Simple array checking?

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

I have a simple checking function which i am having trouble with and cannot find the solution. I am basically checking to see if an array has any content. The following code works:

my @modifiedFiles   = ();  


#assuming that this array is empty
if (!@modifiedFiles) 
{ 
   exit;
}
[download]

The follwoing code does not work and gives the error:


my $modifiedFiles   = {};  

#assuming this array is empty
if(!$modifiedFiles->{$streamID}}) 
{ 
    print "EXIT";
    exit; 
};

ERROR: Can't use an undefined value as an ARRAY reference at E:\Inetpu
+b\cgi-bin\wms.pl line 163.
[download]

THis is quite an obvious error since $modifiedFiles->{$streamID} has no value. Is there however a function i can use to check wether this array exists at all without throwing up an error like this?

costas

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Re: Simple array checking? by broquaint (Abbot) on Apr 12, 2002 at 13:28 UTC
You can check whether a key in a hash exists with the `exists()` function, and you can check if an array has anything it by comparing it's length to 0. If you want to check whether a given value in a hash is an array you could use `ref()`, and if so, just dereference it to check it's length. `my %hash = qw(foo bar baz quux); my $key = "baz"; my @array = qw(x y z); $hash{$key} = \@array; print "$key exists in %hash\n" if exists $hash{$key}; print "\@array has contents\n" if @array > 0; print "\$hash{\$key} is an array\n" if ref($hash{$key}) eq 'ARRAY'; print "\$hash{\$key} is an array with contents\n" if @{$hash{$key}} > 0; __output__ baz exists in %hash @array has contents $hash{$key} is an array $hash{$key} is an array with contents` [download] HTH broquaint	[reply] [d/l]
Re: Simple array checking? by perlplexer (Hermit) on Apr 12, 2002 at 13:26 UTC
Well, first of all, you have an unmatched curly in there. Second, you can't get that error from the code that you posted ($modifiedFiles is a hash ref, not an array ref). What is on line 163 in your script? --perlplexer	[reply]
Re: Simple array checking? by tachyon (Chancellor) on Apr 12, 2002 at 13:45 UTC
As perlplexer points out `$modifiedFiles` is a HASH ref not an ARRAY ref thus the code you post is not the cause of your troubles. You wish to check if a HASH KEY exists, and here is how you do it (you need the defined otherwise the key can exist but have a value of '' or 0 which will be false). unless(blah) is the same as if(! blah) `my $hash_ref = {}; unless ( defined $hash_ref->{'some_key'}) { print "some_key is not defined\n"; } my $other_hash_ref = { 'some_key' => 0 }; if ( ! defined $other_hash_ref->{'some_key'}) { print "some_key is not defined\n"; } else { print "some_key is defined\n"; }` [download] cheers tachyon s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print	[reply] [d/l]
Re: Simple array checking? by costas (Scribe) on Apr 12, 2002 at 13:39 UTC
Sorry about the unmatched bracket, mistyped into perlmonks!! line 163= `if(!(@{$createdFiles->{$streamID}}\|\|@{$modifiedFiles->{$streamID}}\|\|@{ +$deletedFiles->{$streamID}})) { exit; }; Can't use an undefined value as an ARRAY reference at E:\Inetpub\cgi-b +in\wms.pl line 163.` [download] I have already declared $createdFiles a a hash, the undefined value is i assume the key. I just want to know if they exit or not, i will try the exists function.	[reply] [d/l]
Re: Re: Simple array checking? by perlplexer (Hermit) on Apr 12, 2002 at 13:56 UTC
Alright, now we are getting somewhere. :) Here's what you can do: `# !@{$array_ref} <-- Perl will complain about this if $array_ref is un +defined # $#{$array_ref} <-- but not this if($#{$createdFiles->{$streamID}} == -1){ print "Empty\n"; }` [download] --perlpelxer	[reply] [d/l]