benchmark.rb

require 'benchmark/ips'

def products
  [
    {"dt" => "2016-01-01",
     "quantity" => 122,
     "amount" => 123000
    },
    {"dt" => "2016-01-02",
     "quantity" => 97,
     "amount" => 97800
    },
    {"dt" => "2016-01-03",
     "quantity" => 142,
     "amount" => 163000
    }
  ]
end

def array_each
  rowqty = 0
  rowamount = 0

  products.each do |row|
    rowqty += row['quantity']
    rowamount += row['amount']
    row['total_quantity'] = rowqty
    row['total_amount'] = rowamount
  end
end

def array_inject
  products.inject([0, 0]) do |data, product|
    data[0] += product['quantity']
    data[1] += product['amount']
    product['total_quantity'] = data[0]
    product['total_amount'] = data[1]
    data
  end
end

Benchmark.ips do |x|
  x.report('Using Array#each') do
    array_each
  end
  x.report('Using Array#inject') do
    array_inject
  end
  x.compare!
end