Welcome to our exploration of Compound Data Structures in Ruby! Having navigated through Sets, Arrays, and using freeze for immutability, we’ll now dive into nested hashes and nested arrays. These structures allow us to manage complex, hierarchical data, which is essential in many real-world scenarios. This lesson will guide you through a recap of the basics, as well as the creation and modification of nested hashes and arrays.

As a quick recap, Arrays are ordered collections of elements, while Hashes store data in key-value pairs. Both of these structures can be nested to represent more complex data.

Here’s a simple example of a school directory using a hash where each grade level contains an array of student names:

Ruby
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1school_directory = {
2  'Grade1' => ['Amy', 'Bobby', 'Charlie'],
3  'Grade2' => ['David', 'Eve', 'Frank'],
4  'Grade3' => ['George', 'Hannah', 'Ivy']
5}
6
7puts school_directory['Grade1']  # Output: ["Amy", "Bobby", "Charlie"]

This nested structure organizes student names by grade level, making it easy to access the list of students in each grade.

Creating nested structures in Ruby is straightforward, following the same syntax as non-nested versions.

Nested Hash: A hash that contains other hashes as values. This structure is useful for organizing data into categories.

Ruby
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1nested_hash = {
2  'fruit' => {
3    'apple' => 'red',
4    'banana' => 'yellow'
5  },
6  'vegetable' => {
7    'carrot' => 'orange',
8    'spinach' => 'green'
9  }
10}
11
12puts nested_hash.inspect
13# Output: {"fruit"=>{"apple"=>"red", "banana"=>"yellow"}, "vegetable"=>{"carrot"=>"orange", "spinach"=>"green"}}

In this example, nested_hash is a hash with categories "fruit" and "vegetable," each containing its own key-value pairs.

Nested Array: An array that contains other arrays as elements. This structure is useful when organizing lists within a larger list.

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1nested_array = [
2  [1, 2, 3],
3  [4, 5, 6],
4  [7, 8, 9]
5]
6
7puts nested_array.inspect  # Output: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Here, nested_array contains three inner arrays, each with a set of numbers.

Combining Hashes and Arrays: Hashes can store arrays as values, allowing for a hybrid data structure that combines lists and key-value pairs.

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1array_hash = {
2  'numbers' => [1, 2, 3],
3  'letters' => ['a', 'b', 'c']
4}
5
6puts array_hash.inspect  # Output: {"numbers"=>[1, 2, 3], "letters"=>["a", "b", "c"]}

In this example, array_hash stores arrays under the keys "numbers" and "letters," making it possible to access organized lists by key.

Retrieving values from nested hashes or arrays involves using multiple indices or keys to drill down to the desired element.

• Nested Hash Access: To access a nested hash, chain keys to retrieve a specific value.

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  1puts nested_hash['fruit']['apple']  # Output: "red"

  Here, we access the color of "apple" by chaining the keys 'fruit' and 'apple'.

• Nested Array Access: Access elements in a nested array by specifying indices.

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  1puts nested_array[1][2]  # Output: 6

  This example retrieves the element "6" from the second array inside nested_array.

• Combined Structure Access: For hashes containing arrays, chain the key and index to access an element.

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  1puts array_hash['letters'][1]  # Output: "b"

  In this example, we access the second element in the "letters" array within array_hash.

You can modify nested arrays and hashes in much the same way as their non-nested counterparts. Here are some examples of common operations:

• Modifying Nested Hashes and Arrays: Change existing values, add new elements, or modify entire structures.

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  1# Change the color of 'spinach' in nested_hash
  2nested_hash['vegetable']['spinach'] = 'red'
  3
  4# Add an element to the first array in nested_array
  5nested_array[0] << 10
  6
  7# Add a new key-value pair to 'fruit' in nested_hash
  8nested_hash['fruit']['cherry'] = 'red'

  In these examples, we update the color of "spinach," add a new number to the first array in nested_array, and add "cherry" to the "fruit" hash.

• Deleting Elements: Remove specific elements or key-value pairs to keep the structure manageable.

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  1# Remove '8' from the third array in nested_array
  2nested_array[2].delete_at(1)
  3
  4# Delete 'apple' from the 'fruit' hash in nested_hash
  5nested_hash['fruit'].delete('apple')

  Here, we remove "8" from the third array in nested_array and delete the key-value pair for "apple" in nested_hash.

These examples demonstrate how to dynamically update and manage complex data structures in Ruby.

Congratulations! You've explored the essentials of nested arrays and nested hashes in Ruby, which are crucial for managing complex data structures. You’ve learned how to create, access, and modify values in these nested structures, enabling you to handle more intricate data in Ruby applications.

Up next, you’ll have hands-on practice to reinforce your understanding of these compound data structures. Happy coding!