在ruby中访问嵌套哈希的元素 [英] Accessing elements of nested hashes in ruby

问题描述

我正在开发一个用ruby编写的实用程序，它广泛使用嵌套哈希。目前，我正在检查对嵌套散列元素的访问，如下所示：

  structure = {：a => {：b => 'foo'}} 
 
＃我想要structure [：a] [：b] 
 
 value = nil 
 
如果structure.has_key？（ ：a）&& structure [：a] .has_key？（：b）then 
 value = structure [：a] [：b] 
 end 
  

有没有更好的方法来做到这一点？我希望能够说：

$ p $ value = structure [：a] [：b]


获得 nil 如果：a不是 structure 等

解决方案

我通常这样做的方式是：

  h = Hash.new {| h，k | h [k] = {}} 
  

这会给你一个散列，输入一个缺失的密钥，但返回nil为第二级密钥：

  h ['foo']  - > {} 
 h ['foo'] ['bar']  - > nil 
  

你可以嵌套这个来添加可以用这种方式处理的多个图层：

  h = Hash.new {| h，k | h [k] = Hash.new {| hh，kk | hh [kk] = {}}} 
 
 h ['bar']  - > {} 
 h ['tar'] ['zar']  - > {} 
 h ['scar'] ['far'] ['mar']  - > nil 
  

您也可以通过使用 default_proc 方法：

  h = Hash.new {| h，k | h [k] = Hash.new（& h.default_proc）} 
 
 h ['bar']  - > {} 
 h ['tar'] ['star'] ['par']  - > {} 
  

上面的代码创建了一个散列，其默认proc使用相同的默认proc创建一个新的散列。因此，在查找不可见密钥时，创建默认值的哈希将具有相同的默认行为。

编辑：更多详细信息

Ruby哈希允许您控制在查找新密钥时如何创建默认值。指定时，此行为被封装为 Proc 对象，并可通过 default_proc 和 default_proc = 方法。默认的proc也可以通过将一个块传递给 Hash.new 。

让我们稍微分解一下这段代码。这不是惯用的ruby，但更容易分成多行：

  1。 recursive_hash = Hash.new do | h，k | 
 2. h [k] = Hash.new（& h.default_proc）
 3. end 
  

第1行声明一个变量 recursive_hash 为一个新的 Hash 并开始一个块为 recursive_hash 的 default_proc 。该块传递两个对象： h ，这是执行密钥查找的 Hash 实例，而<

第2行将散列中的默认值设置为一个新的<$ c k ，该键被查找。 $ c> Hash 实例。这个散列的缺省行为是通过传递从 default_proc 创建的 Proc ;即，块本身正在定义的默认proc。

以下是IRB会话的示例：

  IRB（主）：011：0> recursive_hash = Hash.new do | h，k | 
 irb（main）：012：1 * h [k] = Hash.new（& h.default_proc）
 irb（main）：013：1>结束
 => {} 
 irb（main）：014：0> recursive_hash [：foo] 
 => {} 
 irb（main）：015：0> recursive_hash 
 => {：foo => {}} 
  

在 recursive_hash [ ：foo] 被创建，它的 default_proc 由 recursive_hash 的<$ c提供$ C> default_proc 。这有两个作用：

1. recursive_hash [：foo] 的默认行为是与相同。recursive_hash 。


2. 由 recursive_hash [：foo] 的 default_proc 将与 recursive_hash 相同。

ol>



因此，继续在IRB中，我们得到以下结果：



  irb主要）：016：0> recursive_hash [：foo] [：bar] 
 => {} 
 irb（main）：017：0> recursive_hash 
 => {：foo => {：bar => {}}} 
 irb（main）：018：0> recursive_hash [：foo] [：bar] [：zap] 
 => {} 
 irb（main）：019：0> recursive_hash 
 => {：foo => {：bar => {：zap => {}}}} 
  




I'm working a little utility written in ruby that makes extensive use of nested hashes. Currently, I'm checking access to nested hash elements as follows:

structure = { :a => { :b => 'foo' }}

# I want structure[:a][:b]

value = nil

if structure.has_key?(:a) && structure[:a].has_key?(:b) then
  value = structure[:a][:b]
end

Is there a better way to do this? I'd like to be able to say:

value = structure[:a][:b]

And get nil if :a is not a key in structure, etc.

解决方案

The way I usually do this these days is:

h = Hash.new { |h,k| h[k] = {} }

This will give you a hash that creates a new hash as the entry for a missing key, but returns nil for the second level of key:

h['foo'] -> {}
h['foo']['bar'] -> nil

You can nest this to add multiple layers that can be addressed this way:

h = Hash.new { |h, k| h[k] = Hash.new { |hh, kk| hh[kk] = {} } }

h['bar'] -> {}
h['tar']['zar'] -> {}
h['scar']['far']['mar'] -> nil

You can also chain indefinitely by using the default_proc method:

h = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }

h['bar'] -> {}
h['tar']['star']['par'] -> {}

The above code creates a hash whose default proc creates a new Hash with the same default proc. So, a hash created as a default value when a lookup for an unseen key occurs will have the same default behavior.

EDIT: More details

Ruby hashes allow you to control how default values are created when a lookup occurs for a new key. When specified, this behavior is encapsulated as a Proc object and is reachable via the default_proc and default_proc= methods. The default proc can also be specified by passing a block to Hash.new.

Let's break this code down a little. This is not idiomatic ruby, but it's easier to break it out into multiple lines:

1. recursive_hash = Hash.new do |h, k|
2.   h[k] = Hash.new(&h.default_proc)
3. end

Line 1 declares a variable recursive_hash to be a new Hash and begins a block to be recursive_hash's default_proc. The block is passed two objects: h, which is the Hash instance the key lookup is being performed on, and k, the key being looked up.

Line 2 sets the default value in the hash to a new Hash instance. The default behavior for this hash is supplied by passing a Proc created from the default_proc of the hash the lookup is occurring in; ie, the default proc the block itself is defining.

Here's an example from an IRB session:

irb(main):011:0> recursive_hash = Hash.new do |h,k|
irb(main):012:1* h[k] = Hash.new(&h.default_proc)
irb(main):013:1> end
=> {}
irb(main):014:0> recursive_hash[:foo]
=> {}
irb(main):015:0> recursive_hash
=> {:foo=>{}}

When the hash at recursive_hash[:foo] was created, its default_proc was supplied by recursive_hash's default_proc. This has two effects:

1. The default behavior for recursive_hash[:foo] is the same as recursive_hash.
2. The default behavior for hashes created by recursive_hash[:foo]'s default_proc will be the same as recursive_hash.

So, continuing in IRB, we get the following:

irb(main):016:0> recursive_hash[:foo][:bar]
=> {}
irb(main):017:0> recursive_hash
=> {:foo=>{:bar=>{}}}
irb(main):018:0> recursive_hash[:foo][:bar][:zap]
=> {}
irb(main):019:0> recursive_hash
=> {:foo=>{:bar=>{:zap=>{}}}}

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