7 daily use cases of Ruby Hash

ruby_hash_use_case.rb

1. How to convert a JSON into a Hash?

You’ve just retrieved a Twitter profile as a JSON:
data = '{
  "name": "Aaron Patterson",
  "screen_name": "tenderlove",
  "location": "Seattle, WA"
}'

You want to transform it as a Hash for easier data manipulation:
require 'json'
 
profile = JSON.parse(data)

IRB Output:
=> {
  "name"=>"Aaron Patterson",
  "screen_name"=>"tenderlove",
  "location"=>"Seattle, WA"
}

Reference: JSON#parse

2. How to convert a Hash into a JSON?

In your web application, you track the number of signups per day for the current week:
signups_of_the_week = {
    monday: 2,
    tuesday: 3,
    wednesday: 4,
    thursday: 20,
    friday: 5,
    saturday: 2,
    sunday: 5
}

You can provide them through an API as JSON data:
require 'json'
 
signups_of_the_week.to_json

IRB Output:
=> "{\"monday\":2,\"tuesday\":3,\"wednesday\":4,\"thursday\":20,\"friday\":5,\"saturday\":2,\"sunday\":5}"

Reference: JSON#generate
Side note: JSON#pretty_generate is really helpful for pretty printing and debugging.
3. How to set default value for a nested Hash?

You have collection of contacts indexed by name, a nested Hash:
contacts = {
  'John' => {
    name: 'John',
    email: 'john@doe.com'
  },
  'Freddy' => {
    name 'Freddy',
    email: 'freddy@mercury.com'
  }
}

When working with a contact, you don’t want to check every time if it exists or not. You just want to write:
contacts['Jane'][:email] = 'jane@doe.com'
puts contacts['Jane']

IRB Output:
=> {:name=>"Jane", :email=>"jane@doe.com"}

You can do it by setting a complex value when creating the Hash
contacts = Hash.new do |hsh, key|
  hsh[key] = {
    name: key,
    email: ''
  }
end

or later with
contacts.default_proc = Proc.new do |hsh, key|
  hsh[key] = {
    name: key,
    email: ''
  }
end

References: Hash#new, Hash#default_proc=
4. How to merge two nested Hashes?

In an online shop, you want to merge a wish list with the current basket, both indexed by product id:
wish_list = {
  8 => {
    title: "The Color of Magic",
  },
  42 => {
    title: "The Hitch-Hiker's Guide to the Galaxy",
    price: 5
  }
}
 
basket = {
  8 => {
    price: 10
  },
  1729 => {
    title: "Ramanujan:  Twelve Lectures on Subjects Suggested by His Life and Work",
  price: 28
  }
}

With ActiveSupport, you can simply do it with:
require 'active_support/core_ext/hash' # not necessary if in Rails
 
basket.deep_merge(wish_list)

or without ActiveSupport
def deep_merge(h1, h2)
  h1.merge(h2) { |key, h1_elem, h2_elem| deep_merge(h1_elem, h2_elem) }
end
 
deep_merge(basket, wish_list)

IRB Output:
=> {
  8=>{:price=>10, :title=>"The Color of Magic"},
  1729=>{:title=>"Ramanujan:  Twelve Lectures on Subjects Suggested by His Life and Work", :price=>28},
  42=>{:title=>"The Hitch-Hiker's Guide to the Galaxy", :price=>5}
}

References: Hash#merge, Hash#deep_merge
5. How to filter out some keys of a Hash?

You’ve built an histogram of daily sales and stored it in a Hash, the key being the day:
histogram = {
  monday: 5,
  tuesday: 7,
  wednesday: 10,
  thursday: 18,
  friday: 7,
  saturday: 2,
  sunday: 0
}

You want to filter out Saturday and Sunday. With ActiveSupport,  you can do it as following
require 'active_support/core_ext/hash' # not necessary if Rails
 
histogram.except(:saturday, :sunday)

or without ActiveSupport
def filter(hsh, *keys)
  hsh.dup.tap do |h|
    keys.each { |k| h.delete(k) }
  end
end
 
filter(histogram, :saturday, :sunday)

Another clearer implementation is based on reject (credits to Thiago A.):
def filter2(hsh, *keys)
  hsh.reject { |k, _| keys.include? k }
end

Note that if you deal with large collection you should benchmark your implementation to select the best one.

IRB Output:
=> {:monday=>5, :tuesday=>7, :wednesday=>10, :thursday=>18, :friday=>7}

Reference: Hash#except, Hash#delete, Hash#reject, Object#dup, Object#tap
6. How to “sort” a Hash by value?

In a dices game, you keep the score per player in a Hash:
scores = {
  'The Lady' => 3,
  'Fate' => 2,
  'Death' => 10
}

You want to to sort them by scores. You can do it with:
leaderboard = scores.sort_by { |_, score| -score }

IRB Output:
=> [["Death", 10], ["The Lady", 3], ["Fate", 2]]

Reference: Enumerable#sort_by
Side note: Hash enumerates their value in their insertion order.
7. How to find the differences between two Hashes?

You regularly retrieve data from RSS feeds and put them into a Hash:
entries = {
  1372284000 => "CVE-2013-4073",
  1368482400 => "CVE-2013-2065"
}

When doing an update, you get another Hash:
updated_entries = {
  1385074800 => "CVE-2013-4164",
  1372284000 => "CVE-2013-4073",
  1368482400 => "CVE-2013-2065"
}

You want to spot which entries are new so you can for instance send them by email. You can do it with:

new_entries = updated_entries.dup.delete_if {|k| entries.keys.include? k }

but it’s better and clearer to do with
new_entries = updated_entries.reject { |k, _| entries.include? k }

as you benefit from the fast lookup of Hash (Credits to apeiros)

IRB Output:
=> {1385074800=>"CVE-2013-4164"}