Presence of Key in Hash of Arrays

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

Greetings:
I have contructed an array of hashes iteratively like so:

while(<DATA>){
  ($key,$value)=split;
  push(@$myhash{$key}},$value);
}
[download]

Twice, in following code I test for the presence of a key in this hash. I notice that the second time i perform the test, the script always observes the key exists. I am wondering if I am inadvertently defining a key in my first check? The check follows...It is identical each time performed:

if($myhash{$key)){
  do something...
}
[download]

Does this inadvertently define a key? If so, is there an easier, more accurate way?
Thanks for your help,
Jason

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Re: Presence of Key in Hash of Arrays by dpuu (Chaplain) on Aug 02, 2002 at 20:14 UTC
Try: `if (exists $myhash{$key}) { ... }` This will avoid the "autovivication" of the hash element. However, I'm a bit confused: Your check might create the element, but it would give it an undefined value. So the next time you perform your check you should get the same value. --Dave	[reply] [d/l]
Re: Presence of Key in Hash of Arrays by DamnDirtyApe (Curate) on Aug 02, 2002 at 20:41 UTC
I tried to recreate your results, but the following code does not behave as you described: `#! perl use strict ; use warnings ; $\|++ ; my %myhash = () ; while ( <DATA> ) { my ( $key, $value ) = split ; push( @{ $myhash{ $key } }, $value ) ; } for my $key ( qw/ ni perch shrubbery / ) { if ( $myhash{ $key } ) { print "Found $key on pass 1.\n" ; } } for my $key ( qw/ ni perch shrubbery / ) { if ( $myhash{ $key } ) { print "Found $key on pass 2.\n" ; } } __DATA__ foo 1 bar 2 zoot 3 perl 4` [download] Could you post some working code that produces the results you mentioned? _______________ D a m n D i r t y A p e Home Node \| Email	[reply] [d/l]
Re: Presence of Key in Hash of Arrays by insensate (Hermit) on Aug 02, 2002 at 20:12 UTC
Sorry...let me restate 2 lines: In the first block: `push(@{$myhash{$key}},$value);` [download] In the second: `if($myhash{$key}){` [download] Thanks, Jason	[reply] [d/l] [select]
Re: Presence of Key in Hash of Arrays by tosh (Scribe) on Aug 02, 2002 at 22:31 UTC
Yes this does define a key, it made me very very VERY angry when I finally tracked down my errors to this. exists might work, I've never tried it, I just had to wuss-out and check for a value not just existence, although I'm sure there's a better way. Tosh	[reply]