Collections (Array, Dictionary, Set) in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Array in Swift?

An Array is an ordered collection of values. You can access elements by their position (index). Arrays keep the order of items.

beginner

How does a Dictionary work in Swift?

A Dictionary stores key-value pairs. Each key is unique and maps to a value. You use the key to find the value quickly.

beginner

What is a Set in Swift and when to use it?

A Set is an unordered collection of unique values. Use it when you want to store items without duplicates and order does not matter.

beginner

How do you add an element to a Swift Array?

Use the append() method. For example: myArray.append(5) adds the number 5 to the end of the array.

beginner

How do you check if a Set contains a value in Swift?

Use the contains() method. For example: mySet.contains(3) returns true if 3 is in the set, otherwise false.

Which Swift collection type keeps elements in order and allows duplicates?

A Tuple

B Dictionary

C Set

D Array

Arrays keep elements in order and allow duplicates. Sets do not keep order and do not allow duplicates.

How do you access a value in a Dictionary?

A By key

B By value

C By position

D By index number

Dictionaries use keys to access their values, not index numbers.

What happens if you add a duplicate element to a Set?

A It adds the duplicate

B It ignores the duplicate

C It replaces the old element

D It causes an error

Sets ignore duplicates and keep only unique elements.

Which method adds an element to the end of an Array?

A append()

B insert()

C add()

D push()

The append() method adds an element to the end of an Array.

Which collection type is best to store unique items without caring about order?

A Array

B Dictionary

C Set

D String

Set stores unique items and does not keep order.

Explain the differences between Array, Dictionary, and Set in Swift.

Think about order, uniqueness, and how you access elements.

Describe a real-life example where you would use a Set instead of an Array.

Imagine a guest list where each person should appear only once.

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