Equatable, Hashable, Comparable protocols in Swift

Introduction

These protocols help you compare and organize your data easily. They let you check if things are equal, use them in sets or dictionaries, and sort them.

When you want to check if two items are the same, like comparing two people by name.

When you want to store custom objects in a set or dictionary for fast lookup.

When you want to sort a list of your custom objects, like sorting players by score.

When you want to remove duplicates from a list of your objects.

When you want to use your objects as keys in a dictionary.

Syntax

Swift

struct MyType: Equatable, Hashable, Comparable {
    static func == (lhs: MyType, rhs: MyType) -> Bool {
        // return true if lhs and rhs are equal
        return true // placeholder implementation
    }

    func hash(into hasher: inout Hasher) {
        // combine properties to create a hash
    }

    static func < (lhs: MyType, rhs: MyType) -> Bool {
        // return true if lhs is less than rhs
        return false // placeholder implementation
    }
}

Equatable requires you to define how to check if two values are equal.

Hashable requires you to provide a way to create a unique number (hash) for your object.

Comparable requires you to define how to compare two values to sort them.

Examples

Defines equality by checking if both name and age are the same.

Swift

struct Person: Equatable {
    var name: String
    var age: Int

    static func == (lhs: Person, rhs: Person) -> Bool {
        return lhs.name == rhs.name && lhs.age == rhs.age
    }
}

Creates a hash by combining name and age, so Person can be used in sets or dictionaries.

Swift

struct Person: Equatable, Hashable {
    var name: String
    var age: Int

    static func == (lhs: Person, rhs: Person) -> Bool {
        return lhs.name == rhs.name && lhs.age == rhs.age
    }

    func hash(into hasher: inout Hasher) {
        hasher.combine(name)
        hasher.combine(age)
    }
}

Allows sorting people by their age.

Swift

struct Person: Equatable, Comparable {
    var name: String
    var age: Int

    static func == (lhs: Person, rhs: Person) -> Bool {
        return lhs.name == rhs.name && lhs.age == rhs.age
    }

    static func < (lhs: Person, rhs: Person) -> Bool {
        return lhs.age < rhs.age
    }
}

Sample Program

This program defines a Person struct that can be compared, hashed, and checked for equality. It creates a list with duplicates, removes duplicates by converting to a Set, then sorts the unique people by age and prints their names and ages.

Swift

struct Person: Equatable, Hashable, Comparable {
    var name: String
    var age: Int

    static func == (lhs: Person, rhs: Person) -> Bool {
        return lhs.name == rhs.name && lhs.age == rhs.age
    }

    func hash(into hasher: inout Hasher) {
        hasher.combine(name)
        hasher.combine(age)
    }

    static func < (lhs: Person, rhs: Person) -> Bool {
        return lhs.age < rhs.age
    }
}

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

// Remove duplicates using a Set
let uniquePeople = Set(people)

// Sort unique people by age
let sortedPeople = uniquePeople.sorted()

for person in sortedPeople {
    print("\(person.name), \(person.age)")
}

Important Notes

Swift can automatically generate Equatable and Hashable implementations for structs if all properties conform.

Comparable requires you to define the < operator to decide sorting order.

Hashable is important for using your objects in sets and as dictionary keys.

Summary

Equatable lets you check if two values are equal.

Hashable lets you use your objects in sets and dictionaries.

Comparable lets you sort your objects.

Practice

1.

Which protocol in Swift allows you to check if two instances of a type are equal using ==?

easy

A. Hashable

B. Codable

C. Comparable

D. Equatable

Solution

1. Step 1: Understand the purpose of Equatable

   Equatable protocol allows comparing two instances for equality using ==.

2. Step 2: Compare with other protocols

   Hashable is for hashing, Comparable is for ordering, Codable is for encoding/decoding.

3. Final Answer:

   Equatable -> Option D

4. Quick Check:

   Equality check = Equatable [OK]

Hint: Equality means Equatable protocol [OK]

Common Mistakes:

• Confusing Hashable with Equatable
• Thinking Comparable checks equality
• Mixing Codable with Equatable

2.

Which of the following is the correct way to declare a struct Person that conforms to Equatable and Hashable?

struct Person: ??? {
    var name: String
    var age: Int
}

easy

A. Equatable, Hashable

B. Comparable, Hashable

C. Equatable, Comparable

D. Codable, Hashable

Solution

1. Step 1: Identify needed protocols

   To compare equality and use in sets/dictionaries, use Equatable and Hashable.

2. Step 2: Check syntax for protocol conformance

   Protocols are listed separated by commas after colon in struct declaration.

3. Final Answer:

   Equatable, Hashable -> Option A

4. Quick Check:

   Equality + hashing = Equatable, Hashable [OK]

Hint: List protocols comma-separated after colon [OK]

Common Mistakes:

• Using Comparable instead of Equatable
• Forgetting commas between protocols
• Using Codable instead of Equatable

3.

What will be the output of the following Swift code?

struct Item: Comparable {
    var value: Int
    static func < (lhs: Item, rhs: Item) -> Bool {
        return lhs.value < rhs.value
    }
}

let items = [Item(value: 3), Item(value: 1), Item(value: 2)]
let sortedValues = items.sorted().map { $0.value }
print(sortedValues)

medium

A. [3, 2, 1]

B. [1, 2, 3]

C. [2, 3, 1]

D. Compilation error

Solution

1. Step 1: Understand Comparable conformance

   The < operator compares value properties to sort ascending.

2. Step 2: Analyze sorting and mapping

   Sorting the array orders items by value: 1, 2, 3. Mapping extracts values in this order.

3. Final Answer:

   [1, 2, 3] -> Option B

4. Quick Check:

   Sorted values ascending = [1, 2, 3] [OK]

Hint: Comparable sorts ascending by < operator [OK]

Common Mistakes:

• Assuming descending order by default
• Confusing map output with original array
• Forgetting to implement < operator

4.

Identify the error in this Swift struct that tries to conform to Hashable:

struct Book: Hashable {
    var title: String
    var pages: Int

    func hash(into hasher: inout Hasher) {
        hasher.combine(title)
    }
}

medium

A. Missing combine call for all properties in hash(into:)

B. Struct cannot conform to Hashable without Codable

C. Incorrect method signature for hash(into:)

D. Missing == operator implementation for Equatable

Solution

1. Step 1: Check Hashable requirements

   Hashable requires hashing all properties that affect equality.

2. Step 2: Analyze hash(into:) implementation

   Only title is combined; pages is missing, causing incorrect hashing.

3. Final Answer:

   Missing combine call for all properties in hash(into:) -> Option A

4. Quick Check:

   Hash all relevant properties [OK]

Hint: Hash all properties used in equality [OK]

Common Mistakes:

• Hashing only some properties
• Forgetting to implement == operator if needed
• Wrong method signature for hash(into:)

5.

You want to create a struct Player that can be stored in a Set, compared for equality, and sorted by score descending. Which protocols should Player conform to, and how should you implement the sorting?

hard

A. Conform to Equatable and Hashable; implement < to compare scores descending

B. Conform to Comparable only; implement < to compare scores ascending

C. Conform to Equatable, Hashable, Comparable; implement < to compare scores descending

D. Conform to Hashable only; sorting is automatic

Solution

1. Step 1: Identify needed protocols

   To store in Set and check equality, conform to Equatable and Hashable. To sort, conform to Comparable.

2. Step 2: Implement sorting logic

   Implement < operator to compare scores descending by reversing comparison.

3. Final Answer:

   Conform to Equatable, Hashable, Comparable; implement < for descending scores -> Option C

4. Quick Check:

   Set + equality + custom sort = Equatable, Hashable, Comparable [OK]

Hint: Use all three protocols; reverse < for descending sort [OK]

Common Mistakes:

• Forgetting Comparable for sorting
• Sorting ascending instead of descending
• Missing Equatable or Hashable for Set storage