UserDefaults for simple values in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output after saving and retrieving a value with UserDefaults?

Consider this Swift code that saves a string to UserDefaults and then retrieves it. What will be printed?

iOS Swift

let defaults = UserDefaults.standard
let key = "username"
defaults.set("Alice", forKey: key)
if let name = defaults.string(forKey: key) {
  print(name)
} else {
  print("No value")
}

A Alice

B No value

C nil

D Crash due to missing key

💡 Hint

UserDefaults stores simple values persistently and can retrieve them by the same key.

The code saves the string "Alice" under the key "username" and then retrieves it successfully, so it prints "Alice".

📝 Syntax

intermediate

Which option correctly saves an integer to UserDefaults?

You want to save the integer 42 to UserDefaults under the key "score". Which code snippet does this correctly?

A UserDefaults.standard.setInt(42, forKey: "score")

B UserDefaults.standard["score"] = 42

C UserDefaults.set(42, forKey: "score")

D UserDefaults.standard.set(42, forKey: "score")

💡 Hint

Use the standard UserDefaults instance and the set(_:forKey:) method.

Option D uses the correct method and instance to save an integer value to UserDefaults.

❓ lifecycle

advanced

When are UserDefaults values saved to disk?

You save a value to UserDefaults using set(_:forKey:). When is this value guaranteed to be saved to disk?

A Never saved automatically; must call save() explicitly

B When the app terminates or goes to background

C Only when synchronize() is called

D Immediately after calling set(_:forKey:)

💡 Hint

UserDefaults batches writes for efficiency and saves at certain app lifecycle events.

UserDefaults automatically saves changes when the app goes to background or terminates. The synchronize() method is deprecated and not needed.

🔧 Debug

advanced

Why does retrieving a saved Bool from UserDefaults return false unexpectedly?

This code saves a Bool value true but later retrieving it returns false. What is the likely cause? let defaults = UserDefaults.standard defaults.set(true, forKey: "isLoggedIn") let loggedIn = defaults.bool(forKey: "isLoggedIn") print(loggedIn)

A The key "isLoggedIn" was never saved before retrieval

B The value was saved but the retrieval key is misspelled

C UserDefaults.bool(forKey:) returns false if the key does not exist

D UserDefaults cannot store Bool values

💡 Hint

Check if the key exists before relying on bool(forKey:).

bool(forKey:) returns false if the key does not exist, so if the value was not saved or the key is wrong, it returns false by default.

🧠 Conceptual

expert

What is the best practice for storing sensitive data in UserDefaults?

You want to store a user's password securely. Which statement is true about storing it in UserDefaults?

A UserDefaults is not secure; use Keychain for sensitive data instead

B UserDefaults encrypts data automatically, so it's safe for passwords

C Storing passwords in UserDefaults is fine if the app uses HTTPS

D UserDefaults is secure and recommended for storing passwords

💡 Hint

Think about data encryption and security best practices on iOS.

UserDefaults stores data in plain text and is not encrypted. Sensitive data like passwords should be stored in the Keychain, which is designed for secure storage.

Practice

1. What is the main purpose of UserDefaults in an iOS app?

easy

A. To store small pieces of data that persist between app launches

B. To create complex database structures

C. To handle network requests

D. To design the app's user interface

Solution

1. Step 1: Understand UserDefaults role

   UserDefaults is designed to save simple data like settings or preferences that stay saved even after the app closes.

2. Step 2: Compare options

   Options B, C, and D describe other app functions unrelated to UserDefaults.

3. Final Answer:

   To store small pieces of data that persist between app launches -> Option A

4. Quick Check:

   UserDefaults = persistent small data storage [OK]

Hint: Remember UserDefaults saves simple data persistently [OK]

Common Mistakes:

• Thinking UserDefaults is for large databases
• Confusing UserDefaults with UI design
• Assuming UserDefaults handles networking

2. Which of the following is the correct syntax to save a Boolean value true for the key "isLoggedIn" using UserDefaults?

easy

A. UserDefaults.standard.save(true, "isLoggedIn")

B. UserDefaults.set(true, key: "isLoggedIn")

C. UserDefaults.standard.store(true, forKey: "isLoggedIn")

D. UserDefaults.standard.set(true, forKey: "isLoggedIn")

Solution

1. Step 1: Recall UserDefaults saving syntax

   The correct method to save a value is set(_:forKey:) called on UserDefaults.standard.

2. Step 2: Match syntax with options

   Only UserDefaults.standard.set(true, forKey: "isLoggedIn") uses the correct method and parameters.

3. Final Answer:

   UserDefaults.standard.set(true, forKey: "isLoggedIn") -> Option D

4. Quick Check:

   Save bool = set(_:forKey:) [OK]

Hint: Use UserDefaults.standard.set(value, forKey: key) to save [OK]

Common Mistakes:

• Omitting .standard before set
• Using wrong method names like save or store
• Swapping parameter labels

3. What will be the output of this Swift code snippet?

let defaults = UserDefaults.standard
defaults.set(42, forKey: "score")
let savedScore = defaults.integer(forKey: "score")
print(savedScore)

medium

A. 42

B. Optional(42)

C. nil

D. 0

Solution

1. Step 1: Understand saving and reading integer

   The code saves integer 42 under key "score" and then reads it back using integer(forKey:).

2. Step 2: Know integer(forKey:) behavior

   This method returns the stored integer value or 0 if none exists. Since 42 was saved, it returns 42.

3. Final Answer:

   42 -> Option A

4. Quick Check:

   integer(forKey:) returns saved int or 0 [OK]

Hint: integer(forKey:) returns Int, not Optional [OK]

Common Mistakes:

• Expecting Optional(42) instead of 42
• Confusing integer(forKey:) with object(forKey:)
• Thinking it returns nil if key missing

4. Identify the error in this code snippet that tries to save a string to UserDefaults:

let defaults = UserDefaults.standard
defaults.set("John", forKey: "username")
let name = defaults.string(forKey: "user_name")
print(name ?? "No name")

medium

A. The print statement syntax is incorrect

B. The key used to save and read the string is different

C. The set method is missing .standard

D. UserDefaults cannot save strings

Solution

1. Step 1: Compare keys used for saving and reading

   The string is saved with key "username" but read with key "user_name" which are different.

2. Step 2: Understand impact of key mismatch

   Because keys differ, reading returns nil and prints "No name" instead of "John".

3. Final Answer:

   The key used to save and read the string is different -> Option B

4. Quick Check:

   Matching keys needed for save/read [OK]

Hint: Check keys match exactly when saving and reading [OK]

Common Mistakes:

• Assuming UserDefaults can't save strings
• Forgetting .standard before set
• Misreading print statement syntax

5. You want to save a user's preferred theme setting as a Boolean in UserDefaults and then read it safely with a default of false if not set. Which code snippet correctly does this?

hard

A. UserDefaults.standard.set(true, forKey: "darkMode")
let isDark = UserDefaults.standard.object(forKey: "darkMode") as? Bool ?? false

B. UserDefaults.standard.set("true", forKey: "darkMode")
let isDark = UserDefaults.standard.bool(forKey: "darkMode")

C. UserDefaults.standard.set(true, forKey: "darkMode")
let isDark = UserDefaults.standard.bool(forKey: "darkMode")

D. UserDefaults.standard.set(true, forKey: "darkMode")
let isDark = UserDefaults.standard.string(forKey: "darkMode") == "true"

Solution

1. Step 1: Save Boolean correctly

   UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.bool(forKey: "darkMode") saves a Boolean true directly using set(_:forKey:) with a Bool value.

2. Step 2: Read Boolean safely with default

   Using bool(forKey:) returns the saved value or false if none exists, matching the requirement.

3. Step 3: Analyze other options

   UserDefaults.standard.set("true", forKey: "darkMode") let isDark = UserDefaults.standard.bool(forKey: "darkMode") saves a String "true" which is incorrect for bool reading. UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.object(forKey: "darkMode") as? Bool ?? false uses object casting which fails because NSNumber(true) as? Bool returns nil, so ?? false gives false even when set to true. UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.string(forKey: "darkMode") == "true" reads as String and compares, but string(forKey:) returns nil for NSNumber causing false.

4. Final Answer:

   UserDefaults.standard.set(true, forKey: "darkMode") let isDark = UserDefaults.standard.bool(forKey: "darkMode") -> Option C

5. Quick Check:

   Save/read Bool with set and bool(forKey:) [OK]

Hint: Use bool(forKey:) to read Bool with default false [OK]

Common Mistakes:

• Saving Bool as String and reading as Bool
• Using object(forKey:) cast instead of bool(forKey:)
• Reading Bool as String and comparing