blocks|key|3833934|text|您必须使用exists函数|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3833935|3833936|+exists+EXPR|blockquote|3833937|3833938|给定一个指定哈希元素的表达式，如果哈希中指定的元素已初始化，则返回true，即使相应的值未定义。|3833939|请注意，EXPR可以是任意复杂的，只要最后的操作是散列或数组键查找或子例程名称：|3833940|3833941|3833942|if+(exists+$ref->{A}->{B}->{$key})+{+}|ordered-list-item|3833943|if+(exists+$hash{A}{B}{$key})+{+}|3833944|3833945|entityMap|0|LINK|mutability|MUTABLE|url|http://perldoc.perl.org/functions/exists.html^0|5|6|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]]|C|$]]|$1|D|3|-4|5|6|7|16|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|17|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|18|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|19|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|C|$]]|$1|M|3|-4|5|6|7|1B|8|@]|9|@]|C|$]]|$1|N|3|-4|5|6|7|1C|8|@]|9|@]|C|$]]|$1|O|3|P|5|Q|7|1D|8|@]|9|@]|C|$]]|$1|R|3|S|5|Q|7|1E|8|@]|9|@]|C|$]]|$1|T|3|-4|5|6|7|1F|8|@]|9|@]|C|$]]|$1|U|3|-4|5|6|7|1G|8|@]|9|@]|C|$]]]|V|$W|$5|X|Y|Z|C|$10|11]]]]

You have to use the <a href="http://perldoc.perl.org/functions/exists.html" rel="nofollow">exists</a> function

<blockquote>
 exists EXPR
 
 Given an expression that specifies an
 element of a hash, returns true if the
 specified element in the hash has ever
 been initialized, even if the
 corresponding value is undefined.
 
 Note that the EXPR can be arbitrarily
 complicated as long as the final
 operation is a hash or array key
 lookup or subroutine name:
 
 <ol>
 <li>if (exists $ref->{A}->{B}->{$key}) { }</li>
 <li>if (exists $hash{A}{B}{$key}) { }</li>
 </ol>
</blockquote>

blocks|key|2237455|text|use+Data::Dumper;

my+%25HoH;

$HoH{A}{B}+=+1;

if(exists+$HoH{C}+&&+exists+$HoH{C}{D})+{
+++print+"exists\n";
}

print+Dumper(\%25HoH);

if(exists+$HoH{C}{D})+{
+++print+"exists\n";
}

print+Dumper(\%25HoH);|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2237456|输出：|unstyled|2237457|$VAR1+=+{
++++++++++'A'+=>+{
+++++++++++++++++++'B'+=>+1
+++++++++++++++++}
++++++++};
$VAR1+=+{
++++++++++'A'+=>+{
+++++++++++++++++++'B'+=>+1
+++++++++++++++++},
++++++++++'C'+=>+{}
++++++++};|2237458|自动生成是创建密钥的原因。我的第二个示例中的"exists“显示了这一点，因此第一个示例分别检查这两个键。|2237459|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|N|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|F|7|Q|8|@]|9|@]|A|$]]]|L|$]]

<pre><code>use Data::Dumper;

my %HoH;

$HoH{A}{B} = 1;

if(exists $HoH{C} &amp;&amp; exists $HoH{C}{D}) {
 print "exists\n";
}

print Dumper(\%HoH);

if(exists $HoH{C}{D}) {
 print "exists\n";
}

print Dumper(\%HoH);
</code></pre>

Output:

<pre><code>$VAR1 = {
 'A' =&gt; {
 'B' =&gt; 1
 }
 };
$VAR1 = {
 'A' =&gt; {
 'B' =&gt; 1
 },
 'C' =&gt; {}
 };
</code></pre>

Autovivification is causing the keys to be created. "exists" in my second example shows this so the first example checks both keys individually.

blocks|key|1434548|text|写作|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1434549|if+(defined+$HoH{$c}{$d})+{...}|code-block|syntax|javascript|1434550|将会“工作”，因为它会告诉你$HoH{$c}{$d}是否有一个定义的值。问题是，如果$HoH{$c}不存在，它将被创建(使用适当的值)，以便可以测试$HoH{$c}{$d}。这个过程被称为“自生动”。在设置值时很方便，例如|offset|length|style|CODE|1434551|my+%25hoh;
$hoh{a}{b}+=+1;+#+Don't+need+to+set+'$hoh{a}+=+{}'+first|1434552|但在检索可能不存在的值时不方便。我希望Perl足够聪明，只对用作左值的表达式执行自动形象化，并短路返回r值的undef，但是，天哪，它并没有那么神奇。autovivification杂注(在CPAN上可用)添加了执行此操作的功能。|1434553|为了避免自动生动，你需要先测试中间值：|1434554|if+(exists+$HoH{$c}+&&+defined+$HoH{$c}{$d})+{
+++++...
}|1434555|entityMap|0|LINK|mutability|MUTABLE|url|http://search.cpan.org/perldoc?autovivification^0|0|0|E|C|16|8|22|C|0|0|1I|5|23|G|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@$I|15|J|16|K|L]|$I|17|J|18|K|L]|$I|19|J|1A|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|1C|8|@$I|1D|J|1E|K|L]]|9|@$I|1F|J|1G|1|1H]]|A|$]]|$1|Q|3|R|5|6|7|1I|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1J|8|@]|9|@]|A|$E|F]]|$1|U|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|V|$W|$5|X|Y|Z|A|$10|11]]]]

Writing

<pre><code>if (defined $HoH{$c}{$d}) {...}
</code></pre>

will "work" insomuch as it will tell you whether or not <code>$HoH{$c}{$d}</code> has a defined value. The problem is that if <code>$HoH{$c}</code> doesn't already exist it will be created (with an appropriate value) so that <code>$HoH{$c}{$d}</code> can be tested. This process is called "autovivification." It's convenient when setting values, e.g.

<pre><code>my %hoh;
$hoh{a}{b} = 1; # Don't need to set '$hoh{a} = {}' first
</code></pre>

but inconvenient when retrieving possibly non-existent values. I wish that Perl was smart enough to only perform autovivification for expressions used as lvalues and short-circuit to return <code>undef</code> for rvalues but, alas, it's not that magical. The <a href="http://search.cpan.org/perldoc?autovivification" rel="nofollow">autovivification</a> pragma (available on CPAN) adds the functionality to do this.

To avoid autovivification you need to test the intermediate values first:

<pre><code>if (exists $HoH{$c} &amp;&amp; defined $HoH{$c}{$d}) {
 ...
}
</code></pre>

blocks|key|1435966|text|以下几种方式：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1435967|if+(+$HoH{$c}+&&+defined+$HoH{$c}{$d}+)+{...}|code-block|syntax|javascript|1435968|或|1435969|if+(+defined+${+$HoH{$c}+%7C%7C+{}+}{$d}+)+{...}|1435970|1435971|no+autovivification;
if+(defined+$HoH{$c}{$d})+{...}|1435972|1435973|use+Data::Diver;
if+(+defined+Data::Diver::Dive(+\%25HoH,+$c,+$d+)+)+{...}|1435974|entityMap^0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|L|3|M|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|N|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|R|$]]

Several ways:

<pre><code>if ( $HoH{$c} &amp;&amp; defined $HoH{$c}{$d} ) {...}
</code></pre>

or

<pre><code>if ( defined ${ $HoH{$c} || {} }{$d} ) {...}
</code></pre>

or

<pre><code>no autovivification;
if (defined $HoH{$c}{$d}) {...}
</code></pre>

or

<pre><code>use Data::Diver;
if ( defined Data::Diver::Dive( \%HoH, $c, $d ) ) {...}
</code></pre>

blocks|key|169518|text|我的观点是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|169519|use+List::Util+++qw<first>;
use+Params::Util+qw<_HASH>;

sub+exists_deep+(\[%25$]@)+{
++++my+$ref+=+shift;
++++return+unless+my+$h+=+_HASH(+$ref+)+//+_HASH(+$$ref+)
++++++++++++++and+defined(+my+$last_key+=+pop+)
++++++++++++++;
++++#+Note+that+this+*must*+be+a+hash+ref,+for+anything+else+to+make+sense.
++++return+if+first+{+!(+$h+=+_HASH(+$h->{+$_+}+))+}+@_;
++++return+exists+$h->{+$last_key+};
}|code-block|syntax|javascript|169520|您也可以递归地执行此操作。您还可以创建一个下降结构，只需一点额外的编码就可以实现中间甚至是终端arrayref。|169521|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

My take:

<pre><code>use List::Util qw&lt;first&gt;;
use Params::Util qw&lt;_HASH&gt;;

sub exists_deep (\[%$]@) {
 my $ref = shift;
 return unless my $h = _HASH( $ref ) // _HASH( $$ref )
 and defined( my $last_key = pop )
 ;
 # Note that this *must* be a hash ref, for anything else to make sense.
 return if first { !( $h = _HASH( $h-&gt;{ $_ } )) } @_;
 return exists $h-&gt;{ $last_key };
}
</code></pre>

You could also do this recursively. You could also create a descent structure allowing intermediate and even terminal arrayref with just a little additional coding.

In Perl, I have a hash of hashes created with a loop similar to the following

<pre><code> my %HoH
 for my $i (1..10) {
 $HoH{$a}{$b} = $i; 
 }
</code></pre>

$a and $b are variables that do have some value when the HoH gets filled in. After creating the HoH, how can I check if a particular pair ($c, $d) exists in the HoH? The following does not work

<pre><code>if (defined $HoH{$c}{$d}) {...}
</code></pre>

because if $c does not exist in HoH already, it will be created as a key without a value.

Perl, check if pair exists in hash of hashes

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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在Perl中，我有一个用类似于下面的循环创建的散列散列    my %HoH    for my $i (1..10) {      $HoH{$a}{$b} = $i;     }$a和$b是在填充HoH时确实有一些值的变量。创建HoH后，如何检查HoH中是否存在特定的对($c、$d)？以下代码不起作用if (def...

问Perl，检查散列的哈希中是否存在对
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问Perl，检查散列的哈希中是否存在对EN