hashes

Hashes

Jump start: Lesson 13

Learning Goals

• Vocabulary: data structure, hash, key-value pair, default value
• To understand what a data structure is and how it can be used
• To be able to create a new hash data structure
• To be able to access data in a hash
• To be able to add data to a hash
• To understand the difference between an array and a hash

Activities

• Review the notes in this section
• Complete Hash Worksheet
• Complete Account Generator Continued
• Complete Walk-a-thon

Notes

Overview

A Hash is another very popular collection type in Ruby. It is similar to an Array, except that indexing is done via unordered keys of any object type, and not an ordered integer index.

{ "key1" => "key1sAssociatedValue", "key2" => "key2sAssociatedValue" ...}

Creating Hashes

We initialize an empty Hash by using {} (pronounced brace). A hash can be assigned to a variable in the same or a similar way that other types of data are assigned to a variable:

Empty

my_hash = {}

my_hash = Hash.new

We know that this hash is empty because the hash definition starts with the {(left brace) and ends with the }(right brace), and there is nothing between those two symbols.

With Data

By using the new syntax and specifying a default value, all keys retrieved with no corresponding value will return the default value. Without the default value, accessing an undefined key will return nil.

my_hash = Hash.new
my_hash["gibberish"] # => nil

default_data = Hash.new("def")

# By creating a hash with a default value,
# all data retrievals will return the default value if not specified otherwise
default_data["gibberish"] # => "def"

You can also create a new hash with key/value pairs populated.

my_dog = {
  "name" => "barkly",
  "breed" => "beagle",
  "age" => 2
}

Accessing Data

To access data from within a hash, we use a syntax that is similar to accessing data in an Array. The difference is that we use the strings that correspond to the key to retrieve the associated value.

my_dog["age"] # => 2
my_dog["breed"] # => "spaniel"
my_dog["name"] # => "barkly"

# by default, will return nil for keys that don't exist
my_dog["color"] # => nil

Assigning Data

We can utilize our understanding of reassigning values in Arrays to assign and reassign values in a Hash.

Assuming that we want to add a new key value pair to an existing hash, we can choose the key that we want to set the value for. In the case below, we have decided that we want to set the value "blenheim" for a new key "color".

# Add a new key value pair for color
my_dog["color"] = "blenheim"

In addition, we can reset the value associated with an existing key using the same syntax. In the case below, we would like to set the "age" value to 3 from it's original 2.

# Retrieve the value currently set
my_dog["age"] # => 2

# Reassign the value associated with the age key
my_dog["age"] = 3

# Retrieve the updated value
my_dog["age"] # => 3

Using Built-In Methods

length

This method returns the number of items in the hash. An item corresponds to a key/value pair.

my_dog = {
  "name" => "barkly",
  "breed" => "beagle",
  "age" => 2
}

my_dog.length # => 3

keys

This method returns an array of all the defined keys for the hash.

my_dog.keys #=> ["name", "breed", "age"]

values

This method returns an array of all the values of all defined keys for the hash.

my_dog.values #=> ["barkly", "beagle", 3]