Codable protocol for encoding/decoding in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Codable encoding example?

Consider this Swift code that encodes a struct to JSON. What will be printed?

Swift

import Foundation

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

let user = User(name: "Alice", age: 30)
let encoder = JSONEncoder()
encoder.outputFormatting = .sortedKeys
if let jsonData = try? encoder.encode(user), let jsonString = String(data: jsonData, encoding: .utf8) {
    print(jsonString)
}

A {"name":"Alice","age":30}

B {"age":30,"name":"Alice"}

C {"name":30,"age":"Alice"}

D Runtime error: Encoding failed

💡 Hint

Remember that outputFormatting = .sortedKeys sorts keys alphabetically.

The JSONEncoder with .sortedKeys outputs keys in alphabetical order, so age comes before name. The values match the struct properties.

❓ Predict Output

intermediate

What error does this decoding code produce?

Given this Swift code decoding JSON into a struct, what error will occur?

Swift

import Foundation

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

let json = "{\"id\": \"101\", \"name\": \"Book\"}".data(using: .utf8)!

let decoder = JSONDecoder()
let product = try? decoder.decode(Product.self, from: json)
print(product != nil ? "Success" : "Failure")

A Success

B Syntax error

C Runtime error: type mismatch during decoding

D Failure

💡 Hint

Check the JSON types vs struct property types.

The JSON has the id as a string, but the struct expects an Int. This mismatch causes decoding to fail, so product is nil and prints "Failure".

🧠 Conceptual

advanced

Which option correctly implements custom decoding with Codable?

You want to decode a JSON where the key full_name maps to the property name in your struct. Which code snippet correctly implements this?

A

struct Person: Codable {
    var name: String
    enum CodingKeys: String, CodingKey {
        case name = "full_name"
    }
}

B

struct Person: Codable {
    var name: String
    enum CodingKeys: String, CodingKey {
        case full_name = "name"
    }
}

C

struct Person: Codable {
    var name: String
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .full_name)
    }
    enum CodingKeys: String, CodingKey {
        case full_name
    }
}

D

struct Person: Codable {
    var name: String
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
    }
    enum CodingKeys: String, CodingKey {
        case name = "full_name"
    }
}

💡 Hint

Look carefully at how CodingKeys maps JSON keys to struct properties.

Option A correctly maps the JSON key full_name to the property name using case name = "full_name". Option A misses the mapping in CodingKeys so decoding will fail. Option A tries to decode using .name key which doesn't exist in JSON. Option A reverses the mapping incorrectly.

🔧 Debug

advanced

Why does this Codable struct fail to encode?

Examine this Swift struct and encoding code. Why does encoding fail?

Swift

import Foundation

struct Item: Codable {
    var id: Int
    var description: String
    var date: Date
}

let item = Item(id: 1, description: "Test", date: Date())
let encoder = JSONEncoder()
encoder.dateEncodingStrategy = .iso8601
let jsonData = try? encoder.encode(item)
print(jsonData != nil ? "Encoded" : "Failed")

A Encoding succeeds and prints "Encoded"

B Encoding fails because Date is not Codable

C Encoding fails because .iso8601 strategy is not supported on JSONEncoder

D Encoding fails because description is a reserved keyword

💡 Hint

Check if Date supports Codable and if .iso8601 is valid.

Date conforms to Codable and JSONEncoder supports .iso8601 dateEncodingStrategy. The property name description is allowed. So encoding succeeds.

🚀 Application

expert

How many keys are in the decoded dictionary from this JSON?

Given this JSON string decoded into a Swift dictionary, how many keys does the resulting dictionary have?

Swift

import Foundation

let jsonString = "{\"a\": 1, \"b\": {\"c\": 2, \"d\": 3}, \"e\": [4,5]}"
let jsonData = jsonString.data(using: .utf8)!

let decoded = try? JSONSerialization.jsonObject(with: jsonData) as? [String: Any]
print(decoded?.count ?? 0)

A 2

B 5

C 3

D 1

💡 Hint

Count only the top-level keys in the dictionary.

The JSON has three top-level keys: a, b, and e. Nested keys inside b are not counted at this level.