Array methods (length, include?, flatten) in Ruby - Mini Project: Build & Apply

Array methods (length, include?, flatten)

📖 Scenario: You are organizing a small library's book collection. The books are grouped by genre, and you want to learn how to use Ruby array methods to explore this collection.

🎯 Goal: Build a Ruby program that uses array methods length, include?, and flatten to find out how many genres there are, check if a specific book is in the collection, and combine all books into one list.

📋 What You'll Learn

Create an array of arrays called library with exact genres and books

Create a variable called genre_count to store the number of genres

Use include? method to check if the book 'The Hobbit' is in the library

Use flatten method to combine all books into one array called all_books

Print the results exactly as instructed

💡 Why This Matters

🌍 Real World

Libraries, bookstores, and personal collections often organize items in groups. Knowing how to count groups, check for items, and combine lists helps manage collections easily.

💼 Career

Understanding array methods is essential for data handling in programming jobs, especially when working with lists, databases, or APIs that return nested data.

1

Create the library array

Create an array called library with these exact genres and books:
[['Fantasy', 'The Hobbit', 'Harry Potter'], ['Science Fiction', 'Dune', 'Neuromancer'], ['Mystery', 'Sherlock Holmes', 'Gone Girl']]

Ruby

# Create the array called library with the given genres and books
# Your code here

Hint

Remember to use square brackets [] to create arrays and separate items with commas.

2

Count the number of genres

Create a variable called genre_count and set it to the length of the library array using the length method.

Ruby

library = [['Fantasy', 'The Hobbit', 'Harry Potter'], ['Science Fiction', 'Dune', 'Neuromancer'], ['Mystery', 'Sherlock Holmes', 'Gone Girl']]
# Create genre_count using library.length
# Your code here

Hint

Use array.length to find how many items are in an array.

3

Check if 'The Hobbit' is in the library

Create a variable called has_hobbit and set it to true or false by checking if the string 'The Hobbit' is included anywhere in the library array using the include? method on a flattened version of library.

Ruby

library = [['Fantasy', 'The Hobbit', 'Harry Potter'], ['Science Fiction', 'Dune', 'Neuromancer'], ['Mystery', 'Sherlock Holmes', 'Gone Girl']]
genre_count = library.length
# Create has_hobbit by checking if 'The Hobbit' is included in the flattened library
# Your code here

Hint

Use flatten to make one array from nested arrays, then use include? to check for an item.

4

Flatten the library and print results

Create a variable called all_books that is the flattened version of library. Then print genre_count, has_hobbit, and all_books each on its own line using puts.

Ruby

library = [['Fantasy', 'The Hobbit', 'Harry Potter'], ['Science Fiction', 'Dune', 'Neuromancer'], ['Mystery', 'Sherlock Holmes', 'Gone Girl']]
genre_count = library.length
has_hobbit = library.flatten.include?('The Hobbit')
# Create all_books as the flattened library
# Print genre_count, has_hobbit, and all_books each on its own line
# Your code here

Hint

Use puts to print each value. Use inspect to print the array as a string.

Practice

1. What does the length method do when called on an array in Ruby?

easy

A. Returns the number of elements in the array

B. Checks if an element exists in the array

C. Flattens nested arrays into a single array

D. Removes duplicate elements from the array

Solution

1. Step 1: Understand the purpose of length

   The length method counts how many items are inside an array.

2. Step 2: Compare with other methods

   include? checks for presence, flatten merges nested arrays, so they do different things.

3. Final Answer:

   Returns the number of elements in the array -> Option A

4. Quick Check:

   length = count items [OK]

Hint: Remember length counts items, not checks or changes arrays [OK]

Common Mistakes:

• Confusing length with include? method
• Thinking length flattens nested arrays
• Assuming length removes duplicates

2. Which of the following is the correct syntax to check if the array arr includes the number 5?

easy

A. arr.include(5)

B. arr.includes?(5)

C. arr.has?(5)

D. arr.include?(5)

Solution

1. Step 1: Recall the correct method name and syntax

   The method to check presence is include? with a question mark at the end.

2. Step 2: Verify method call on array

   It should be called as arr.include?(5), not include or includes?.

3. Final Answer:

   arr.include?(5) -> Option D

4. Quick Check:

   Correct method is include? with question mark [OK]

Hint: Remember Ruby methods with ? end with a question mark [OK]

Common Mistakes:

• Omitting the question mark in include?
• Using incorrect method names like includes? or has?
• Using parentheses incorrectly

3. What is the output of the following Ruby code?

arr = [1, [2, 3], [4, [5]]]
puts arr.flatten.length

medium

A. 5

B. 3

C. 6

D. 7

Solution

1. Step 1: Apply flatten to the array

   The nested array [1, [2, 3], [4, [5]]] becomes [1, 2, 3, 4, 5] after flattening.

2. Step 2: Calculate the length of the flattened array

   The flattened array has 5 elements, so length returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Flattened array length = 5 [OK]

Hint: Flatten first, then count elements with length [OK]

Common Mistakes:

• Counting nested arrays as single elements
• Forgetting flatten changes nested arrays
• Miscounting elements after flatten

4. The following code is intended to check if the array nums contains the number 10, but it raises an error. What is the problem?

nums = [5, 10, 15]
if nums.include(10)
  puts "Found 10"
end

medium

A. The array should be flattened before checking

B. The method should be include? with a question mark

C. The number 10 should be a string "10"

D. The if statement syntax is incorrect

Solution

1. Step 1: Identify the method name error

   The method to check presence is include? with a question mark, not include.

2. Step 2: Understand why error occurs

   Calling include without question mark is undefined, causing an error.

3. Final Answer:

   The method should be include? with a question mark -> Option B

4. Quick Check:

   Use include? to check presence [OK]

Hint: Check method names carefully; include? needs question mark [OK]

Common Mistakes:

• Using include without question mark
• Thinking flatten is needed to check presence
• Confusing number and string types
• Miswriting if statement syntax

5. Given the nested array data = [[1, 2], [3, [4, 5]], 6], which Ruby code correctly flattens it and checks if the number 4 is included?

hard

A. data.include?(4.flatten)

B. data.include?(4).flatten

C. data.flatten.include?(4)

D. data.flatten.include(4)

Solution

1. Step 1: Flatten the nested array first

   Use data.flatten to convert nested arrays into a single-level array.

2. Step 2: Check if 4 is included in the flattened array

   Call include?(4) on the flattened array to get true or false.

3. Final Answer:

   data.flatten.include?(4) -> Option C

4. Quick Check:

   Flatten then include? is correct order [OK]

Hint: Flatten first, then check inclusion with include? [OK]

Common Mistakes:

• Calling include? before flatten
• Using include without question mark
• Trying to flatten the number 4
• Incorrect method chaining order