What is a Ruby hash?

A hash is a data structure used to store data in the form of UNIQUE key-value pairs. Unlike arrays, there are no numerical indexes, you access the hash values with keys.

Examples include:

• A list of country names & their corresponding country codes (like ES => Spain)
• A dictionary, where every word has a list of possible definitions
• A domain name & it’s IP address

Hashes can speed up your code when you have data that is (or can be transformed into) this dictionary-like format.

Ok.

We get the general idea of what a hash is, but how do you create one?

Like this:

{}

That’s an empty hash!

A hash with three key/value pairs looks like this:

{ a: 1, b: 2, c: 3 }

Where a is a key, and 1 is the corresponding value for that key. Notice that the key-value pairs are separated by commas.

Now:

Let’s look at how you can use hashes in your Ruby projects with common hash methods.

Storing Values in a Ruby Hash

You can create a hash with a set of initial values, as we have already seen.

Here’s another example:

fruits = { coconut: 1, apple: 2, banana: 3 }

Another option is to add new values into an existing hash.

Like this:

fruits[:orange] = 4

This is :orange as the hash key, and 4 as its corresponding value.

Why is the colon before the word :orange when we access a value & after the word orange: when we create a hash? It’s a nicer syntax that allows you to create hashes without the hash-rocket (=>) symbol, which is a valid, but older way to do it.

Values can be any Ruby object.

Including:

• Strings
• Integers & Floats
• Arrays

Keys can also be anything, but symbols (like :banana) & strings are the most common type of keys you’ll find.

Remember that…

Keys are unique, we can only have one :orange key, or one :apple key.

When you add the same key twice you change its value.

How to Access Values From a Hash

You access a hash by key.

If you need to access the values directly, then a hash may not be the right structure for your data.

Example:

fruits[:orange]
# 4

This is the whole point of hashes, to quickly look up an item by its key.

If a key doesn’t exist, you’ll get nil.

fruits[:peach]
# nil

As an alternative, you can use the fetch method, which allows you to provide a default value.

Example:

fruits.fetch(:peach, 0)

If you use fetch without a default value (the 2nd argument), Ruby will raise a KeyError exception.

That’s helpful because you’ll know what key is missing.

How to Merge Two Ruby Hashes

You can take two hashes & merge them together into a new hash.

What method does this?

Well, it’s not hard to guess this one. The method’s name is merge.

Here’s how to use it:

defaults    = { a: 1, b: 2, c: 3 }
preferences = { c: 4 }

defaults.merge!(preferences)
# {:a=>1, :b=>2, :c=>4}

Notice that because keys are unique, newer values overwrite older values.

You could use this fact for interesting solutions, like creating a “defaults” hash that users can override by passing their own hash.

If you need more control over how keys are merged you can pass a block.

Like this:

defaults.merge!(preferences) { |key, old, new| [old, new].max }

Where old are the values coming from defaults, and new are the values coming from preferences.

Multiple Values For One Key

In a dictionary…

Words are unique, but they can have multiple values (definitions) associated with them.

You can do this in Ruby!

Example:

dictionary = {
  opportunity: [
    "a set of circumstances that makes it possible to do something",
    "a situation or condition favorable for attainment of a goal"
  ],
  creativity: [
    "the use of imagination or original ideas to create something new",
    "the ability to create",
    "the process where new ideas emerge from combinings existing ideas in new ways"
  ]
}

dictionary[:creativity][1]

Where dictionary[:creativity] gives you an array & [1] gives you the 2nd element from that array.

In other words:

The key is a symbol & the values are arrays. When you access the hash you get an array back which you access normally, like any other array.

How to Sort a Hash

You can sort arrays. But did you know that you can also sort hashes?

When you sort a hash, it’s sorted by key.

Example:

{ b: 1, a: 2 }.sort

# [[:a, 2], [:b, 1]]

But you can also sort them by value:

{ c: 3, b: 1, a: 2 }.sort_by(&:last)

You’ll notice that what you get from sorting a hash is not another hash…

It’s an array!

But you can convert this array back into a hash, using the to_h method.

Get All Keys & Values From a Hash

If you want a list of all the keys, good news, there is a method for that!

Here it is:

{ apple: 1, banana: 2 }.keys

# [:apple, :banana]

There’s also a method for values:

{ apple: 1, banana: 2 }.values

# [1, 2]

If you want to know if a key exists in a hash, use the key? method.

Summary

You’ve learned about Ruby hashes, a helpful data structure which is composed of key-value pairs. You also learned how to access a hash by key, and how to store new data in a hash.

Now open up irb (or pry) & start playing with hashes!

Thanks for reading

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