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    <p>The rule of thumb is to use the function most suited to your needs.</p> <p>If you just want the keys and do not plan to ever <em>read</em> any of the values, use keys():</p> <pre><code>foreach my $key (keys %hash) { ... } </code></pre> <p>If you just want the values, use values():</p> <pre><code>foreach my $val (values %hash) { ... } </code></pre> <p>If you need the keys <em>and</em> the values, use each():</p> <pre><code>keys %hash; # reset the internal iterator so a prior each() doesn't affect the loop while(my($k, $v) = each %hash) { ... } </code></pre> <p>If you plan to change the keys of the hash in any way <em>except</em> for deleting the current key during the iteration, then you must not use each(). For example, this code to create a new set of uppercase keys with doubled values works fine using keys():</p> <pre><code>%h = (a =&gt; 1, b =&gt; 2); foreach my $k (keys %h) { $h{uc $k} = $h{$k} * 2; } </code></pre> <p>producing the expected resulting hash:</p> <pre><code>(a =&gt; 1, A =&gt; 2, b =&gt; 2, B =&gt; 4) </code></pre> <p>But using each() to do the same thing:</p> <pre><code>%h = (a =&gt; 1, b =&gt; 2); keys %h; while(my($k, $v) = each %h) { $h{uc $k} = $h{$k} * 2; # BAD IDEA! } </code></pre> <p>produces incorrect results in hard-to-predict ways. For example:</p> <pre><code>(a =&gt; 1, A =&gt; 2, b =&gt; 2, B =&gt; 8) </code></pre> <p>This, however, is safe:</p> <pre><code>keys %h; while(my($k, $v) = each %h) { if(...) { delete $h{$k}; # This is safe } } </code></pre> <p>All of this is described in the perl documentation:</p> <pre><code>% perldoc -f keys % perldoc -f each </code></pre>
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    1. COPlease add a void-context keys %h; before each each loop to show safely using the iterator.
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