Hashes

Ruby hashes are collections of data, similar to objects in JavaScript and dictionaries in Python.

Introduction

Now that you've learned about arrays, it's time to get into the array's supercharged cousin: the hash. Ruby hashes are more advanced collections of data and are similar to objects in JavaScript and dictionaries in Python if you're familiar with those.

Learning outcomes

By the end of this lesson, you should be able to do the following:

• Explain what a hash is.

• Describe how to create a new hash and how to add information to it.

• Explain what keys and values are.

• Describe how to change and delete data inside a hash.

• Describe how to merge two hashes.

Arrays vs. hashes

You're sitting in your cubicle, diligently working away (because you would never dream of slacking off at work), when lunch time rolls around. You need to grab a bite to eat, but how are you going to go about requesting food? For the purposes of this parable, you have two options: a vending machine or a nice restaurant.

If you were to go to the vending machine, you would see nice, orderly rows of food where each option is labeled with a number. These labels are the indices of the vending machine array. It's important to note that the indices are not interchangeable: "12" will always come before "13" and after "11". You request your food by using an index to tell the vending machine what you want. It understands the index and returns whatever lives in that spot. Mmmmm, nothing like a lunch of Flamin' Hot Cheetos and Diet Coke! You are a programmer, after all.

Your other option is to sit yourself down at a table covered with a nice white tablecloth, where a pleasant waiter will see to your every need. The first thing they will do is bring you a menu, which for those of you that have only been eating out of vending machines so far in your life, lists out all of your dining options labeled with the name of the dish, such as "sublimated artichoke frittata" or "whole pork belly, market acorns, and activated shell bean". In this menu hash, the dish names are called keys: they are the labels that are used to identify your dining options. The food that those dish names represent are the values that the keys point to. To order your food, you give your waiter the key (you tell him the name of the dish you want), and he returns the value of that key (food that matches the description on the menu). Mmmmm, nothing like a lunch of free-range bison with corn and peach compote and an IPA! You are a programmer, after all.

There are two important differences to note between the vending machine array and the menu hash. First, it's far easier for us to use the names of things to find what we're looking for than to have to translate what we want into numerical indices. This is a huge advantage of using a hash: no more having to count out array elements to request what we want! Second, the items on a menu can appear in any order, and we'll still get exactly what we want as long as we use the correct name. This unordered aspect of hashes isn't true for arrays, which are highly dependent on order.

Creating hashes

Let's dive in and create a hash!

my_hash = { 
  "a random word" => "ahoy", 
  "Dorothy's math test score" => 94, 
  "an array" => [1, 2, 3],
  "an empty hash within a hash" => {} 
}

This example shows the most basic way to create a hash, which is to use the hash literal of curly braces ({}).

The above hash has four keys that point to four different values. For example, the first key, "a random word", points to the value "ahoy". As you can see, the values of a hash can be a number, a string, an array, or even another hash. Keys and values are associated with a special operator called a hash rocket: =>.

Just like with an array, you can also create a new hash by calling the good old #new method on the Hash class.

my_hash = Hash.new
my_hash               #=> {}

Of course, hashes don't only take strings as keys and values. Ruby is a pretty flexible language, so you can jam any old thing in there and it'll work just fine.

hash = { 9 => "nine", :six => 6 }

Accessing values

You can access values in a hash the same way that you access elements in an array. When you call a hash's value by key, the key goes inside a pair of brackets, just like when you're calling an array by index.

shoes = {
  "summer" => "sandals",
  "winter" => "boots"
}

shoes["summer"]   #=> "sandals"

If you try to access a key that doesn't exist in the hash, it will return nil:

shoes["hiking"]   #=> nil

Sometimes, this behavior can be problematic for you if silently returning a nil value could potentially wreak havoc in your program. Luckily, hashes have a fetch method that will raise an error when you try to access a key that is not in your hash.

shoes.fetch("hiking")   #=> KeyError: key not found: "hiking"

Alternatively, this method can return a default value instead of raising an error if the given key is not found.

shoes.fetch("hiking", "hiking boots") #=> "hiking boots"

Adding and changing data

You can add a key-value pair to a hash by calling the key and setting the value, just like you would with any other variable.

shoes["fall"] = "sneakers"

shoes     #=> {"summer"=>"sandals", "winter"=>"boots", "fall"=>"sneakers"}

You can also use this approach to change the value of an existing key.

shoes["summer"] = "flip-flops"
shoes     #=> {"summer"=>"flip-flops", "winter"=>"boots", "fall"=>"sneakers"}

Removing data

Deleting data from a hash is simple with the hash's #delete method, which provides the cool functionality of returning the value of the key-value pair that was deleted from the hash.

shoes.delete("summer")    #=> "flip-flops"
shoes                     #=> {"winter"=>"boots", "fall"=>"sneakers"}

Methods

Hashes respond to many of the same methods as arrays do since they both employ Ruby's Enumerable module. In the next lesson, we'll go into far more detail on the Enumerable module, including the differences in how the Enumerable methods behave for arrays and hashes.

A couple of useful methods that are specific to hashes are the #keys and #values methods, which very unsurprisingly return the keys and values of a hash, respectively. Note that both of these methods return arrays.

books = { 
  "Infinite Jest" => "David Foster Wallace", 
  "Into the Wild" => "Jon Krakauer" 
}

books.keys      #=> ["Infinite Jest", "Into the Wild"]
books.values    #=> ["David Foster Wallace", "Jon Krakauer"]

Merging two hashes

Occasionally, you'll come across a situation where two hashes wish to come together in holy union. Luckily, there's a method for that. (No ordained minister required!)

hash1 = { "a" => 100, "b" => 200 }
hash2 = { "b" => 254, "c" => 300 }
hash1.merge(hash2)      #=> { "a" => 100, "b" => 254, "c" => 300 }

Notice that the values from the hash getting merged in (in this case, the values in hash2) overwrite the values of the hash getting... uh, merged at (hash1 here) when the two hashes have a key that's the same.

For a full list of the methods that work on hashes, check out the Ruby Docs

Symbols as hash keys

In this lesson, we mostly used strings for hash keys, but in the real world, you'll almost always see symbols (like :this_guy) used as keys. This is predominantly because symbols are far more performant than strings in Ruby, but they also allow for a much cleaner syntax when defining hashes. Behold the beauty:

# 'Rocket' syntax 
american_cars = { 
  :chevrolet => "Corvette", 
  :ford => "Mustang", 
  :dodge => "Ram" 
}
# 'Symbols' syntax
japanese_cars = { 
  honda: "Accord", 
  toyota: "Corolla", 
  nissan: "Altima" 
}

That last example brings a tear to the eye, doesn't it? Notice that the hash rocket and the colon that represents a symbol have been mashed together. This unfortunately only works for symbols, though, so don't try { 9: "value" } or you'll get a syntax error.

When you use the cleaner 'symbols' syntax to create a hash, you'll still need to use the standard symbol syntax when you're trying to access a value. In other words, regardless of which of the above two syntax options you use when creating a hash, they both create symbol keys that are accessed the same way.

american_cars[:ford]    #=> "Mustang"
japanese_cars[:honda]   #=> "Accord"

Assignment

1. Read Launch School's chapter on Hashes, and go through the exercises using irb or any other REPL, such as repl.it.

2. Complete the hash exercises from the ruby-exercises repo that you previously cloned.

Additional resources

This section contains helpful links to other content. It isn't required, so consider it supplemental for if you need to dive deeper into something.

• Read the article Ruby Explained: Hashes by Erik Trautman. One of the delete examples is incorrect: deleting a key by setting its value to nil does not delete the key-value pair.

Knowledge check

This section contains questions for you to check your understanding of this lesson. If you're having trouble answering the questions below on your own, review the material above to find the answer.

• What are the differences between hashes and arrays?

• What are keys and values in a hash?

• How can you create a new hash?

• How can you populate a hash with data?

• How can you change existing values within a hash?

• How can you delete existing data from a hash?

• How can you merge two hashes together?

• Why is it preferred to use symbols as hash keys?