Collections (Array, Dictionary, Set) in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty array of integers.

iOS Swift

var numbers: [Int] = [1]

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

AArray()

B{}

C()

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which are for dictionaries or closures.

Using parentheses () which are for tuples or function calls.

The correct way to create an empty array is with square brackets [].

2fill in blank

medium

Complete the code to add a new key-value pair to the dictionary.

iOS Swift

var capitals = ["France": "Paris"]
capitals["Japan"] = [1]

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

A"Tokyo"

BTokyo

C'Tokyo'

Dtokyo

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string value.

Using single quotes which are not valid for strings in Swift.

Dictionary values must be strings in quotes. "Tokyo" is the correct string literal.

3fill in blank

hard

Fix the error in the code to create a set of strings.

iOS Swift

let fruits: Set<String> = [1]

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

A["apple", "banana", "cherry"]

BSet(["apple", "banana", "cherry"])

C{"apple", "banana", "cherry"}

D("apple", "banana", "cherry")

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets alone creates an array, not a set.

Using parentheses or curly braces incorrectly.

To create a set with values, use Set([...]) with an array inside.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters keys with values greater than 10.

iOS Swift

let scores = ["Alice": 12, "Bob": 8, "Eve": 15]
let filtered = scores.filter { $0.value [1] [2] }

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

A>

B10

C<

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than < instead of greater than >.

Using the wrong number for comparison.

The filter checks if the value is greater than 10 using > 10.

5fill in blank

hard

Fill all three blanks to create a dictionary from an array filtering words longer than 4 characters.

iOS Swift

let words = ["apple", "dog", "banana", "cat"]
let dict = Dictionary(uniqueKeysWithValues: words.filter { $0.count [1] [2] }.map { [3] in ([3], [3].count) })

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

A>

B4

C$0

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using $0 as the key variable name in the map instead of a named variable.

Using less than < instead of greater than > in the filter.

The filter uses > 4 to select words longer than 4 letters. The map uses 'word' as the key.

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