Sorted and custom comparators in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift sorting code?

Consider the following Swift code that sorts an array of strings by their length in ascending order. What will be printed?

Swift

let words = ["apple", "banana", "fig", "date"]
let sortedWords = words.sorted { $0.count < $1.count }
print(sortedWords)

A ["apple", "banana", "date", "fig"]

B ["banana", "apple", "date", "fig"]

C ["fig", "date", "apple", "banana"]

D ["fig", "apple", "date", "banana"]

💡 Hint

Think about sorting by the number of letters in each word.

The code sorts the array by the length of each string from shortest to longest. "fig" has 3 letters, "date" 4, "apple" 5, and "banana" 6, so the sorted array is ["fig", "date", "apple", "banana"].

🧠 Conceptual

intermediate

Which option correctly sorts integers descending using a custom comparator?

In Swift, you want to sort an array of integers from largest to smallest. Which sorting call correctly achieves this?

A numbers.sorted(by: >)

B numbers.sorted(by: <)

C numbers.sorted { $0 < $1 }

D numbers.sorted { $0 == $1 }

💡 Hint

Think about the comparison operator that returns true when the first number is bigger.

Using the greater-than operator (>) sorts the array descending. The less-than operator (<) sorts ascending. The closure with equality (==) does not define a strict order and will not sort properly.

🔧 Debug

advanced

What error does this Swift sorting code produce?

Examine this Swift code snippet. What error will it cause when compiled?

Swift

let names = ["Anna", "Bob", "Charlie"]
let sortedNames = names.sorted { $0 > }
print(sortedNames)

A SyntaxError: Missing second operand in closure

B TypeError: Cannot compare String and Int

C RuntimeError: Index out of range

D No error, prints ["Charlie", "Bob", "Anna"]

💡 Hint

Look carefully at the closure syntax inside the sorted method.

The closure { $0 > } is incomplete because it lacks the second operand to compare with $0. This causes a syntax error.

❓ Predict Output

advanced

What is the output of this Swift code sorting tuples by second element?

Given the array of tuples, what will be printed after sorting by the second element ascending?

Swift

let pairs = [("a", 3), ("b", 1), ("c", 2)]
let sortedPairs = pairs.sorted { $0.1 < $1.1 }
print(sortedPairs)

A [("c", 2), ("b", 1), ("a", 3)]

B [("b", 1), ("c", 2), ("a", 3)]

C [("b", 1), ("a", 3), ("c", 2)]

D [("a", 3), ("c", 2), ("b", 1)]

💡 Hint

Focus on sorting by the second value in each tuple.

The sorting closure compares the second element of each tuple. The order by second element ascending is 1, 2, then 3, so the sorted array is [("b", 1), ("c", 2), ("a", 3)].

🚀 Application

expert

How many items are in the resulting array after this custom sort and filter?

Consider this Swift code that filters and sorts an array of integers. How many items remain after running it?

Swift

let numbers = [5, 3, 8, 1, 9, 2]
let filteredSorted = numbers.filter { $0 % 2 == 0 }.sorted(by: >)
print(filteredSorted.count)

A 5

B 3

C 4

D 2

💡 Hint

First find even numbers, then count how many remain after sorting.

The filter keeps only even numbers from [5, 3, 8, 1, 9, 2]: 8 and 2. After sorting descending: [8, 2]. The count is 2.