Sorted and custom comparators in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the array in ascending order.

Swift

let numbers = [5, 3, 8, 1]
let sortedNumbers = numbers.[1]()

Drag options to blanks, or click blank then click option'

Amap

Bsorted

Creversed

Dsort

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sort()' which sorts the array in place and returns void.

Using 'reversed()' which reverses the array order.

Using 'map()' which transforms elements but does not sort.

The 'sorted()' method returns a new array sorted in ascending order.

2fill in blank

medium

Complete the code to sort the array in descending order using a closure.

Swift

let numbers = [5, 3, 8, 1]
let sortedNumbers = numbers.sorted(by: { $0 [1] $1 })

Drag options to blanks, or click blank then click option'

A>

B<

C==

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which sorts ascending.

Using '==' which does not sort properly.

Using '<=' which sorts ascending with duplicates.

Using '>' in the closure sorts the array in descending order.

3fill in blank

hard

Fix the error in the code to sort an array of strings by their length.

Swift

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

Drag options to blanks, or click blank then click option'

Asize

Blength()

Ccount

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'length' which is not a Swift String property.

Using 'size' which does not exist for strings.

Using 'length()' which is not a valid method.

Strings in Swift use 'count' property to get their length.

4fill in blank

hard

Fill both blanks to create a dictionary of words and their lengths, including only words longer than 4 characters.

Swift

let words = ["apple", "banana", "fig", "date"]
let wordLengths = Dictionary(uniqueKeysWithValues: words.filter { $0.count [2] 4 }.map { word in (word, [1]) })

Drag options to blanks, or click blank then click option'

Aword.count

Bcount

C>

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'count' alone without 'word.' prefix.

Using '<' instead of '>' for filtering.

Using 'length' which is not a Swift property.

Use 'word.count' to get length and '>' to filter words longer than 4.

5fill in blank

hard

Fill all three blanks to create a sorted array of tuples (word, length) for words with length at least 5, sorted by length descending.

Swift

let words = ["apple", "banana", "fig", "date"]
let filteredSorted = words.filter { $0.count [1] 5 }.map { [2] in ([2], [2].count) }.sorted { $0.1 [3] $1.1 }

Drag options to blanks, or click blank then click option'

A>=

Bword

Cw

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>=' for filtering.

Using 'word' instead of 'w' for the map variable.

Using '<' in sorted closure which sorts ascending.

Filter words with length >= 5, map to tuples (w, w.count), and sort descending by length using '>' operator.

Practice

1. What does the sorted(by:) method do in Swift?

easy

A. It sorts an array using a custom rule you provide.

B. It removes duplicate elements from an array.

C. It reverses the order of elements in an array.

D. It filters elements based on a condition.

Solution

1. Step 1: Understand the purpose of sorted(by:)

   This method sorts elements of an array based on a closure that defines the order.

2. Step 2: Compare with other array methods

   Unlike filtering or reversing, sorted(by:) changes order based on a custom rule.

3. Final Answer:

   It sorts an array using a custom rule you provide. -> Option A

4. Quick Check:

   sorted(by:) = custom sorting [OK]

Hint: Remember: sorted(by:) arranges items your way [OK]

Common Mistakes:

• Confusing sorting with filtering
• Thinking it removes duplicates
• Assuming it reverses the array

2. Which of the following is the correct syntax to sort an array of integers in descending order using sorted(by:)?

easy

A. let sortedArray = numbers.sorted(by: { $1 > $0 })

B. let sortedArray = numbers.sorted(by: { $0 < $1 })

C. let sortedArray = numbers.sorted()

D. let sortedArray = numbers.sorted(by: { $0 > $1 })

Solution

1. Step 1: Understand sorting order

   To sort descending, the comparator must return true when the first element is greater than the second.

2. Step 2: Check each option

   let sortedArray = numbers.sorted(by: { $0 > $1 }) uses { $0 > $1 } which sorts descending. let sortedArray = numbers.sorted(by: { $0 < $1 }) sorts ascending. let sortedArray = numbers.sorted() sorts ascending by default. let sortedArray = numbers.sorted(by: { $1 > $0 }) reverses the comparison incorrectly.

3. Final Answer:

   let sortedArray = numbers.sorted(by: { $0 > $1 }) -> Option D

4. Quick Check:

   Descending sort uses $0 > $1 [OK]

Hint: Use $0 > $1 for descending sort [OK]

Common Mistakes:

• Using $0 < $1 for descending
• Forgetting to provide closure for custom order
• Confusing parameter order in closure

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

let names = ["Anna", "Bob", "Charlie"]
let sortedNames = names.sorted(by: { $0.count < $1.count })
print(sortedNames)

medium

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

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

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

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

Solution

1. Step 1: Understand the sorting rule

   The closure sorts by string length ascending: shorter names come first.

2. Step 2: Apply sorting to the array

   "Bob" has 3 letters, "Anna" 4, "Charlie" 7. So order is ["Bob", "Anna", "Charlie"].

3. Final Answer:

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

4. Quick Check:

   Sort by length ascending = ["Bob", "Anna", "Charlie"] [OK]

Hint: Sort by $0.count < $1.count for shortest first [OK]

Common Mistakes:

• Sorting alphabetically instead of by length
• Confusing < with > in closure
• Expecting original order

4. Identify the error in this Swift code that tries to sort an array of tuples by the second value:

let items = [("a", 3), ("b", 1), ("c", 2)]
let sortedItems = items.sorted(by: { $0.1 > $1.1 })
print(sortedItems)

medium

A. There is no error; the code sorts descending by second value.

B. The tuple elements cannot be accessed by index like $0.1.

C. The closure parameters should be $1.1 > $0.1 for ascending order.

D. The sorting closure must return an Int, not Bool.

Solution

1. Step 1: Check tuple access syntax

   Accessing tuple elements by $0.1 and $1.1 is correct in Swift closures.

2. Step 2: Verify sorting closure logic

   The closure returns true if first element's second value is greater, sorting descending by second value.

3. Final Answer:

   There is no error; the code sorts descending by second value. -> Option A

4. Quick Check:

   Tuple access and Bool return are correct [OK]

Hint: Tuple elements accessed as $0.1; closure returns Bool [OK]

Common Mistakes:

• Thinking tuple elements can't be accessed by index
• Expecting closure to return Int
• Confusing ascending and descending order

5. You have an array of dictionaries representing people: let people = [["name": "Alice", "age": 30], ["name": "Bob", "age": 25], ["name": "Charlie", "age": 30]]. How do you sort this array first by age ascending, then by name ascending if ages are equal?

hard

A. people.sorted { $0["age"] as! Int < $1["age"] as! Int && $0["name"] as! String < $1["name"] as! String }

B. people.sorted(by: { $0["age"] as! Int > $1["age"] as! Int || $0["name"] as! String > $1["name"] as! String })

C. people.sorted { ($0["age"] as! Int, $0["name"] as! String) < ($1["age"] as! Int, $1["name"] as! String) }

D. people.sorted(by: { $0["name"] as! String < $1["name"] as! String })

Solution

1. Step 1: Understand sorting by multiple keys

   Sorting by age ascending, then name ascending means comparing tuples of (age, name).

2. Step 2: Check each option's logic

   people.sorted { ($0["age"] as! Int, $0["name"] as! String) < ($1["age"] as! Int, $1["name"] as! String) } compares tuples directly, which sorts by age first, then name if ages equal. people.sorted { $0["age"] as! Int < $1["age"] as! Int && $0["name"] as! String < $1["name"] as! String } uses && which fails for correct ordering. people.sorted(by: { $0["age"] as! Int > $1["age"] as! Int || $0["name"] as! String > $1["name"] as! String }) sorts descending and mixes conditions incorrectly. people.sorted(by: { $0["name"] as! String < $1["name"] as! String }) sorts only by name.

3. Final Answer:

   people.sorted { ($0["age"] as! Int, $0["name"] as! String) < ($1["age"] as! Int, $1["name"] as! String) } -> Option C

4. Quick Check:

   Tuple comparison sorts by multiple keys [OK]

Hint: Compare tuples (age, name) for multi-level sorting [OK]

Common Mistakes:

• Using && instead of tuple comparison
• Sorting only by one key
• Mixing ascending and descending incorrectly