How to create a hash based on two attributes of objects in an array?

Question

Suppose I have the following class, product.rb:

require 'yaml'

class Product
    attr_accessor :id, :price

    def initialize args = {}
        options = {:id => "", :price => 0}.merge(args)
        self.id, self.price = options[:id], options[:price]
    end

    def inspect
        self.to_yaml        
    end
end

And the following example program, example.rb:

require_relative 'product'

products = Array.new

(1..5).each do |n| # Create 5 products with random id and price
    products << Product.new(
        :id => rand(1..5),
        :price => rand(10..50))
end

products.each { |p| puts p.inspect }

Example output from running example.rb:

C:\>ruby example.rb
--- !ruby/object:Product
id: 3
price: 48
--- !ruby/object:Product
id: 2
price: 47
--- !ruby/object:Product
id: 5
price: 32
--- !ruby/object:Product
id: 5
price: 49
--- !ruby/object:Product
id: 3
price: 33

The result is an array of objects of type Product, with duplicate ids.

How can I create a hash with id as the key and the sum of price as the value?

Example of desired result:

{ "2" => 47, "3" => 81, "5" => 81 } # { id => sum of price }

Ordering by id is not necessary in this particular case, but it would be a nice bonus. I can always sort the resulting hash after it has been created. — leifericf
– leifericf, Commented Jan 3, 2017 at 7:28

Maxim Krizhanovsky · Accepted Answer · 2017-01-02 14:51:19Z

4

You can use each_with_object to inject a Hash with default value of 0, like this:

products.each_with_object(Hash.new(0)) { |p, result| result[p.id] += p.price }

answered Jan 2, 2017 at 14:51

Maxim Krizhanovsky

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1 Comment

leifericf Over a year ago

Perfect, I wasn't aware of each_with_object and was fumbling around with map and lambdas. Ruby being Ruby, I knew there was probably an easier way.

user1351104 · Accepted Answer · 2017-01-02 14:51:33Z

1

products = Array.new
sum_hash = Hash.new(0) # new hash with default 0 value

(1..5).each do |n| # Create 5 products with random id and price
  id = rand(1..5)
  price = rand(10..50)

  sum_hash[id] += price

  products << Product.new(
    :id => id,
    :price => price)
end

products.each { |p| puts p.inspect }

answered Jan 2, 2017 at 14:51

user1351104

Comments

Eric Duminil · Accepted Answer · 2017-01-02 16:25:58Z

0

Another possibility would be to use group_by :

products.group_by(&:id)
        .map{|id, ps| [id, ps.map(&:price).inject(:+)] }
        .to_h

answered Jan 2, 2017 at 16:25

Eric Duminil

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