user7754069 user7754069 -4 years ago 130
Ruby Question

Group array of hashes by multiple keys to find average in Ruby

I have found solution for my problem done in JS but i need it done in Ruby (RoR). Here is link to problem and solution: Find average value for array of hashes using multiple group by

So i have array of hashes that needs to be grouped by keys (first

subject_id
then
element_id
) and then find average values for them. Number of hashes in array is not fixed.

Below is input array:

a=[
{:subject_id=>1, :element_id=>2, :value=>55},
{:subject_id=>1, :element_id=>4, :value=>33},
{:subject_id=>1, :element_id=>2, :value=>33},
{:subject_id=>1, :element_id=>4, :value=>1},
{:subject_id=>1, :element_id=>2, :value=>7},
{:subject_id=>1, :element_id=>4, :value=>4},
{:subject_id=>2, :element_id=>2, :value=>3},
{:subject_id=>2, :element_id=>2, :value=>5},
{:subject_id=>2, :element_id=>4, :value=>9}
]


Result:

b=[
{:subject_id=>1, :element_id=>2, :value=>95},
{:subject_id=>1, :element_id=>4, :value=>38},
{:subject_id=>2, :element_id=>2, :value=>8},
{:subject_id=>2, :element_id=>4, :value=>9}
]

Answer Source

The result shown in the question isn't the average, it's the sum, so the result will be different:

  def groupByAndAverage(a)
    b = []

    a.each_with_index do |element, key|
      index = b.index do |x| 
        x != element && 
          x[:subject_id] == element[:subject_id] && 
            x[:element_id] == element[:element_id] 
      end
      if index
        b[index][:value] += element[:value]
        b[index][:amount] += 1
      else
        b.push a[key].merge(amount: 1)
      end

      true
    end
    b.map do |element| 
      element[:value] = element[:value] / element[:amount]
      element.delete(:amount)
      element
    end
    b
  end

And the result from this is:

  [{:subject_id=>1, :element_id=>2, :value=>31}, 
    {:subject_id=>1, :element_id=>4, :value=>12}, 
    {:subject_id=>2, :element_id=>2, :value=>4}, 
    {:subject_id=>2, :element_id=>4, :value=>9}]
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