Accessing elements (indexing, first, last) in Ruby - Mini Project: Build & Apply

Accessing elements (indexing, first, last)

📖 Scenario: You are helping a small bookstore organize a list of book titles. You want to practice how to get specific books from the list by their position.

🎯 Goal: Learn how to access elements in a list by using index numbers, and how to get the first and last items easily.

📋 What You'll Learn

Create a list of book titles with exact names

Create a variable to hold the index number to access

Use the index to get the correct book from the list

Print the first book, the book at the index, and the last book

💡 Why This Matters

🌍 Real World

Accessing elements by position is common when working with lists of items like books, songs, or products.

💼 Career

Knowing how to get specific data from lists is a basic skill for programming jobs that handle collections of information.

1

Create the list of books

Create an array called books with these exact titles in this order: 'The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby'

Ruby

# Create the array called books with the exact titles
# Your code here

Hint

Use square brackets [] to create the array and separate titles with commas.

2

Set the index to access

Create a variable called index and set it to 2 to choose the third book in the list

Ruby

books = ['The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby']
# Create the variable index and set it to 2
# Your code here

Hint

Remember, arrays start counting at 0, so 2 means the third item.

3

Access the book at the index

Create a variable called selected_book and set it to the book at position index in the books array

Ruby

books = ['The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby']
index = 2
# Set selected_book to the book at position index
# Your code here

Hint

Use square brackets with the variable index to get the book.

4

Print first, selected, and last books

Print the first book using books.first, the selected book using selected_book, and the last book using books.last each on its own line

Ruby

books = ['The Hobbit', '1984', 'Pride and Prejudice', 'To Kill a Mockingbird', 'The Great Gatsby']
index = 2
selected_book = books[index]
# Print the first book, the selected book, and the last book
# Your code here

Hint

Use puts to print each book on its own line.

Practice

1. Which method or syntax in Ruby returns the first element of an array arr?

easy

A. arr.last

B. arr[1]

C. arr[-1]

D. arr.first

Solution

1. Step 1: Understand the purpose of .first

   The .first method returns the first element of an array in Ruby.

2. Step 2: Compare with other options

   arr[1] returns the second element (indexing starts at 0), arr.last and arr[-1] return the last element.

3. Final Answer:

   arr.first -> Option D

4. Quick Check:

   First element = arr.first [OK]

Hint: Use .first to get the first element easily [OK]

Common Mistakes:

• Using arr[1] instead of arr[0] for first element
• Confusing .last with .first
• Using negative index for first element

2. Which of the following is the correct syntax to get the last element of an array arr in Ruby?

easy

A. arr.first

B. arr[0]

C. arr[-1]

D. arr[1]

Solution

1. Step 1: Understand negative indexing in Ruby arrays

   Negative indexes count from the end, so arr[-1] returns the last element.

2. Step 2: Check other options

   arr[0] is the first element, arr.first also returns first, and arr[1] is second element.

3. Final Answer:

   arr[-1] -> Option C

4. Quick Check:

   Last element = arr[-1] [OK]

Hint: Use negative index -1 to get last element [OK]

Common Mistakes:

• Using arr[1] thinking it's last element
• Using arr.first instead of arr.last
• Using arr[-0] which is invalid

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

arr = [10, 20, 30, 40, 50]
puts arr[2]
puts arr.first
puts arr[-2]

medium

A. 40
10
30

B. 30
10
40

C. 30
50
40

D. 20
10
50

Solution

1. Step 1: Evaluate arr[2]

   Index 2 is the third element, which is 30.

2. Step 2: Evaluate arr.first and arr[-2]

   arr.first is 10 (first element), arr[-2] is second last element, 40.

3. Final Answer:

   30 10 40 -> Option B

4. Quick Check:

   Index 2 = 30, first = 10, second last = 40 [OK]

Hint: Remember index starts at 0; negative counts from end [OK]

Common Mistakes:

• Confusing index 2 with index 3
• Mixing up first and last elements
• Misunderstanding negative indexes

4. The following Ruby code throws an error. What is the problem?

arr = [5, 10, 15]
puts arr.first()
puts arr[-4]

medium

A. Negative index -4 is out of range and returns nil, no error

B. Missing argument in first() causes error

C. Array is empty, so first() fails

D. Syntax error in calling first()

Solution

1. Step 1: Check arr.first() usage

   first() can be called with or without parentheses; no error here.

2. Step 2: Analyze arr[-4]

   Array has 3 elements, so index -4 is out of range and returns nil, which prints as empty line, no error.

3. Final Answer:

   Negative index -4 is out of range and returns nil, no error -> Option A

4. Quick Check:

   Out of range negative index returns nil [OK]

Hint: Out of range negative index returns nil, no error [OK]

Common Mistakes:

• Assuming out of range index causes error
• Thinking first() needs arguments
• Confusing nil with error

5. Given the array arr = ["a", "b", "c", "d", "e"], which Ruby expression returns a new array with only the first and last elements?

hard

A. [arr.first, arr.last]

B. arr[0, 1] + arr[-1]

C. arr[0] + arr[-1]

D. arr.first + arr.last

Solution

1. Step 1: Understand the goal

   We want a new array containing only the first and last elements.

2. Step 2: Evaluate each option

   [arr.first, arr.last] creates an array with arr.first and arr.last. arr[0, 1] + arr[-1] tries to add an array and a string (wrong). arr[0] + arr[-1] adds two strings, resulting in concatenation, not array. arr.first + arr.last adds two strings, also concatenation.

3. Final Answer:

   [arr.first, arr.last] -> Option A

4. Quick Check:

   Array with first and last elements = [arr.first, arr.last] [OK]

Hint: Use array brackets to combine first and last elements [OK]

Common Mistakes:

• Adding strings instead of creating an array
• Using + with array and string causing error
• Forgetting to wrap elements in array brackets