Collections (Array, Dictionary, Set) in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Swift code snippet?

Consider this Swift code that uses a dictionary to count occurrences of words. What will be printed?

iOS Swift

let words = ["apple", "banana", "apple", "orange", "banana", "apple"]
var counts = [String: Int]()
for word in words {
  counts[word, default: 0] += 1
}
print(counts["apple"] ?? 0)

A 2

B 1

C 3

D 0

💡 Hint

Count how many times "apple" appears in the array.

The code counts each word's occurrences. "apple" appears 3 times, so counts["apple"] is 3.

📝 Syntax

intermediate

Which option correctly creates a Set of unique integers in Swift?

You want to create a Set containing the numbers 1, 2, 3, 3, 4. Which code snippet does this correctly?

A let numbers = Set([1, 2, 3, 3, 4])

B let numbers = [1, 2, 3, 3, 4] as Set

C let numbers = Set(1, 2, 3, 3, 4)

D let numbers: Set = [1, 2, 3, 3, 4]

💡 Hint

Sets can be created from arrays using the Set initializer.

Option A uses the Set initializer with an array, correctly creating a Set with unique values.

🧠 Conceptual

advanced

What happens when you access a dictionary key that does not exist in Swift?

Given a dictionary let dict = ["a": 1, "b": 2], what is the result of dict["c"]?

A It causes a runtime error

B It returns 0

C It returns an empty string

D It returns nil

💡 Hint

Think about optional values in Swift.

Accessing a non-existent key returns nil because dictionary lookups return optional values.

❓ lifecycle

advanced

What is the effect of modifying an Array while iterating over it in Swift?

Consider this code: var numbers = [1, 2, 3] for number in numbers { numbers.append(number + 3) } What happens when this runs?

iOS Swift

var numbers = [1, 2, 3]
for number in numbers {
  numbers.append(number + 3)
}

A It throws a runtime error

B It causes an infinite loop

C It appends new elements only once and stops

D It ignores the appended elements during iteration

💡 Hint

Think about how the for-in loop works with arrays.

Appending elements during iteration extends the array, so the loop never ends, causing an infinite loop.

🔧 Debug

expert

Why does this Swift code cause a crash?

Examine this code: var dict = ["key1": "value1"] let value = dict["key2"]! print(value) Why does it crash?

iOS Swift

var dict = ["key1": "value1"]
let value = dict["key2"]!
print(value)

A Because dict["key2"] is nil and force unwrapping causes a crash

B Because the dictionary is empty

C Because "key2" is not a valid key type

D Because print cannot print optional values

💡 Hint

Force unwrapping nil causes a crash.

Accessing a missing key returns nil. Using ! to force unwrap nil causes a runtime crash.

Practice

1. Which Swift collection type keeps the order of items and allows duplicates?

easy

A. Array

B. Dictionary

C. Set

D. Tuple

Solution

1. Step 1: Understand collection types

   Arrays keep the order of elements and allow duplicates, dictionaries store key-value pairs, and sets store unique unordered items.

2. Step 2: Match collection behavior

   Since the question asks for ordered and duplicate-allowed collection, Array fits this description.

3. Final Answer:

   Array -> Option A

4. Quick Check:

   Ordered + duplicates = Array [OK]

Hint: Remember: Arrays keep order and duplicates [OK]

Common Mistakes:

• Confusing sets with arrays because both store multiple items
• Thinking dictionaries keep order like arrays
• Assuming tuples are collections like arrays

2. Which of the following is the correct way to declare an empty dictionary in Swift?

easy

A. var dict = Set<String>()

B. var dict = [Int]()

C. var dict = [String: Int]()

D. var dict = Array<Int>()

Solution

1. Step 1: Identify dictionary syntax

   In Swift, dictionaries are declared with key and value types inside square brackets separated by a colon, e.g., [KeyType: ValueType]().

2. Step 2: Check options

   var dict = [String: Int]() uses [String: Int]() which is correct for an empty dictionary with String keys and Int values. Others are array or set declarations.

3. Final Answer:

   var dict = [String: Int]() -> Option C

4. Quick Check:

   Dictionary syntax = [Key: Value]() [OK]

Hint: Dictionaries use [Key: Value]() syntax [OK]

Common Mistakes:

• Using [Int]() which is an array, not dictionary
• Confusing sets with dictionaries
• Missing colon between key and value types

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

let fruits: Set<String> = ["apple", "banana", "apple", "orange"]
print(fruits.count)

medium

A. 4

B. Error

C. 2

D. 3

Solution

1. Step 1: Understand Set behavior

   Sets store unique elements only, so duplicates are removed automatically.

2. Step 2: Count unique fruits

   The array has "apple" twice, so unique items are "apple", "banana", "orange" which is 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Set removes duplicates, count = 3 [OK]

Hint: Sets remove duplicates, count unique items only [OK]

Common Mistakes:

• Counting duplicates as separate items
• Thinking sets keep order and duplicates
• Expecting a runtime error for duplicates

4. Identify the error in this Swift dictionary code:

var capitals = ["France": "Paris", "Japan": "Tokyo"]
capitals["France"] = "Lyon"
capitals["Germany"] = "Berlin"
print(capitals["Germany"])

medium

A. Adding new key-value pair is not allowed

B. No error, code runs and prints Optional("Berlin")

C. Cannot assign new value to existing key

D. Dictionary keys must be Int, not String

Solution

1. Step 1: Check dictionary mutation rules

   Dictionaries in Swift are mutable if declared with var, so you can update existing keys and add new ones.

2. Step 2: Analyze code behavior

   Updating "France" to "Lyon" and adding "Germany" with "Berlin" is valid. Printing capitals["Germany"] returns Optional("Berlin") because dictionary lookup returns an optional.

3. Final Answer:

   No error, code runs and prints Optional("Berlin") -> Option B

4. Quick Check:

   Mutable dictionary updates and adds keys [OK]

Hint: Mutable dictionaries allow adding and updating keys [OK]

Common Mistakes:

• Thinking dictionaries are immutable by default
• Expecting print to show value without Optional
• Believing keys must be Int type

5. Given this code, what is the resulting dictionary?

let names = ["Anna", "Bob", "Anna", "Cara"]
var counts = [String: Int]()
for name in names {
  counts[name, default: 0] += 1
}
print(counts)

hard

A. ["Anna": 2, "Bob": 1, "Cara": 1]

B. ["Anna": 1, "Bob": 1, "Cara": 1]

C. ["Anna": 3, "Bob": 1, "Cara": 1]

D. Syntax error due to default value

Solution

1. Step 1: Understand dictionary default value usage

   The syntax counts[name, default: 0] returns the current value for key or 0 if key is missing, allowing safe increment.

2. Step 2: Count occurrences in array

   "Anna" appears twice, "Bob" and "Cara" once each, so counts dictionary will have Anna: 2, Bob: 1, Cara: 1.

3. Final Answer:

   ["Anna": 2, "Bob": 1, "Cara": 1] -> Option A

4. Quick Check:

   Counting duplicates with default 0 increments [OK]

Hint: Use dictionary[key, default: 0] += 1 to count items [OK]

Common Mistakes:

• Expecting syntax error from default value usage
• Counting duplicates as 1 only
• Confusing dictionary keys and values