Set creation and operations in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you create an empty set of integers in Swift?

Use var numbers = Set<Int>() to create an empty set of integers.

beginner

What is the difference between a Set and an Array in Swift?

A Set stores unique values with no defined order, while an Array stores ordered values and can have duplicates.

beginner

How do you add an element to a Swift set?

Use the insert() method, for example: mySet.insert(5) adds the number 5 to the set.

intermediate

What does the union() operation do on two Swift sets?

It combines all unique elements from both sets into a new set, without duplicates.

intermediate

How can you find common elements between two sets in Swift?

Use the intersection() method to get a new set with elements found in both sets.

Which Swift code creates a set of strings?

A var fruits = ["apple", "banana"]

B var fruits = {"apple", "banana"}

C var fruits = ("apple", "banana")

D var fruits: Set<String> = ["apple", "banana"]

Option D correctly creates a Set of strings using Set<String> and array literal syntax.

What happens if you insert a duplicate element into a Swift set?

A The set ignores the duplicate and remains unchanged.

B The set converts to an array.

C The set throws a runtime error.

D The set adds the duplicate element again.

Sets only store unique elements, so inserting a duplicate does nothing.

Which method returns elements in one set but not in another?

A subtracting()

B intersection()

C union()

D insert()

subtracting() returns elements in the first set that are not in the second.

How do you check if a Swift set contains a specific element?

A set.has(element)

B set.includes(element)

C set.contains(element)

D set.find(element)

Use contains() to check if an element is in the set.

What is the result of setA.union(setB)?

A A set with elements only in setB

B A set with all unique elements from setA and setB

C A set with elements only in setA

D A set with elements common to setA and setB

union() combines all unique elements from both sets.

Explain how to create a set in Swift and add elements to it.

Think about starting empty and then putting things inside.

Describe the difference between union, intersection, and subtracting operations on sets.

Imagine mixing, matching, and removing items from two baskets.

Practice

1. What is true about a Set in Swift?

easy

A. It can only store numbers, not strings.

B. It stores duplicate values in the order they were added.

C. It stores unique values without any specific order.

D. It always keeps elements sorted alphabetically.

Solution

1. Step 1: Understand Set properties

   A Set in Swift stores unique values and does not keep any order.

2. Step 2: Compare options with Set behavior

   It stores unique values without any specific order. matches the unique and unordered nature of Sets. Others describe arrays or sorted collections.

3. Final Answer:

   It stores unique values without any specific order. -> Option C

4. Quick Check:

   Set = unique + unordered [OK]

Hint: Remember: Sets never keep duplicates or order [OK]

Common Mistakes:

• Thinking Sets keep order like arrays
• Assuming Sets allow duplicates
• Believing Sets only store numbers

2. Which of the following is the correct way to create a Set of integers in Swift?

easy

A. let numbers = {1, 2, 3, 4} as Set<Int>

B. let numbers: Set<Int> = [1, 2, 3, 4]

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

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

Solution

1. Step 1: Recall Set creation syntax

   In Swift, a Set can be created by specifying type and using square brackets with values.

2. Step 2: Check each option's syntax

   let numbers: Set<Int> = [1, 2, 3, 4] uses correct syntax: Set<Int> = [1, 2, 3, 4]. Others have syntax errors or wrong casting.

3. Final Answer:

   let numbers: Set<Int> = [1, 2, 3, 4] -> Option B

4. Quick Check:

   Set creation = type + brackets [OK]

Hint: Use Set<Type> with brackets for values [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Trying to cast arrays directly to Set without syntax
• Missing type declaration when needed

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

let setA: Set<Int> = [1, 2, 3, 4]
let setB: Set<Int> = [3, 4, 5, 6]
let result = setA.intersection(setB)
print(result.sorted())

medium

A. [3, 4]

B. [1, 2]

C. [5, 6]

D. [1, 2, 3, 4, 5, 6]

Solution

1. Step 1: Understand intersection operation

   Intersection returns elements common to both sets. Here, common elements are 3 and 4.

2. Step 2: Check printed output

   Result is converted to a sorted array, so output is [3, 4].

3. Final Answer:

   [3, 4] -> Option A

4. Quick Check:

   Intersection = common elements [OK]

Hint: Intersection = only items in both sets [OK]

Common Mistakes:

• Confusing intersection with union
• Forgetting to sort before printing
• Expecting all elements combined

4. Identify the error in this Swift code snippet:

var fruits: Set<String> = ["apple", "banana", "orange"]
fruits.append("grape")

medium

A. The brackets [] cannot be used to create a Set.

B. You cannot add strings to a Set of strings.

C. The Set must be declared as a constant, not variable.

D. Sets do not have an append method.

Solution

1. Step 1: Check Set modification methods

   Sets use insert() to add elements, not append().

2. Step 2: Analyze the code error

   Calling append() on a Set causes a compile error because it is not defined for Set.

3. Final Answer:

   Sets do not have an append method. -> Option D

4. Quick Check:

   Use insert() to add to Set [OK]

Hint: Use insert(), not append(), to add Set items [OK]

Common Mistakes:

• Using append() instead of insert()
• Thinking Sets are immutable by default
• Confusing array and Set methods

5. Given two sets:

let setX: Set<Int> = [1, 2, 3, 4, 5]
let setY: Set<Int> = [4, 5, 6, 7]

Which Swift code correctly creates a new set containing elements in either setX or setY but not in both?

hard

A. let result = setX.symmetricDifference(setY)

B. let result = setX.union(setY)

C. let result = setX.intersection(setY)

D. let result = setX.subtracting(setY)

Solution

1. Step 1: Understand symmetricDifference

   Symmetric difference returns elements in either set but not in both.

2. Step 2: Compare with other operations

   Union combines all elements, intersection finds common, subtracting removes elements of second from first.

3. Final Answer:

   let result = setX.symmetricDifference(setY) -> Option A

4. Quick Check:

   Symmetric difference = exclusive elements [OK]

Hint: SymmetricDifference = items in one set only [OK]

Common Mistakes:

• Using union instead of symmetricDifference
• Confusing intersection with symmetricDifference
• Using subtracting which is one-sided