Equatable, Hashable, Comparable protocols in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Equatable protocol do in Swift?

The Equatable protocol allows you to check if two values are equal using the == operator. It helps compare if two instances are the same.

beginner

Why is the Hashable protocol important?

The Hashable protocol lets a type be used in collections like Set or as keys in Dictionary. It provides a way to create a hash value for each instance.

beginner

What does the Comparable protocol allow you to do?

The Comparable protocol lets you compare values to see which is smaller, equal, or larger using operators like <, <=, >, and >=. It helps sort values.

intermediate

How do you make a custom Swift struct conform to Equatable?

You add Equatable to the struct declaration. Swift can automatically create the == function if all properties are Equatable. Example:
struct Point: Equatable { var x: Int; var y: Int }

intermediate

What must you implement when conforming to Hashable?

You must provide a hash(into:) method that feeds the essential properties into the hasher. This creates a hash value for the instance.

Which operator is required by the Equatable protocol?

A ==

B +

C <

D !=

The Equatable protocol requires the == operator to check equality.

What does conforming to Hashable enable for a type?

A Use in sets and dictionary keys

B Automatic sorting

C Printing to console

D Inheritance from other classes

Conforming to Hashable allows a type to be stored in sets and used as dictionary keys.

Which protocol lets you compare if one value is less than another?

A Equatable

B Hashable

C Codable

D Comparable

Comparable provides operators like < to compare values.

If a struct has only Equatable properties, what happens when you add Equatable conformance?

A You must write the <code>==</code> function manually

B Swift automatically generates the <code>==</code> function

C The struct cannot conform to Equatable

D You must implement <code>hash(into:)</code>

Swift can automatically create the == function if all properties are Equatable.

What method must you implement for Hashable conformance?

A func equals()

B func compare(to other: Self)

C func hash(into hasher: inout Hasher)

D func encode()

You must implement hash(into:) to provide a hash value.

Explain the purpose of the Equatable, Hashable, and Comparable protocols in Swift and how they differ.

Think about equality, uniqueness, and order.

Describe how you would make a custom struct conform to Equatable and Hashable in Swift.

Focus on protocol conformance and required methods.

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