Set algebra (union, intersection, difference) in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the union of two sets in Swift?

The union of two sets is a new set containing all unique elements from both sets combined.

beginner

How do you find the intersection of two sets in Swift?

The intersection of two sets is a new set containing only the elements that appear in both sets.

beginner

What does the difference of two sets represent?

The difference of two sets is a new set containing elements that are in the first set but not in the second set.

beginner

Swift code: What does this do? <br><pre>let a: Set&lt;Int&gt; = [1, 2, 3]<br>let b: Set&lt;Int&gt; = [3, 4, 5]<br>let result = a.union(b)</pre>

It creates a new set with all unique elements from sets a and b: {1, 2, 3, 4, 5}.

beginner

Swift code: What is the result of this? <br><pre>let a: Set&lt;Int&gt; = [1, 2, 3, 4]<br>let b: Set&lt;Int&gt; = [3, 4, 5]<br>let result = a.subtracting(b)</pre>

It creates a new set with elements in a but not in b: {1, 2}.

Which Swift method returns elements common to both sets?

A union()

B subtracting()

C intersection()

D adding()

The intersection() method returns elements that appear in both sets.

What does the union() method do in Swift sets?

A Returns elements only in the first set

B Returns all unique elements from both sets

C Returns elements only in the second set

D Removes elements from the first set

union() combines all unique elements from both sets.

If you want elements in set A but not in set B, which method do you use?

A symmetricDifference()

B intersection()

C union()

D subtracting()

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

What is the result of Set([1, 2, 3]).intersection(Set([3, 4, 5])) in Swift?

A {3}

B {1, 2}

C {4, 5}

D {1, 2, 3, 4, 5}

Only 3 is common to both sets.

Which method would you use to find elements in either set but not in both?

A symmetricDifference()

B intersection()

C subtracting()

D union()

symmetricDifference() returns elements in either set but not in both.

Explain how to use union, intersection, and difference with Swift sets.

Think about combining, overlapping, and excluding elements.

Describe a real-life example where set difference is useful.

Imagine two lists and what is unique to one.

Practice

1. What does the union operation do when applied to two Swift sets?

easy

A. Combines all unique elements from both sets into one set.

B. Finds elements that are only in the first set.

C. Finds elements common to both sets.

D. Removes all elements from both sets.

Solution

1. Step 1: Understand the union operation

   The union of two sets combines all unique elements from both sets without duplicates.

2. Step 2: Compare with other set operations

   Intersection finds common elements, difference finds unique elements in one set, so union is the only one combining all unique elements.

3. Final Answer:

   Combines all unique elements from both sets into one set. -> Option A

4. Quick Check:

   union = all unique elements combined [OK]

Hint: Union means all unique items from both sets combined [OK]

Common Mistakes:

• Confusing union with intersection
• Thinking union removes elements
• Mixing union with difference

2. Which of the following is the correct Swift syntax to find the intersection of two sets a and b?

easy

A. let result = a + b

B. let result = a.intersection(b)

C. let result = a.union(b)

D. let result = a - b

Solution

1. Step 1: Recall Swift set intersection syntax

   In Swift, the method to find common elements is intersection(), called as a.intersection(b).

2. Step 2: Check other options

   a + b is not valid syntax, a.union(b) finds all unique elements, and a - b finds difference, so only a.intersection(b) is correct.

3. Final Answer:

   let result = a.intersection(b) -> Option B

4. Quick Check:

   intersection uses .intersection() method [OK]

Hint: Use .intersection() method to find common elements [OK]

Common Mistakes:

• Using + instead of .intersection()
• Confusing union and intersection methods
• Using subtraction for intersection

3. What is the output of the following Swift code?

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

medium

A. [5, 6]

B. [3, 4]

C. [1, 2]

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

Solution

1. Step 1: Understand subtracting operation

   setA.subtracting(setB) returns elements in setA that are NOT in setB. Here, setA is [1,2,3,4], setB is [3,4,5,6].

2. Step 2: Calculate difference

   Elements 3 and 4 are in both sets, so they are removed. Remaining elements in setA are 1 and 2.

3. Final Answer:

   [1, 2] -> Option C

4. Quick Check:

   subtracting = elements only in first set [OK]

Hint: Subtracting removes elements found in second set from first [OK]

Common Mistakes:

• Confusing subtracting with intersection
• Expecting union instead of difference
• Not sorting output before printing

4. The following Swift code is intended to find the union of two sets setX and setY. What is the error?

let setX: Set<String> = ["apple", "banana"]
let setY: Set<String> = ["banana", "cherry"]
let combined = setX.union setY
print(combined)

medium

A. union cannot be used with string sets.

B. Sets must be arrays to use union.

C. The print statement is incorrect.

D. Missing parentheses after union method call.

Solution

1. Step 1: Check method call syntax

   In Swift, methods require parentheses even if they take parameters. The code uses setX.union setY without parentheses.

2. Step 2: Correct syntax for union

   The correct call is setX.union(setY). Other options are incorrect because union works on sets of strings and print is valid.

3. Final Answer:

   Missing parentheses after union method call. -> Option D

4. Quick Check:

   Method calls need parentheses [OK]

Hint: Always use parentheses when calling methods [OK]

Common Mistakes:

• Omitting parentheses on method calls
• Thinking union only works on numbers
• Misunderstanding print syntax

5. Given two sets of integers:

let set1: Set<Int> = [2, 4, 6, 8, 10]
let set2: Set<Int> = [1, 2, 3, 4, 5]

Write Swift code to find elements that are in either set but NOT in both (symmetric difference). What is the correct way to do this?

hard

A. All of the above

B. let result = set1.symmetricDifference(set2)

C. let result = set1.subtracting(set2).union(set2.subtracting(set1))

D. let result = set1.union(set2).subtracting(set1.intersection(set2))

Solution

1. Step 1: Understand symmetric difference

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

2. Step 2: Analyze each option

   let result = set1.symmetricDifference(set2) uses built-in symmetricDifference() method. let result = set1.union(set2).subtracting(set1.intersection(set2)) finds union then removes intersection, which equals symmetric difference. let result = set1.subtracting(set2).union(set2.subtracting(set1)) finds difference both ways then unions them, also symmetric difference.

3. Step 3: Confirm all options produce same result

   All three approaches correctly compute symmetric difference.

4. Final Answer:

   All of the above -> Option A

5. Quick Check:

   Multiple ways to get symmetric difference [OK]

Hint: Symmetric difference = union minus intersection or use built-in method [OK]

Common Mistakes:

• Using only union or intersection alone
• Confusing difference with symmetric difference
• Not combining differences both ways