Sort_by for custom sorting in Ruby - Mini Project: Build & Apply

Sort_by for custom sorting

📖 Scenario: You work in a bookstore. You have a list of books with their titles and number of pages. You want to sort the books by their page count to help customers find shorter or longer books easily.

🎯 Goal: Build a Ruby program that sorts a list of books by their page count using sort_by.

📋 What You'll Learn

Create an array of hashes called books with exact entries for title and pages

Create a variable called max_pages with the value 300

Use sort_by on books to sort by the number of pages

Print the sorted list of book titles and their page counts

💡 Why This Matters

🌍 Real World

Sorting lists of items by a property is common in apps like bookstores, libraries, or online shops to help users find what they want quickly.

💼 Career

Knowing how to use sort_by for custom sorting is a basic but important skill for Ruby developers working with collections of data.

1

Create the books array

Create an array called books with these exact hashes: { title: 'Ruby Basics', pages: 150 }, { title: 'Advanced Ruby', pages: 320 }, { title: 'Rails Guide', pages: 280 }, { title: 'JavaScript Essentials', pages: 200 }

Ruby

# Create the books array with hashes for title and pages
# Your code here

Hint

Remember to create an array with hashes inside, each hash having keys :title and :pages.

2

Add max_pages variable

Create a variable called max_pages and set it to 300

Ruby

books = [
  { title: 'Ruby Basics', pages: 150 },
  { title: 'Advanced Ruby', pages: 320 },
  { title: 'Rails Guide', pages: 280 },
  { title: 'JavaScript Essentials', pages: 200 }
]
# Create max_pages variable and set it to 300
# Your code here

Hint

Just create a variable named max_pages and assign 300 to it.

3

Sort books by pages using sort_by

Use sort_by on books to create a new array called sorted_books sorted by the :pages key

Ruby

books = [
  { title: 'Ruby Basics', pages: 150 },
  { title: 'Advanced Ruby', pages: 320 },
  { title: 'Rails Guide', pages: 280 },
  { title: 'JavaScript Essentials', pages: 200 }
]
max_pages = 300
# Use sort_by to sort books by pages into sorted_books
# Your code here

Hint

Use sort_by with a block that returns book[:pages].

4

Print the sorted books

Use a for loop with variable book to iterate over sorted_books and print each book's title and pages in the format: "Title: Ruby Basics, Pages: 150"

Ruby

books = [
  { title: 'Ruby Basics', pages: 150 },
  { title: 'Advanced Ruby', pages: 320 },
  { title: 'Rails Guide', pages: 280 },
  { title: 'JavaScript Essentials', pages: 200 }
]
max_pages = 300
sorted_books = books.sort_by { |book| book[:pages] }
# Print each book's title and pages from sorted_books
# Your code here

Hint

Use a for loop and puts with string interpolation to print each book's details.

Practice

1. What does the Ruby method sort_by do?

easy

A. Sorts elements based on a value you specify for each element.

B. Changes the original array to a sorted one.

C. Removes duplicate elements from an array.

D. Reverses the order of elements in an array.

Solution

1. Step 1: Understand the purpose of sort_by

   sort_by sorts elements by a value you choose for each element, like length or age.

2. Step 2: Compare with other array methods

   Unlike sort!, sort_by returns a new sorted array without changing the original.

3. Final Answer:

   Sorts elements based on a value you specify for each element. -> Option A

4. Quick Check:

   sort_by sorts by chosen value [OK]

Hint: Remember: sort_by sorts by a custom value, not in-place [OK]

Common Mistakes:

• Thinking sort_by changes the original array
• Confusing sort_by with reverse or uniq
• Assuming sort_by removes duplicates

2. Which of the following is the correct syntax to sort an array words by their length using sort_by?

easy

A. words.sort_by(word.length)

B. words.sort_by => word.length

C. words.sort_by { |word| word.length }

D. words.sort_by(word) { length }

Solution

1. Step 1: Recall Ruby block syntax for sort_by

   The correct syntax uses a block with curly braces and a block variable: array.sort_by { |item| expression }.

2. Step 2: Match syntax with the options

   words.sort_by { |word| word.length } correctly uses words.sort_by { |word| word.length }. Others have syntax errors or wrong usage.

3. Final Answer:

   words.sort_by { |word| word.length } -> Option C

4. Quick Check:

   sort_by needs a block with |variable| [OK]

Hint: Use curly braces and |var| for sort_by blocks [OK]

Common Mistakes:

• Passing arguments directly instead of using a block
• Using => instead of block braces
• Misplacing parentheses around block

3. What will be the output of this Ruby code?

people = [{name: "Anna", age: 30}, {name: "Bob", age: 25}, {name: "Cara", age: 35}]
sorted = people.sort_by { |person| person[:age] }
puts sorted.map { |p| p[:name] }

medium

A. ["Anna", "Bob", "Cara"]

B. ["Bob", "Anna", "Cara"]

C. ["Cara", "Anna", "Bob"]

D. ["Bob", "Cara", "Anna"]

Solution

1. Step 1: Understand sorting by age

   The array is sorted by the :age key in ascending order: 25, 30, 35.

2. Step 2: Map sorted array to names

   After sorting, the order is Bob (25), Anna (30), Cara (35). Mapping names gives ["Bob", "Anna", "Cara"].

3. Final Answer:

   ["Bob", "Anna", "Cara"] -> Option B

4. Quick Check:

   sort_by age ascending = [Bob, Anna, Cara] [OK]

Hint: Sort_by returns new array sorted by block value [OK]

Common Mistakes:

• Assuming sort_by sorts descending by default
• Confusing keys in hash access
• Not mapping to names after sorting

4. Identify the error in this code snippet:

items = ["apple", "pear", "banana"]
sorted = items.sort_by do |item|
  item.length
end
puts sorted

medium

A. No error; code works correctly

B. Incorrect block syntax for sort_by

C. Missing parentheses in sort_by call

D. Using sort_by instead of sort

Solution

1. Step 1: Check block syntax

   The block uses do |item| ... end, which is valid for sort_by.

2. Step 2: Verify sort_by usage

   sort_by accepts a block and returns a new array sorted by item length. No syntax or logic errors.

3. Final Answer:

   No error; code works correctly -> Option A

4. Quick Check:

   sort_by with do-end block is valid [OK]

Hint: do-end blocks work fine with sort_by [OK]

Common Mistakes:

• Thinking parentheses are mandatory around block
• Confusing sort_by with sort!
• Assuming sort_by modifies original array

5. Given an array of hashes representing products:

products = [
  {name: "Pen", price: 1.5, stock: 10},
  {name: "Notebook", price: 2.0, stock: 0},
  {name: "Eraser", price: 0.5, stock: 5}
]

How would you use sort_by to get products sorted by price but only include those with stock greater than zero?

hard

A. products.sort_by { |p| p[:stock] }.select { |p| p[:price] > 0 }

B. products.sort_by { |p| p[:price] if p[:stock] > 0 }

C. products.sort_by { |p| p[:stock] > 0 ? p[:price] : 0 }

D. products.select { |p| p[:stock] > 0 }.sort_by { |p| p[:price] }

Solution

1. Step 1: Filter products with stock > 0

   Use select to keep only products where stock is greater than zero.

2. Step 2: Sort filtered products by price

   Apply sort_by on the filtered array to sort by price.

3. Final Answer:

   products.select { |p| p[:stock] > 0 }.sort_by { |p| p[:price] } -> Option D

4. Quick Check:

   Filter then sort_by for correct result [OK]

Hint: Filter first, then sort_by for combined conditions [OK]

Common Mistakes:

• Trying to filter inside sort_by block
• Sorting by stock instead of price
• Selecting after sorting, which keeps unwanted items