Sorted and custom comparators in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sorted() method do in Swift?

The sorted() method returns a new array with the elements sorted in ascending order by default.

beginner

How do you use a custom comparator with sorted(by:) in Swift?

You provide a closure that takes two elements and returns true if the first element should be ordered before the second, otherwise false.

beginner

Example: Sort an array of integers in descending order using sorted(by:).

Use <code>let sortedArray = array.sorted(by: { $0 > $1 })</code> to sort from highest to lowest.

intermediate

Why use a custom comparator instead of the default sorted()?

A custom comparator lets you define your own sorting rules, like sorting strings by length or sorting objects by a property.

intermediate

What is the type signature of the closure used in sorted(by:)?

The closure has type (Element, Element) -> Bool, where it compares two elements and returns true if the first should come before the second.

What does array.sorted() return in Swift?

A The original array sorted in place

B An error if the array is not sorted

C A Boolean indicating if the array is sorted

D A new array sorted in ascending order

sorted() returns a new sorted array and does not change the original.

Which closure signature is correct for sorted(by:)?

A (Element, Element) -> Bool

B (Element) -> Bool

C (Element, Element) -> Element

D (Element) -> Element

The closure compares two elements and returns true if the first should come before the second.

How do you sort an array of strings by their length in ascending order?

A array.sorted()

B array.sorted(by: { $0.count < $1.count })

C array.sorted(by: { $0 > $1 })

D array.sorted(by: { $0.count > $1.count })

Use a custom comparator comparing the count property to sort by length.

What happens if you use sorted(by: { $0 < $1 })?

A Causes a runtime error

B Sorts the array in descending order

C Sorts the array in ascending order

D Leaves the array unsorted

This comparator sorts elements from smallest to largest (ascending).

Can sorted(by:) be used to sort arrays of custom objects?

A Yes, by providing a closure comparing object properties

B No, only basic types can be sorted

C Only if the objects conform to <code>Comparable</code>

D Only if the array is mutable

You can sort custom objects by defining how to compare them in the closure.

Explain how to sort an array in Swift using a custom comparator.

Think about how you tell Swift the order you want.

Describe the difference between sorted() and sorted(by:).

One is simple, the other lets you decide the order.

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