Codable protocol for encoding/decoding in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the struct conform to Codable.

Swift

struct User: [1] {
    var name: String
    var age: Int
}

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

ACodable

BEquatable

CHashable

DIdentifiable

Attempts:

3 left

💡 Hint

Common Mistakes

Using Equatable instead of Codable.

Forgetting to conform to any protocol.

The Codable protocol allows the struct to be encoded and decoded easily.

2fill in blank

medium

Complete the code to encode the user instance to JSON data.

Swift

let user = User(name: "Alice", age: 30)
let encoder = JSONEncoder()
let data = try? encoder.[1](user)

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

Aencode

Bserialize

Cdecode

Dconvert

Attempts:

3 left

💡 Hint

Common Mistakes

Using decode instead of encode.

Trying to use serialize or convert which are not methods of JSONEncoder.

The encode method converts the instance into JSON data.

3fill in blank

hard

Fix the error in decoding JSON data to a User instance.

Swift

let decoder = JSONDecoder()
let user = try? decoder.[1](User.self, from: jsonData)

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

Aencode

Bserialize

Cdecode

Dconvert

Attempts:

3 left

💡 Hint

Common Mistakes

Using encode instead of decode.

Trying to use serialize or convert which are not methods of JSONDecoder.

The decode method is used to convert JSON data back into a Swift instance.

4fill in blank

hard

Fill both blanks to create a dictionary from an array of users with their names as keys.

Swift

let users = [User(name: "Bob", age: 25), User(name: "Carol", age: 28)]
let userDict = Dictionary(uniqueKeysWithValues: users.map { [1] in ([1].[2], [1].age) })

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

Auser

Bname

Cage

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using property names instead of variable names in the closure parameters.

Mixing up the order of variables.

We use user and name as names in the closure to map users to a dictionary with names as keys and ages as values.

5fill in blank

hard

Fill all three blanks to encode a User instance and convert it to a JSON string.

Swift

let user = User(name: "Dave", age: 40)
let encoder = JSONEncoder()
if let data = try? encoder.[1](user) {
    let jsonString = String(data: data, encoding: [2])
    print(jsonString ?? [3])
}

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

Aencode

Butf8

C"Invalid JSON"

Ddecode

Attempts:

3 left

💡 Hint

Common Mistakes

Using decode instead of encode.

Using wrong encoding like ascii.

Not providing a fallback string.

We encode the user, convert data to a UTF-8 string, and print a fallback message if conversion fails.

Practice

1. What is the main purpose of the Codable protocol in Swift?

easy

A. To create user interfaces quickly

B. To enable easy encoding and decoding of data to formats like JSON

C. To manage memory automatically

D. To handle network requests

Solution

1. Step 1: Understand Codable's role

   The Codable protocol allows Swift types to be encoded to and decoded from external representations like JSON.

2. Step 2: Compare with other options

   Options B, C, and D relate to UI, memory, and networking, which are unrelated to Codable.

3. Final Answer:

   To enable easy encoding and decoding of data to formats like JSON -> Option B

4. Quick Check:

   Codable = encoding/decoding data [OK]

Hint: Codable means convert data to/from JSON easily [OK]

Common Mistakes:

• Confusing Codable with UI or networking protocols
• Thinking Codable manages memory
• Assuming Codable is for database operations

2. Which of the following is the correct way to declare a struct that supports encoding and decoding using Codable?

easy

A. struct User: Codable { var name: String; var age: Int }

B. struct User { var name: String; var age: Int }

C. struct User: Encodable { var name: String; var age: Int }

D. struct User: Decodable { var name: String; var age: Int }

Solution

1. Step 1: Check Codable conformance

   To support both encoding and decoding, the struct must conform to Codable, which combines Encodable and Decodable.

2. Step 2: Analyze options

   struct User: Codable { var name: String; var age: Int } correctly uses : Codable. Options C and D only support encoding or decoding, not both. struct User { var name: String; var age: Int } has no conformance.

3. Final Answer:

   struct User: Codable { var name: String; var age: Int } -> Option A

4. Quick Check:

   Use Codable to encode and decode both ways [OK]

Hint: Use Codable to encode and decode both ways [OK]

Common Mistakes:

• Using only Encodable or Decodable when both needed
• Forgetting to add Codable conformance
• Declaring struct without protocol conformance

3. Given the following code, what will be printed?

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

let jsonData = "{\"name\": \"Anna\", \"age\": 30}".data(using: .utf8)!
let decoder = JSONDecoder()
if let person = try? decoder.decode(Person.self, from: jsonData) {
    print(person.name)
} else {
    print("Failed to decode")
}

medium

A. Anna

B. Failed to decode

C. 30

D. nil

Solution

1. Step 1: Decode JSON string to Person

   The JSON string matches the Person struct keys and types, so decoding succeeds.

2. Step 2: Print the name property

   Since decoding succeeds, person.name is "Anna", which is printed.

3. Final Answer:

   Anna -> Option A

4. Quick Check:

   Decoded name = Anna [OK]

Hint: Match JSON keys exactly to struct properties [OK]

Common Mistakes:

• Mismatching JSON keys and struct properties
• Not unwrapping optional data safely
• Confusing print output with age value

4. What is wrong with this code snippet that tries to decode JSON into a struct conforming to Codable?

struct Product: Codable {
    var id: Int
    var name: String
}

let json = "{\"id\": 101, \"title\": \"Book\"}"
let data = json.data(using: .utf8)!
let decoder = JSONDecoder()
let product = try? decoder.decode(Product.self, from: data)

medium

A. The JSON string is not valid JSON

B. The struct does not conform to Codable

C. The JSON key "title" does not match the struct property "name"

D. The decoder is not initialized properly

Solution

1. Step 1: Compare JSON keys with struct properties

   The JSON has key "title" but the struct expects "name"; this mismatch causes decoding to fail.

2. Step 2: Check other code parts

   The struct conforms to Codable, JSON is valid, and decoder is correctly initialized, so no issues there.

3. Final Answer:

   The JSON key "title" does not match the struct property "name" -> Option C

4. Quick Check:

   JSON keys must match struct properties exactly [OK]

Hint: JSON keys must match struct property names exactly [OK]

Common Mistakes:

• Ignoring key name mismatches
• Assuming decoder setup causes error
• Not validating JSON format

5. You want to encode a struct with an optional property that should be omitted from the JSON if it is nil. Which approach using Codable is correct?

hard

A. Declare the property as optional and use JSONEncoder's default behavior

B. Use Decodable instead of Codable

C. Make the property non-optional and assign a default value

D. Use a custom encode(to:) method to skip encoding nil properties

Solution

1. Step 1: Understand default Codable behavior

   By default, JSONEncoder encodes optional properties as null if nil, not omitting them.

2. Step 2: Use custom encoding to omit nil

   To omit nil optional properties, implement a custom encode(to:) method and encode only non-nil values.

3. Step 3: Evaluate other options

   Declare the property as optional and use JSONEncoder's default behavior keeps nulls, C changes property type, B is unrelated to encoding.

4. Final Answer:

   Use a custom encode(to:) method to skip encoding nil properties -> Option D

5. Quick Check:

   Custom encode(to:) omits nil optionals [OK]

Hint: Custom encode(to:) skips nil optionals in JSON [OK]

Common Mistakes:

• Expecting default encoder to omit nil optionals
• Making optional properties non-optional unnecessarily
• Confusing Decodable with encoding behavior