Ruby - 访问多维哈希并避免访问零对象 [英] Ruby - Access multidimensional hash and avoid access nil object

问题描述

可能存在重复：

Ruby：在IF语句中使用Nils

假设我尝试访问这样的哈希：

  my_hash ['key1'] ['key2'] ['key3'] 
  

如果key1，key2和key3存在于散列表中，那么这很好，但是如果key1不存在呢？

然后我得到 NoMethodError：undefined method [] for nil：NilClass 。

到目前为止，我正在处理这样一个条件如：

if my_hash ['key1']&& my_hash ['key1'] ['key2'] ...

这是否合适？是否还有其他Rubiest方法？

解决方案

如果您使用Ruby 2.3或更高版本，您可以使用挖掘

  my_hash.dig（'key1'，'key2'，'key3'）
  

很多人坚持使用普通的ruby，并链接&&< / code> guard测试。 / p>



您可以使用stdlib $ rel =noreferrer> Hash＃fetch 也是：



  my_hash.fetch（'key1'，{} ）.fetch（'key2'，{}）。fetch（'key3'，nil）
  

如链接ActiveSupport的 #try 方法。

  my_hash.try（：[]，'key1'）。try（：[]，'key2'）。try（：[]，'key3'） 
  

其他人使用 andand

  myhash ['key1'] .and and ['key2'] .andand [' key3'] 
  

有些人认为自我中心nils 是一个好主意（尽管有人会狩猎你，如果他们发现你这样做会折磨你）。

  class NilClass 
 def method_missing（* args）;零; end 
 end 
 
 my_hash ['key1'] ['key2'] ['key3'] 
  

您可以使用可枚举＃reduce （或别名注入）。

  ['key1'，'key2'，'key3' ] .reduce（my_hash）{| m，k | m&& amp;&或者可能扩展哈希或只是你的目标哈希对象与嵌套查找方法 / p> 
 
 
  module NestedHashLookup 
 def nest * keys 
 keys.reduce（self）{| m，k | m&& amp;& m [k]} 
结束
结束
 
 my_hash.extend（NestedHashLookup）
 my_hash.nest'key1'，'key2'，'key3'
  
哦，我们怎么能忘记 maybe  monad？ 
 
 
  Maybe.new（my_hash）['key1'] ['key2'] ['key3'] 
  
 

  
Possible Duplicate:

  Ruby: Nils in an IF statement

  Is there a clean way to avoid calling a method on nil in a nested params hash?  
Let's say I try to access a hash like this:
my_hash['key1']['key2']['key3']
This is nice if key1, key2 and key3 exist in the hash(es), but what if, for example key1 doesn't exist?

Then I would get NoMethodError: undefined method [] for nil:NilClass. And nobody likes that.

So far I deal with this doing a conditional like:

if my_hash['key1'] && my_hash['key1']['key2'] ...

Is this appropriate, is there any other Rubiest way of doing so?
解决方案
There are many approaches to this.

If you use Ruby 2.3 or above, you can use dig
my_hash.dig('key1', 'key2', 'key3')
Plenty of folks stick to plain ruby and chain the && guard tests.

You could use stdlib Hash#fetch too:
my_hash.fetch('key1', {}).fetch('key2', {}).fetch('key3', nil)
Some like chaining ActiveSupport's #try method.
my_hash.try(:[], 'key1').try(:[], 'key2').try(:[], 'key3')
Others use andand
myhash['key1'].andand['key2'].andand['key3']
Some people think egocentric nils are a good idea (though someone might hunt you down and torture you if they found you do this).
class NilClass
  def method_missing(*args); nil; end
end

my_hash['key1']['key2']['key3']
You could use Enumerable#reduce (or alias inject).
['key1','key2','key3'].reduce(my_hash) {|m,k| m && m[k] }
Or perhaps extend Hash or just your target hash object with a nested lookup method
module NestedHashLookup
  def nest *keys
    keys.reduce(self) {|m,k| m && m[k] }
  end
end

my_hash.extend(NestedHashLookup)
my_hash.nest 'key1', 'key2', 'key3'
Oh, and how could we forget the maybe monad?
Maybe.new(my_hash)['key1']['key2']['key3']


                        
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