使用'exists'进行额外的哈希查找？ [英] Additional hash lookup using 'exists'?

问题描述

  if（exists $ ids {$ name}）{
 $ id = $ ids {$ name}; 
} 
  

这是不错的做法吗？我有点担心它包含两个查找，其中应该完成一个。有没有更好的方法来检查存在和赋值？

解决方案

通过检查存在，您可以防止自动版本化。请参阅 Autovivification：它是什么，为什么我在乎？。

更新：由于以下 trendels 指出，autovivification在您发布的示例中不起作用。我假设实际的代码涉及多级哈希。

下面是一个例子：

 ＃！/ usr / bin / perl 
 
 use strict; 
使用警告; 
 
使用Data :: Dumper; 
 
 my（％hash，$ x）; 
 
 if（exists $ hash {test}  - > {vivify}）{
 $ x = $ hash {test}  - > {vivify}  - > {now}; 
} 
 
打印Dumper \％hash; 
 
 $ x = $ hash {test}  - > {vivify}  - > {now}; 
 
打印Dumper \％hash; 
 
 __END__ 
 
 
 C：\Temp> t 
 $ VAR1 = {
'test'=> {} 
}; 
 $ VAR1 = {
'test'=> {
'vivify'=> {} 
} 
}; 
  

I sometimes access a hash like this:

if(exists $ids{$name}){
    $id = $ids{$name};
}

Is that good practice? I'm a bit concerned that it contains two lookups where really one should be done. Is there a better way to check the existence and assign the value?

解决方案

By checking with exists, you prevent autovivification. See Autovivification : What is it and why do I care?.

UPDATE: As trendels points out below, autovivification does not come into play in the example you posted. I am assuming that the actual code involves multi-level hashes.

Here is an illustration:

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my (%hash, $x);

if ( exists $hash{test}->{vivify} ) {
    $x = $hash{test}->{vivify}->{now};
}

print Dumper \%hash;

$x = $hash{test}->{vivify}->{now};

print Dumper \%hash;

__END__


C:\Temp> t
$VAR1 = {
    'test' => {}
};
$VAR1 = {
    'test' => {
        'vivify' => {}
    }
};

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