question on hash behavior

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

Dear monks, I have a question for you concerning the behavior of exists on hash keys. Please consider this code:

use strict;
use warnings;
use Data::Dumper;

my %hash_data = ();

my $key1 = "a";
my $key2 = "1";
my $key3 = "B";
my $key4 = "_";


if (exists $hash_data{$key1}{$key2}{$key3}{$key4}) {
    print "exists\n";
} else {
    print "DOES NOT EXIST\n"
}

print "-----\n";
print Dumper (\%hash_data);
[download]

prints:

DOES NOT EXIST
-----
$VAR1 = {
          'a' => {
                   '1' => {
                            'B' => {}
                          }
                 }
        };
[download]

It looks like some sub-keys are generated (in this case $k1,$k2,$k3) if they don't exist. How can I avoid that? Thanks for sharing your wisdom!

Comment on question on hash behavior Select or Download Code

Replies are listed 'Best First'.
Re: question on hash behavior by toolic (Bishop) on May 03, 2016 at 16:49 UTC
Googling perl autovivification reveals Preventing autovivification while accessing value, which mentions things like Data::Diver to avoid it.	[reply]
Re: question on hash behavior by AnomalousMonk (Archbishop) on May 03, 2016 at 20:18 UTC
I haven't any real experience with it, but there's also the autovivification pragma. Because I'm not familiar with it, I would tend to use it in the narrowest possible scope: `c:\@Work\Perl\monks>perl -wMstrict -le "use Data::Dump; ;; my %hash; dd \%hash; ;; if (do { no autovivification; exists $hash{foo}{bar}{99}{x}{y} }) { print 'got it'; } ;; dd \%hash; " {} {}` [download] This also works with statement modifiers, e.g.: `print 'got it' if do { no autovivification; exists $hash{foo}{bar}{99}{x}{y} };` Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: question on hash behavior by BillKSmith (Monsignor) on May 03, 2016 at 18:28 UTC
`if ( exists $hash_data{$key1} and exists $hash_data{$key1}{$key2} and exists $hash_data{$key1}{$key2}{$key3} and exists $hash_data{$key1}{$key2}{$key3}{$key4} ) {` [download] Bill	[reply] [d/l]
Re: question on hash behavior by perlingRod (Novice) on May 03, 2016 at 17:25 UTC
The syntax in the if statement is that of a hash of hashes, four deep. Below is what I think you are really trying to do. `use strict; use warnings; use Data::Dumper; my %hash_data = (); #my $key1 = "a"; #my $key2 = "1"; #my $key3 = "B"; #my $key4 = "_"; my @keys = ("a", "1", "B", "_"); $hash_data{a} = "aValue"; for my $key (@keys) { if (exists $hash_data{$key}) { print "exists\n"; } else { print "DOES NOT EXIST\n" } } print "-----\n"; print Dumper (\%hash_data);` [download]	[reply] [d/l]
Re^2: question on hash behavior by shamat (Acolyte) on May 04, 2016 at 06:07 UTC
Thank you all! Autovivification is what I was missing. Today I learned something.	[reply]