Sort_by for custom sorting in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sort_by method do in Ruby?

It sorts elements of a collection based on the value returned by the block you provide. It creates a new sorted array without changing the original.

beginner

How do you use sort_by to sort an array of strings by their length?

You pass a block that returns the length of each string:
array.sort_by { |str| str.length }

intermediate

Why might you choose sort_by over sort for custom sorting?

sort_by is often faster and simpler when sorting by a computed value because it calculates the sort keys once, then sorts by those keys.

beginner

What will this code output?

words = ["apple", "pear", "banana"]
sorted = words.sort_by { |w| w[-1] }
puts sorted

It sorts words by their last letter:
["banana", "apple", "pear"] because 'a' < 'e' < 'r'.

intermediate

Can sort_by be used to sort by multiple criteria? How?

Yes. Return an array of values in the block, like:
array.sort_by { |x| [x.age, x.name] } to sort by age, then name.

What does sort_by return in Ruby?

A A new sorted array

B The original array sorted in place

C A boolean indicating if sorting succeeded

D The number of elements sorted

sort_by returns a new array sorted by the block's values, leaving the original unchanged.

Which block would sort an array of numbers by their absolute value?

A array.sort_by { |n| n.abs }

B array.sort_by { |n| -n }

C array.sort_by { |n| n }

D array.sort_by { |n| n.to_s }

Using n.abs sorts numbers by their absolute value.

What is a benefit of using sort_by instead of sort with a block?

A It modifies the original array

B It is slower but more readable

C It calculates sort keys once for efficiency

D It only works with numbers

sort_by calculates the sorting keys once, making it more efficient for complex sorting.

How would you sort an array of hashes by the value of the key :score?

A array.sort_by { |h| h.score }

B array.sort_by(:score)

C array.sort { |a, b| a[:score] <=> b[:score] }

D array.sort_by { |h| h[:score] }

You access the :score key inside the block to sort by it.

What will ["cat", "dog", "bird"].sort_by { |w| w.length } return?

A ["cat", "bird", "dog"]

B ["cat", "dog", "bird"]

C ["dog", "cat", "bird"]

D ["bird", "cat", "dog"]

It sorts words by length: cat (3), dog (3), bird (4). Since cat and dog have the same length, the stable sort preserves their original relative order.

Explain how sort_by works in Ruby and give an example of sorting strings by their last character.

Think about how you tell Ruby what to sort by using a block.

Describe how to sort an array of objects by multiple attributes using sort_by.

Use an array inside the block to specify multiple sort keys.

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