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POKeys that spring into existence if checked with exists
 

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    <p>If a perl-hash is declared like so:</p> <pre><code>use warnings; use strict; my %h; </code></pre> <p>I can then check for the existence of keys within this hash with <code>exists</code>:</p> <pre><code>if (exists $h{k3}) { print "k3 exists\n"; } else { print "k3 doesn't exist\n"; } </code></pre> <p>Since <code>k3</code> does not exist, the script prints <code>k3 doesn't exist</code>.</p> <p>I can also check for the existence <code>ka</code> in <code>k3</code>:</p> <pre><code>if (exists $h{k3}{ka}) { print "k3 ka exists\n"; } else { print "k3 ka doesn't exist\n"; } </code></pre> <p>Unfortunately, this creates the key <code>k3</code>, so that another </p> <pre><code>if (exists $h{k3}) { print "k3 exists\n"; } else { print "k3 doesn't exist\n"; } </code></pre> <p>now prints <code>k3 exists</code>.</p> <p>This is a bit unfortunate for my purposes. I'd rather not want perl to create the <code>k3</code> key.</p> <p>I can, of course, check for <code>ka</code> within <code>k3</code> with something like </p> <pre><code>if (exists $h{k3} and exists $h{k3}{ka}) </code></pre> <p>which doesn't create the key <code>k3</code>. But I wonder if there is a shorter (and imho cleaner) way to check for <code>ka</code> within <code>k3</code>.</p> <p><strong>Edit</strong> The question was <em>marked as duplicate</em>. Unfortunately, the only answer in the referred question that mentions <code>no autovivification</code> has only one upvote there, and is not marked as accepted answer. For my purposes, the <code>no autovification</code> is the feature I needed to know (and that I was unknowingly after). So, I leave my question here as I think the accepted answer in the other question is not the optimal one.</p> <p>Unfortunatly, nobody so far answered with <code>no autovification</code> which I would like to accept. I might therefore answer my own question.</p>
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    1. USRené Nyffenegger
    1. USRené Nyffenegger
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    1. CO“Autovivification” usually does what you want it to: let intermediate references spring into existence. For testing nested structures with `exists`, this is unfortunate. `and`ing the various levels together is the usual approach, but [`no autovivification`](https://metacpan.org/module/autovivification) may help (I've never used that module).
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    2. CO[`no autovivification`](http://search.cpan.org/perldoc?vivification)
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    3. CO@amon: You should promote that to an answer
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      1. USBorodin
 

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