Sorted and custom comparators in Swift

Introduction

Sorting helps organize data so it's easier to find or use. Custom comparators let you decide exactly how to sort things, like by size or color.

When you want to list names in alphabetical order.

When you need to sort numbers from smallest to largest.

When sorting objects by a specific property, like sorting people by age.

When you want to sort items in a special way, like putting favorites first.

Syntax

Swift

let sortedArray = array.sorted(by: { (a, b) -> Bool in
    return a < b
})

The sorted(by:) method takes a closure that compares two items.

The closure returns true if the first item should come before the second.

Examples

Sorts numbers in ascending order using a simple closure.

Swift

let numbers = [3, 1, 4, 2]
let sortedNumbers = numbers.sorted(by: { $0 < $1 })
print(sortedNumbers)

Sorts words by their length, shortest first.

Swift

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

Sorts a list of people by their age from youngest to oldest.

Swift

struct Person {
    let name: String
    let age: Int
}

let people = [
    Person(name: "Alice", age: 30),
    Person(name: "Bob", age: 25),
    Person(name: "Charlie", age: 35)
]

let sortedByAge = people.sorted(by: { $0.age < $1.age })
for person in sortedByAge {
    print("\(person.name): \(person.age)")
}

Sample Program

This program sorts a list of products by their price from lowest to highest and prints each product with its price.

Swift

struct Product {
    let name: String
    let price: Double
}

let products = [
    Product(name: "Book", price: 12.99),
    Product(name: "Pen", price: 1.99),
    Product(name: "Laptop", price: 999.99),
    Product(name: "Coffee", price: 3.49)
]

// Sort products by price, cheapest first
let sortedProducts = products.sorted(by: { $0.price < $1.price })

for product in sortedProducts {
    print("\(product.name): $\(product.price)")
}

Important Notes

You can use shorthand argument names like $0 and $1 for simpler code.

Sorting does not change the original array; it returns a new sorted array.

Custom comparators let you sort by any rule you want, not just natural order.

Summary

Use sorted(by:) to sort arrays with your own rules.

The comparator closure decides the order by returning true or false.

Sorting helps organize data to make it easier to use or display.

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