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Hashes

Goals

  • Make some hashes and get data back out of them

  • Understand why we might use a Hash

  • Understand the differences between Arrays and Hashes

Step 1

Type this in irb:
states = {"CA" => "California", "DE" => "Delaware"}

Arrays are ideal when we want to store an ordered list of items. We can access elements in an array by their position, but sometimes the position doesn't really help us much. We may want to access items in the list based on a name, or key. A hash stores pairs of items, associating keys with values.

The { character is typically called a 'curly brace', and the => is called a 'rocket' or 'hashrocket'

Type this in irb:
states.keys
states.values

You can ask a hash for an array of just its keys or its values.

Type this in irb:
states['CA']
states['DE']

With arrays, we accessed elements by their index, a number. With a hash, we access elements by their key.

Step 2

Type this in irb:
  bike_1 = {'make' => 'Trek', 'color' => 'Silver'}
  bike_2 = {'make' => 'Cannondale', 'color' => 'Blue'}
  bikes = [bike_1, bike_2]

Hashes are often used to store the properties of some object. Here, each hash stores the properties of a bike.

The keys are often the name of a property (here make, color) while tha values are the value of the property for a given object (here, Trek, silver).

Consider how you might have had to store this data if you didn't have hash. What else might you want to store in a hash?

Type this in irb:
  bikes[0]['make']
  bikes[1]['color']

When objects are nested deeply in arrays and hashes, you can access elements one after the other like this.

For example, on the first line here, we are getting the first bike in the bikes array (bikes[0]) and then getting its make (bikes[0]['make'])

Explanation

Hashes are a fundamental way to group large amounts of related data. In other languages, hashes are called dictionaries, maps, or associative arrays.

Further Reading

Ruby's documentation for Hash

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