Sort_by for custom sorting in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom sort_by with string length

What is the output of this Ruby code?

Ruby

words = ["apple", "pear", "banana", "fig"]
sorted = words.sort_by { |word| word.length }
puts sorted.join(",")

A pear,fig,apple,banana

B fig,pear,apple,banana

C fig,apple,pear,banana

D banana,apple,pear,fig

💡 Hint

Think about sorting words by their length from shortest to longest.

The sort_by block returns the length of each word. The words are sorted from shortest to longest length: "fig" (3), "pear" (4), "apple" (5), "banana" (6).

❓ Predict Output

intermediate

Sorting hashes by a nested value using sort_by

What will this Ruby code print?

Ruby

items = [{name: "a", price: 10}, {name: "b", price: 5}, {name: "c", price: 20}]
sorted = items.sort_by { |item| item[:price] }
puts sorted.map { |i| i[:name] }.join("")

A bac

B abc

C cab

D bca

💡 Hint

Look at the price values and sort from lowest to highest.

The sort_by sorts hashes by their :price key ascending: 5 (b), 10 (a), 20 (c). So the names in order are "b", "a", "c".

🔧 Debug

advanced

Identify the error in this sort_by usage

This Ruby code is intended to sort numbers by their remainder when divided by 3. What error does it raise?

Ruby

numbers = [4, 9, 7, 2]
sorted = numbers.sort_by { |n| n % }
puts sorted.join(",")

A TypeError: no implicit conversion of Symbol into Integer

B No error, outputs: 9,4,7,2

C ArgumentError: wrong number of arguments (given 0, expected 1)

D SyntaxError: unexpected end-of-input, expecting expression

💡 Hint

Check the block syntax and the use of the modulo operator.

The code uses 'n %' which is incomplete syntax for modulo operation. Ruby expects an operand after '%'. This causes a SyntaxError.

❓ Predict Output

advanced

Custom sorting with multiple criteria using sort_by

What is the output of this Ruby code?

Ruby

data = [["apple", 2], ["banana", 1], ["pear", 2], ["fig", 1]]
sorted = data.sort_by { |fruit, count| [count, fruit] }
puts sorted.map(&:first).join(",")

A banana,fig,apple,pear

B fig,banana,apple,pear

C banana,fig,pear,apple

D apple,pear,banana,fig

💡 Hint

Sort first by count ascending, then by fruit name ascending.

The sort_by sorts by count first: 1 then 2. For count=1, fruits are "banana", "fig" sorted alphabetically. For count=2, fruits are "apple", "pear" sorted alphabetically. So order is banana, fig, apple, pear.

🧠 Conceptual

expert

Understanding sort_by stability in Ruby

Given this Ruby code, what will be the output and why?

Ruby

arr = ["b", "a", "c", "a"]
sorted = arr.sort_by { |x| x }
puts sorted.join("")

A abc a (sort_by sorts but reverses equal elements order)

B aa bc (sort_by is unstable, order of equal elements may change)

C aabc (sort_by is stable, original order of equal elements preserved)

D cbaa (sort_by sorts descending)

💡 Hint

Think about whether sort_by keeps the order of elements that compare equal.

Ruby's sort_by is stable, meaning elements that compare equal keep their original order. The two "a" elements remain in the same order as in the original array.

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