Extracting values from array

Question

From a hash like the below one, need to extract the values per one parameter:

array_of_hashes = [{type: "test1", value: 1}, {type: "test1", value: 1}, {type: "test2", value: 1}, {type: "test2", value: 1}]

I would like to achive something like this:

array_of_hashes = [{"test1" => [{type: "test1", value: 1}, {type: "test1", value: 1}}], {"test2" => {type: "test2", value: 1}, {type: "test2", value: 1}}]

What is the most efficient way to transform this array? This array could have something like 2 000 000 values.

shivam · Accepted Answer · 2015-04-03 14:58:36Z

2

You can use group_by:

array_of_hashes.group_by { |h| h[:type] }
# => {"test1"=>[{:type=>"test1", :value=>1}, {:type=>"test1", :value=>1}], "test2"=>[{:type=>"test2", :value=>1}, {:type=>"test2", :value=>1}]}

answered Apr 3, 2015 at 14:58

shivam

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Cary Swoveland · Accepted Answer · 2015-04-03 16:52:38Z

If, as in the example, the hashes are ordered by the value of :type, you might find that Enumerable#chunk (then converted to a hash) is faster than group_by:

Hash[array_of_hashes.chunk { |h| h[:type] }.to_a]

I suggest you compare them. Edit: I tested. As shown below, group_by appears to be faster.

def mk_arr(n,m)
  n.times.with_object([]) { |i,a| m.times { a << { type: "test#{i}", value: 1 } } }
end

mk_arr(3,2)
  #=> [{:type=>"test0", :value=>1}, {:type=>"test0", :value=>1},
  #    {:type=>"test1", :value=>1}, {:type=>"test1", :value=>1},
  #    {:type=>"test2", :value=>1}, {:type=>"test2", :value=>1}] 

require 'benchmark'  
def bench_em(n,m)
  arr = mk_arr(n,m)
  puts "n = #{n}, m = #{m}"
  print "group_by: "
  puts Benchmark.measure { arr.group_by { |h| h[:type] } }     
  print "chunk   : "
  puts Benchmark.measure { Hash[arr.chunk { |h| h[:type] }.to_a] }
  puts
end

              user     system      total        real
n = 100000, m = 2
group_by:   0.090000   0.000000   0.090000 (  0.095301)
chunk   :   0.160000   0.000000   0.160000 (  0.166608)

n = 100000, m = 4
group_by:   0.250000   0.010000   0.260000 (  0.252951)
chunk   :   0.260000   0.000000   0.260000 (  0.261747)

n = 100000, m = 8
group_by:   0.370000   0.010000   0.380000 (  0.386423)
chunk   :   0.450000   0.000000   0.450000 (  0.447134)

n = 1000000, m = 4
group_by:   2.740000   0.040000   2.780000 (  2.775030)
chunk   :   3.690000   0.070000   3.760000 (  3.790305)

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