->{$delay}};

P.S.: You've accidentaly used "symbolic references". They are a bad
idea usualy even if you do want to use them, but in this case they
just confused the matter. You DO want to
use strict;
use warnings;
no warnings 'uninitialized';
on top of all your scripts.

Jenda
===== [email]Jenda@Krynicky.cz[/email] === [url]http://Jenda.Krynicky.cz[/url] =====
When it comes to wine, women and song, wizards are allowed
to get drunk and croon as much as they like.
-- Terry Pratchett in Sourcery


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[Jenda Krynicky]

From: "Paul Harwood" <harwood@nyclimits.org>

> Thanks!
>
> I got it to work using:
>
> $server {$thread}->{$logfile}->{$delay} = [@servers];
>
> But I guess it's better to use an array reference?

You are using an array reference.

The difference is that
$server {$thread}->{$logfile}->{$delay} = [@servers];
creates a copy of the array and stores the reference to the copy
while
$server {$thread}->{$logfile}->{$delay} = \@servers;
stored copy to the original array.

Which of these two do you need I'm not sure.

Try this:

@a = (1,2,3);
$ref1 = \@a;
$ref2 = [@a];

print "(@$ref1) == (@$ref2)\n";

$a[0] = 99;
print "(@$ref1) != (@$ref2)\n";

HTH, Jenda
===== [email]Jenda@Krynicky.cz[/email] === [url]http://Jenda.Krynicky.cz[/url] =====
When it comes to wine, women and song, wizards are allowed
to get drunk and croon as much as they like.
-- Terry Pratchett in Sourcery