Perl中的散列和散列引用有什么区别？ [英] What's the difference between a hash and hash reference in Perl?

问题描述

我想正确理解Perl中的哈希。我不得不间歇地使用Perl一段时间，大多数时候我需要这样做，它主要与文本处理有关。

每次，我都必须处理随着哈希，它会变得混乱。我发现哈希的语法非常模糊

散列和哈希引用的一个很好的解释，它们的区别，当它们被需要时等等，我们将非常感激。

解决方案

简单的哈希函数接近数组。他们的初始化甚至看起来相似。首先是数组：

  @last_name =（
Ward，Cleaver，
Fred ，Flintstone，
Archie，Bunker
）; 
  

现在让我们使用散列表示相同的信息（又名关联数组）：

 ％last_name =（
Ward，Cleaver，
Fred，Flintstone，
 Archie，Bunker
）; 
  

尽管它们具有相同的名称，但数组 @last_name 和散列％last_name 是完全独立的。

有了数组，如果我们想知道Archie的姓氏，我们必须执行线性搜索：

  my $ lname; （$ my $ i = 0; $ i <@last_name; $ i + = 2）{
 $ lname = $ last_name [$ i + 1] 
 if $ last_name [$ i] eq阿尔奇; 
} 
打印Archie $ lname\\\
; 
  

使用散列，它在语法上更直接：

 打印Archie $ last_name {Archie} \\\
; 
  

假设我们只想用稍微丰富的结构表示信息：

• 切割者（姓氏）
  
  
  
  
  • 病房（名）
  
  
  • 6月（配偶的名字）
  
  
  
  • 石碑
    
    
    
    
    • Fred
    
    
    • Wilma
    
    
  
  
  • Bunker
    
    
    
    
    • Archie
    
    
    • Edith
    
    
  
  
  
  
  

  在引用出现之前，平键值散列是我们可以做的最好的，但引用允许

    my％ personal_info =（
  Cleaver，{
  FIRST，Ward，
  SPOUSE，June，
  }，
  Flintstone， {
  FIRST，Fred，
  SPOUSE，Wilma，
  }，
  Bunker，{
  FIRST，Archie ，
  SPOUSE，Edith，
  }，
  ）; 
    

  在内部，％personal_info 都是标量，但这些值是一种特殊的标量：使用 {} 创建的哈希引用。这些参考使我们能够模拟多维哈希。例如，我们可以通过
  

    $ personal_info {Flintstone}  - > {SPOUSE} 
    

  请注意，Perl允许我们在下标之间省略箭头，所以上面的内容相当于



    $ personal_info {Flintstone} {SPOUSE} 
    

  如果您想了解有关Fred的更多信息，那么您可以输入很多内容，因此您可以使用某种游标获取引用：



    $ fred = $ personal_info {Flintstone}; 
   printFred的妻子是$ fred-> {SPOUSE} \\\
  ; 
    

  因为上面代码片段中的 $ fred 是一个hashref，箭头是必要的。如果你放弃它，但明智地启用使用strict 来帮助你捕捉这些类型的错误，编译器会抱怨：

   全局符号％fred需要显式包名称... 
    

  Perl引用与C和C ++中的指针类似，但它们不能为空。 C和C ++中的指针需要解引用，因此需要在Perl中引用。

  
  
  

  C和C ++函数参数具有按值传递的语义：它们只是副本，不要回到调用者。如果你想看到更改，你必须传递一个指针。您可以在Perl中使用引用来获得这种效果：
  
  

    sub add_barney {
   my（$ personal_info）= @_; 
   
   $ personal_info-> {Rubble} = {
   FIRST => Barney，
   SPOUSE => 贝蒂，
  }; 
  } 
   
   add_barney \％personal_info; 
    

  没有反斜线， add_barney 一个副本在子回来后立即抛出。

  
  
  

  还要注意使用胖逗号（ => ）以上。它会自动引用左侧的字符串，并使得哈希初始化的语法更少。

  
  

  I would like to properly understand hashes in Perl. I've had to use Perl intermittently for quite some time and mostly whenever I need to do it, it's mostly related to text processing.

  And everytime, I have to deal with hashes, it gets messed up. I find the syntax very cryptic for hashes

  A good explanation of hashes and hash references, their differences, when they are required etc. would be much appreciated.

  解决方案

  A simple hash is close to an array. Their initializations even look similar. First the array:

  @last_name = (
    "Ward",   "Cleaver",
    "Fred",   "Flintstone",
    "Archie", "Bunker"
  );
  

  Now let's represent the same information with a hash (aka associative array):

  %last_name = (
    "Ward",   "Cleaver",
    "Fred",   "Flintstone",
    "Archie", "Bunker"
  );
  

  Although they have the same name, the array @last_name and the hash %last_name are completely independent.

  With the array, if we want to know Archie's last name, we have to perform a linear search:

  my $lname;
  for (my $i = 0; $i < @last_name; $i += 2) {
    $lname = $last_name[$i+1] if $last_name[$i] eq "Archie";
  }
  print "Archie $lname\n";
  

  With the hash, it's much more direct syntactically:

  print "Archie $last_name{Archie}\n";
  

  Say we want to represent information with only slightly richer structure:

  • Cleaver (last name)
    • Ward (first name)
    • June (spouse's first name)
  • Flintstone
    • Fred
    • Wilma
  • Bunker
    • Archie
    • Edith

  Before references came along, flat key-value hashes were about the best we could do, but references allow

  my %personal_info = (
      "Cleaver", {
          "FIRST",  "Ward",
          "SPOUSE", "June",
      },
      "Flintstone", {
          "FIRST",  "Fred",
          "SPOUSE", "Wilma",
      },
      "Bunker", {
          "FIRST",  "Archie",
          "SPOUSE", "Edith",
      },
  );
  

  Internally, the keys and values of %personal_info are all scalars, but the values are a special kind of scalar: hash references, created with {}. The references allow us to simulate "multi-dimensional" hashes. For example, we can get to Wilma via

  $personal_info{Flintstone}->{SPOUSE}
  

  Note that Perl allows us to omit arrows between subscripts, so the above is equivalent to

  $personal_info{Flintstone}{SPOUSE}
  

  That's a lot of typing if you want to know more about Fred, so you might grab a reference as sort of a cursor:

  $fred = $personal_info{Flintstone};
  print "Fred's wife is $fred->{SPOUSE}\n";
  

  Because $fred in the snippet above is a hashref, the arrow is necessary. If you leave it out but wisely enabled use strict to help you catch these sorts of errors, the compiler will complain:

  Global symbol "%fred" requires explicit package name at ...
  

  Perl references are similar to pointers in C and C++, but they can never be null. Pointers in C and C++ require dereferencing and so do references in Perl.

  C and C++ function parameters have pass-by-value semantics: they're just copies, so modifications don't get back to the caller. If you want to see the changes, you have to pass a pointer. You can get this effect with references in Perl:

  sub add_barney {
      my($personal_info) = @_;
  
      $personal_info->{Rubble} = {
          FIRST  => "Barney",
          SPOUSE => "Betty",
      };
  }
  
  add_barney \%personal_info;
  

  Without the backslash, add_barney would have gotten a copy that's thrown away as soon as the sub returns.

  Note also the use of the "fat comma" (=>) above. It autoquotes the string on its left and makes hash initializations less syntactically noisy.

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