Ruby - 访问多维散列并避免访问 nil 对象 [英] Ruby - Access multidimensional hash and avoid access nil object

问题描述

可能的重复:
Ruby:IF 语句中的 Nils
是否有一种干净的方法可以避免在嵌套的 params 哈希中对 nil 调用方法?

假设我尝试访问这样的哈希:

Let's say I try to access a hash like this:

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

如果 key1、key2 和 key3 存在于散列中，这很好，但如果 key1 不存在怎么办?

This is nice if key1, key2 and key3 exist in the hash(es), but what if, for example key1 doesn't exist?

然后我会得到 NoMethodError: undefined method [] for nil:NilClass.没有人喜欢那样.

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

到目前为止，我通过以下条件处理此问题:

So far I deal with this doing a conditional like:

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

这是否合适，有没有其他 Rubyest 方法可以这样做?

Is this appropriate, is there any other Rubiest way of doing so?

推荐答案

有很多方法可以解决这个问题.

There are many approaches to this.

如果您使用 Ruby 2.3 或更高版本，则可以使用 挖掘

If you use Ruby 2.3 or above, you can use dig

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

很多人坚持使用普通的 ruby​​ 并链接 && 防护测试.

Plenty of folks stick to plain ruby and chain the && guard tests.

你可以使用 stdlib Hash#fetch 也是:

You could use stdlib Hash#fetch too:

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

有些人喜欢链接 ActiveSupport 的 #try 方法.>

Some like chaining ActiveSupport's #try method.

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

其他人使用and

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

有些人认为以自我为中心的人一个好主意(尽管如果有人发现你这样做，他们可能会追捕你并折磨你).

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']

您可以使用 Enumerable#reduce(或别名注入).

You could use Enumerable#reduce (or alias inject).

['key1','key2','key3'].reduce(my_hash) {|m,k| m && m[k] }

或者可能使用嵌套查找方法扩展 Hash 或只是您的目标哈希对象

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'

哦，我们怎么能忘记也许 单子?

Oh, and how could we forget the maybe monad?

Maybe.new(my_hash)['key1']['key2']['key3']

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