Ruby 如何合并散列数组以获取值数组的散列

这是Turning a Hash of Arrays into an Array of Hashes in Ruby的反面.

优雅和/或高效地将哈希数组转换为哈希，其中值是所有值的数组:

hs = [
  { a:1, b:2 },
  { a:3, c:4 },
  { b:5, d:6 }
]
collect_values( hs )
#=> { :a=>[1,3], :b=>[2,5], :c=>[4], :d=>[6] }

这段简洁的代码几乎可以工作，但在没有重复的情况下无法创建数组:

def collect_values( hashes )
  hashes.inject({}){ |a,b| a.merge(b){ |_,x,y| [*x,*y] } }
end
collect_values( hs )
#=> { :a=>[1,3], :b=>[2,5], :c=>4, :d=>6 }

这段代码很有效，但是你能写一个更好的版本吗？

def collect_values( hashes )
  # Requires Ruby 1.8.7+ for Object#tap
  Hash.new{ |h,k| h[k]=[] }.tap do |result|
    hashes.each{ |h| h.each{ |k,v| result[k]<<v } }
  end
end

Solutions that only work in Ruby 1.9 are acceptable, but should be noted as such.

以下是使用三种不同的哈希数组对以下各种答案(以及我自己的一些答案)进行基准测试的结果:

• one where each hash has distinct keys, so no merging ever occurs:
  [{:a=>1}, {:b=>2}, {:c=>3}, {:d=>4}, {:e=>5}, {:f=>6}, {:g=>7}, ...]

• one where every hash has the same key, so maximum merging occurs:
  [{:a=>1}, {:a=>2}, {:a=>3}, {:a=>4}, {:a=>5}, {:a=>6}, {:a=>7}, ...]

• and one that is a mix of unique and shared keys:
  [{:c=>1}, {:d=>1}, {:c=>2}, {:f=>1}, {:c=>1, :d=>1}, {:h=>1}, {:c=>3}, ...]

               user     system      total        real
Phrogz 2a  0.577000   0.000000   0.577000 (  0.576000)
Phrogz 2b  0.624000   0.000000   0.624000 (  0.620000)
Glenn 1    0.640000   0.000000   0.640000 (  0.641000)
Phrogz 1   0.671000   0.000000   0.671000 (  0.668000)
Michael 1  0.702000   0.000000   0.702000 (  0.700000)
Michael 2  0.717000   0.000000   0.717000 (  0.726000)
Glenn 2    0.765000   0.000000   0.765000 (  0.764000)
fl00r      0.827000   0.000000   0.827000 (  0.836000)
sawa       0.874000   0.000000   0.874000 (  0.868000)
Tokland 1  0.873000   0.000000   0.873000 (  0.876000)
Tokland 2  1.077000   0.000000   1.077000 (  1.073000)
Phrogz 3   2.106000   0.093000   2.199000 (  2.209000)

最快的代码是我添加的这个方法:

def collect_values(hashes)
  {}.tap{ |r| hashes.each{ |h| h.each{ |k,v| (r[k]||=[]) << v } } }
end

我接受了"glenn mcdonald's answer"，因为它在速度上很有竞争力，相当简洁，但(最重要的是)因为它指出了使用具有自修改默认过程的散列来方便构建的危险，因为这可能会在用户以后索引它时引入不好的更改.

最后，这里是基准代码，以防您想要运行自己的比较:

require 'prime'   # To generate the third hash
require 'facets'  # For tokland1's map_by
AZSYMBOLS = (:a..:z).to_a
TESTS = {
  '26 Distinct Hashes'   => AZSYMBOLS.zip(1..26).map{|a| Hash[*a] },
  '26 Same-Key Hashes'   => ([:a]*26).zip(1..26).map{|a| Hash[*a] },
  '26 Mixed-Keys Hashes' => (2..27).map do |i|
    factors = i.prime_division.transpose
    Hash[AZSYMBOLS.values_at(*factors.first).zip(factors.last)]
  end
}

def phrogz1(hashes)
  Hash.new{ |h,k| h[k]=[] }.tap do |result|
    hashes.each{ |h| h.each{ |k,v| result[k]<<v } }
  end
end
def phrogz2a(hashes)
  {}.tap{ |r| hashes.each{ |h| h.each{ |k,v| (r[k]||=[]) << v } } }
end
def phrogz2b(hashes)
  hashes.each_with_object({}){ |h,r| h.each{ |k,v| (r[k]||=[]) << v } }
end
def phrogz3(hashes)
  result = hashes.inject({}){ |a,b| a.merge(b){ |_,x,y| [*x,*y] } }
  result.each{ |k,v| result[k] = [v] unless v.is_a? Array }
end
def glenn1(hs)
  hs.reduce({}) {|h,pairs| pairs.each {|k,v| (h[k] ||= []) << v}; h}
end
def glenn2(hs)
  hs.map(&:to_a).flatten(1).reduce({}) {|h,(k,v)| (h[k] ||= []) << v; h}
end
def fl00r(hs)
  h = Hash.new{|h,k| h[k]=[]}
  hs.map(&:to_a).flatten(1).each{|v| h[v[0]] << v[1]}
  h
end
def sawa(a)
  a.map(&:to_a).flatten(1).group_by{|k,v| k}.each_value{|v| v.map!{|k,v| v}}
end
def michael1(hashes)
  h = Hash.new{|h,k| h[k]=[]}
  hashes.each_with_object(h) do |h, result|
    h.each{ |k, v| result[k] << v }
  end
end
def michael2(hashes)
  h = Hash.new{|h,k| h[k]=[]}
  hashes.inject(h) do |result, h|
    h.each{ |k, v| result[k] << v }
    result
  end
end
def tokland1(hs)
  hs.map(&:to_a).flatten(1).map_by{ |k, v| [k, v] }
end
def tokland2(hs)
  Hash[hs.map(&:to_a).flatten(1).group_by(&:first).map{ |k, vs|
    [k, vs.map{|o|o[1]}]
  }]
end

require 'benchmark'
N = 10_000
Benchmark.bm do |x|
  x.report('Phrogz 2a'){ TESTS.each{ |n,h| N.times{ phrogz2a(h) } } }
  x.report('Phrogz 2b'){ TESTS.each{ |n,h| N.times{ phrogz2b(h) } } }
  x.report('Glenn 1  '){ TESTS.each{ |n,h| N.times{ glenn1(h)   } } }
  x.report('Phrogz 1 '){ TESTS.each{ |n,h| N.times{ phrogz1(h)  } } }
  x.report('Michael 1'){ TESTS.each{ |n,h| N.times{ michael1(h) } } }
  x.report('Michael 2'){ TESTS.each{ |n,h| N.times{ michael2(h) } } }
  x.report('Glenn 2  '){ TESTS.each{ |n,h| N.times{ glenn2(h)   } } }
  x.report('fl00r    '){ TESTS.each{ |n,h| N.times{ fl00r(h)    } } }
  x.report('sawa     '){ TESTS.each{ |n,h| N.times{ sawa(h)     } } }
  x.report('Tokland 1'){ TESTS.each{ |n,h| N.times{ tokland1(h) } } }
  x.report('Tokland 2'){ TESTS.each{ |n,h| N.times{ tokland2(h) } } }
  x.report('Phrogz 3 '){ TESTS.each{ |n,h| N.times{ phrogz3(h)  } } }

end