我有一个哈希:
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
提取这样的子哈希的最佳方法是什么?
h1.extract_subhash(:b, :d, :e, :f) # => {:b => :B, :d => :D}
h1 #=> {:a => :A, :c => :C}
ActiveSupport
,至少从 2.3.8 开始,提供了四种方便的方法:#slice
、#except
及其破坏性的对应方法:#slice!
和 #except!
。其他答案中提到了它们,但将它们总结在一个地方:
x = {a: 1, b: 2, c: 3, d: 4}
# => {:a=>1, :b=>2, :c=>3, :d=>4}
x.slice(:a, :b)
# => {:a=>1, :b=>2}
x
# => {:a=>1, :b=>2, :c=>3, :d=>4}
x.except(:a, :b)
# => {:c=>3, :d=>4}
x
# => {:a=>1, :b=>2, :c=>3, :d=>4}
注意 bang 方法的返回值。他们不仅会定制现有的哈希值,还会返回已删除(未保留)的条目。
Hash#except!
最适合问题中给出的示例:
x = {a: 1, b: 2, c: 3, d: 4}
# => {:a=>1, :b=>2, :c=>3, :d=>4}
x.except!(:c, :d)
# => {:a=>1, :b=>2}
x
# => {:a=>1, :b=>2}
ActiveSupport
不需要整个Rails,非常轻量级。事实上,很多非 Rails gem 都依赖于它,所以很可能您已经在 Gemfile.lock 中拥有它了。无需自行扩展 Hash 类。
如果您特别希望该方法返回提取的元素但 h1 保持不变:
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
h2 = h1.select {|key, value| [:b, :d, :e, :f].include?(key) } # => {:b=>:B, :d=>:D}
h1 = Hash[h1.to_a - h2.to_a] # => {:a=>:A, :c=>:C}
如果你想将其修补到 Hash 类中:
class Hash
def extract_subhash(*extract)
h2 = self.select{|key, value| extract.include?(key) }
self.delete_if {|key, value| extract.include?(key) }
h2
end
end
如果您只想从哈希中删除指定的元素,使用 delete_if 会更容易。
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
h1.delete_if {|key, value| [:b, :d, :e, :f].include?(key) } # => {:a=>:A, :c=>:C}
h1 # => {:a=>:A, :c=>:C}
h = { a: 100, b: 200, c: 300 }
h.slice(:a) #=> {:a=>100}
h.slice(:b, :c, :d) #=> {:b=>200, :c=>300}
如果您使用rails,则Hash#slice是正确的选择。
{:a => :A, :b => :B, :c => :C, :d => :D}.slice(:a, :c)
# => {:a => :A, :c => :C}
如果您不使用rails,Hash#values_at 将按照您询问的顺序返回值,因此您可以这样做:
def slice(hash, *keys)
Hash[ [keys, hash.values_at(*keys)].transpose]
end
def except(hash, *keys)
desired_keys = hash.keys - keys
Hash[ [desired_keys, hash.values_at(*desired_keys)].transpose]
end
例如:
slice({foo: 'bar', 'bar' => 'foo', 2 => 'two'}, 'bar', 2)
# => {'bar' => 'foo', 2 => 'two'}
except({foo: 'bar', 'bar' => 'foo', 2 => 'two'}, 'bar', 2)
# => {:foo => 'bar'}
说明:
出于
{:a => 1, :b => 2, :c => 3}
我们想要 {:a => 1, :b => 2}
hash = {:a => 1, :b => 2, :c => 3}
keys = [:a, :b]
values = hash.values_at(*keys) #=> [1, 2]
transposed_matrix =[keys, values].transpose #=> [[:a, 1], [:b, 2]]
Hash[transposed_matrix] #=> {:a => 1, :b => 2}
如果你觉得猴子补丁是可行的方法,那么以下就是你想要的:
module MyExtension
module Hash
def slice(*keys)
::Hash[[keys, self.values_at(*keys)].transpose]
end
def except(*keys)
desired_keys = self.keys - keys
::Hash[[desired_keys, self.values_at(*desired_keys)].transpose]
end
end
end
Hash.include MyExtension::Hash
您可以使用 ActiveSupport 核心扩展中提供的 slice!(*keys)
initial_hash = {:a => 1, :b => 2, :c => 3, :d => 4}
extracted_slice = initial_hash.slice!(:a, :c)
initial_hash 现在是
{:b => 2, :d =>4}
extracted_slide 现在是
{:a => 1, :c =>3}
你可以看看
slice.rb in ActiveSupport 3.1.3
module HashExtensions
def subhash(*keys)
keys = keys.select { |k| key?(k) }
Hash[keys.zip(values_at(*keys))]
end
end
Hash.send(:include, HashExtensions)
{:a => :A, :b => :B, :c => :C, :d => :D}.subhash(:a) # => {:a => :A}
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
keys = [:b, :d, :e, :f]
h2 = (h1.keys & keys).each_with_object({}) { |k,h| h.update(k=>h1.delete(k)) }
#=> {:b => :B, :d => :D}
h1
#=> {:a => :A, :c => :C}
delete_if
和keep_if
都是Ruby核心的一部分。在这里,您可以实现您想要的功能,而无需修补 Hash
类型。
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
h2 = h1.clone
p h1.keep_if { |key| [:b, :d, :e, :f].include?(key) } # => {:b => :B, :d => :D}
p h2.delete_if { |key, value| [:b, :d, :e, :f].include?(key) } #=> {:a => :A, :c => :C}
有关更多信息,请查看文档中的以下链接:
如果您还想从数据库记录中提取,最好使用
slice
hash = { a: 1, b: 2, c: 3, d: 4 }
hash.slice!(:a, :b) # => {:c=>3, :d=>4}
hash # => {:a=>1, :b=>2}
https://api.rubyonrails.org/classes/Hash.html#method-i-slice-21
如果你使用rails,使用Hash可能会更方便。除了
h = {a:1, b:2}
h1 = h.except(:a) # {b:2}
正如其他人提到的,Ruby 2.5 添加了 Hash#slice 方法。
Rails 5.2.0beta1 还添加了自己的 Hash#slice 版本,为使用早期版本 Ruby 的框架用户提供功能。 https://github.com/rails/rails/commit/01ae39660243bc5f0a986e20f9c9bff312b1b5f8
如果出于某种原因想要实现自己的,这也是一个很好的衬里:
def slice(*keys)
keys.each_with_object(Hash.new) { |k, hash| hash[k] = self[k] if has_key?(k) }
end unless method_defined?(:slice)
class Hash
def extract(*keys)
key_index = Hash[keys.map{ |k| [k, true] }] # depends on the size of keys
partition{ |k, v| key_index.has_key?(k) }.map{ |group| Hash[group] }
end
end
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
h2, h1 = h1.extract(:b, :d, :e, :f)
这是建议方法的快速性能比较,
#select
似乎是最快的
k = 1_000_000
Benchmark.bmbm do |x|
x.report('select') { k.times { {a: 1, b: 2, c: 3}.select { |k, _v| [:a, :b].include?(k) } } }
x.report('hash transpose') { k.times { Hash[ [[:a, :b], {a: 1, b: 2, c: 3}.fetch_values(:a, :b)].transpose ] } }
x.report('slice') { k.times { {a: 1, b: 2, c: 3}.slice(:a, :b) } }
end
Rehearsal --------------------------------------------------
select 1.640000 0.010000 1.650000 ( 1.651426)
hash transpose 1.720000 0.010000 1.730000 ( 1.729950)
slice 1.740000 0.010000 1.750000 ( 1.748204)
----------------------------------------- total: 5.130000sec
user system total real
select 1.670000 0.010000 1.680000 ( 1.683415)
hash transpose 1.680000 0.010000 1.690000 ( 1.688110)
slice 1.800000 0.010000 1.810000 ( 1.816215)
细化后将如下所示:
module CoreExtensions
module Extractable
refine Hash do
def extract(*keys)
select { |k, _v| keys.include?(k) }
end
end
end
end
并使用它:
using ::CoreExtensions::Extractable
{ a: 1, b: 2, c: 3 }.extract(:a, :b)
此代码将您要求的功能注入到 Hash 类中:
class Hash
def extract_subhash! *keys
to_keep = self.keys.to_a - keys
to_delete = Hash[self.select{|k,v| !to_keep.include? k}]
self.delete_if {|k,v| !to_keep.include? k}
to_delete
end
end
并产生您提供的结果:
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
p h1.extract_subhash!(:b, :d, :e, :f) # => {b => :B, :d => :D}
p h1 #=> {:a => :A, :c => :C}
注意:此方法实际上返回提取的键/值。
如果您不是在 Ruby 2.5 上运行并且您不想通过添加新方法来污染您的 Hash 类,那么这里有一个非常有用的功能解决方案:
slice_hash = -> keys, hash { hash.select { |k, _v| keys.include?(k) } }.curry
然后您甚至可以将其应用于嵌套哈希:
my_hash = [{name: "Joe", age: 34}, {name: "Amy", age: 55}]
my_hash.map(&slice_hash.([:name]))
# => [{:name=>"Joe"}, {:name=>"Amy"}]
只是切片方法的补充,如果您想要与原始哈希分离的子哈希键将是动态的,您可以这样做,
slice(*dynamic_keys) # dynamic_keys should be an array type
我们可以通过仅循环我们想要提取的密钥来完成此操作,只需检查密钥是否存在,然后提取它。
class Hash
def extract(*keys)
extracted_hash = {}
keys.each{|key| extracted_hash[key] = self.delete(key) if self.has_key?(key)}
extracted_hash
end
end
h1 = {:a => :A, :b => :B, :c => :C, :d => :D}
h2 = h1.extract(:b, :d, :e, :f)
我来到这里是为了寻找 ActiveSupport 自 3.0.0 中提供的细微不同的行为,如
Hash#extract!
:
a = {a: 1, b: 2, c: 3}
b = a.extract!(:b)
# => {b: 2}
a
# => {a: 1, c: 3}