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.