CompactMap for optional unwrapping in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does compactMap do in Swift?

compactMap takes a collection and transforms its elements, removing any nil values from the result. It unwraps optionals safely and returns an array of non-optional values.

beginner

How is compactMap different from map when dealing with optionals?

map transforms elements but keeps optionals in the result, while compactMap unwraps optionals and removes nil values, returning only non-optional results.

beginner

Example: What is the result of ["1", "2", "three", "4"].compactMap { Int($0) }?

The result is [1, 2, 4]. The string "three" cannot be converted to an integer, so it becomes nil and is removed.

intermediate

Why use compactMap instead of a loop with manual unwrapping?

compactMap is concise, safe, and expressive. It avoids errors from forced unwrapping and makes code easier to read by handling optionals automatically.

intermediate

Can compactMap be used to filter and transform elements at the same time?

Yes! compactMap lets you transform elements and automatically removes any that become nil, effectively filtering and mapping in one step.

What does compactMap return when applied to an array with some nil values?

A A single optional value

B An array including <code>nil</code> values

C An array without the <code>nil</code> values, unwrapped

D A dictionary of values

compactMap removes nil values and unwraps optionals, returning only non-optional elements.

Which method keeps nil values in the result when transforming an array of optionals?

A compactMap

B map

C filter

D reduce

map transforms elements but keeps nil values as optionals in the result.

What is the output of ["10", "abc", "30"].compactMap { Int($0) }?

A [10, 30]

B ["10", "abc", "30"]

C [10, nil, 30]

D nil

Only "10" and "30" convert to integers; "abc" becomes nil and is removed.

Why is compactMap safer than forced unwrapping when dealing with optionals?

A It automatically removes <code>nil</code> values without crashing

B It forces unwraps all values

C It converts all values to strings

D It ignores all optionals

compactMap safely unwraps optionals and skips nil values, preventing runtime crashes.

Can compactMap be used to both filter and transform an array?

A No, it neither filters nor transforms

B No, it only filters

C No, it only transforms

D Yes, it filters out <code>nil</code> and transforms elements

compactMap transforms elements and removes nil results, combining filtering and mapping.

Explain how compactMap helps in unwrapping optionals in Swift collections.

Think about how it handles nil and non-nil values during transformation.

Describe a real-life scenario where using compactMap is better than using map with forced unwrapping.

Consider converting user input strings to numbers.

Practice

1. What does the compactMap function do when used on an array of optionals in Swift?

easy

A. It unwraps optionals and removes all nil values from the array.

B. It converts all values to strings regardless of their type.

C. It sorts the array in ascending order.

D. It replaces all nil values with zero.

Solution

1. Step 1: Understand what compactMap does

   compactMap takes an array of optionals and unwraps each element, removing any nil values.

2. Step 2: Compare with other options

   It does not convert types, sort, or replace nil with zero. It only filters out nil and unwraps values.

3. Final Answer:

   It unwraps optionals and removes all nil values from the array. -> Option A

4. Quick Check:

   compactMap removes nil = B [OK]

Hint: Remember: compactMap unwraps and drops nils [OK]

Common Mistakes:

• Confusing compactMap with map (which keeps nils)
• Thinking compactMap sorts or changes types
• Assuming compactMap replaces nil with default values

2. Which of the following is the correct syntax to use compactMap on an array numbers of type [Int?] to get an array of non-optional Int values?

easy

A. let result = numbers.compactMap { $0 }

B. let result = numbers.compactMap { Int($0) }

C. let result = numbers.compactMap { $0! }

D. let result = numbers.compactMap { String($0) }

Solution

1. Step 1: Understand the input and output types

   numbers is an array of optional integers [Int?]. We want to unwrap and remove nil values to get [Int].

2. Step 2: Check each option's correctness

   let result = numbers.compactMap { $0 } uses compactMap { $0 } which unwraps optionals and removes nil. let result = numbers.compactMap { Int($0) } tries to convert optional Int to Int again, which is unnecessary. let result = numbers.compactMap { $0! } force unwraps, which can crash if nil. let result = numbers.compactMap { String($0) } converts to String, changing type.

3. Final Answer:

   let result = numbers.compactMap { $0 } -> Option A

4. Quick Check:

   Use compactMap with identity closure to unwrap optionals [OK]

Hint: Use compactMap { $0 } to unwrap optionals safely [OK]

Common Mistakes:

• Using force unwrap inside compactMap causing runtime errors
• Trying to convert optionals unnecessarily
• Changing the type unintentionally

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

let values: [Int?] = [1, nil, 3, nil, 5]
let unwrapped = values.compactMap { $0 }
print(unwrapped)

medium

A. Compilation error

B. [1, 3, 5]

C. [1, nil, 3, nil, 5]

D. [Optional(1), Optional(3), Optional(5)]

Solution

1. Step 1: Understand the input array

   The array values contains optional integers with some nil values.

2. Step 2: Apply compactMap

   compactMap { $0 } unwraps optionals and removes nil values, resulting in [1, 3, 5].

3. Final Answer:

   [1, 3, 5] -> Option B

4. Quick Check:

   compactMap removes nils and unwraps = [1, 3, 5] [OK]

Hint: compactMap removes nils and unwraps optionals [OK]

Common Mistakes:

• Expecting optionals in output instead of unwrapped values
• Thinking nils remain in the output
• Confusing compactMap with map

4. Identify the error in this Swift code snippet and choose the correct fix:

let strings: [String?] = ["apple", nil, "banana"]
let lengths = strings.compactMap { $0.count }
print(lengths)

medium

A. Error: Use force unwrap $0! before accessing count

B. Error: compactMap should be replaced with map

C. No error; code runs correctly

D. Error: Cannot use $0.count on optional; fix by using compactMap { $0?.count }

Solution

1. Step 1: Analyze the closure inside compactMap

   The closure uses $0.count, but $0 is an optional String?. Accessing count directly on optional causes error.

2. Step 2: Correct the closure to safely unwrap

   Using $0?.count returns an optional Int, which compactMap unwraps and removes nils, fixing the error.

3. Final Answer:

   Error: Cannot use $0.count on optional; fix by using compactMap { $0?.count } -> Option D

4. Quick Check:

   Use optional chaining inside compactMap closure [OK]

Hint: Use optional chaining ($0?.property) inside compactMap [OK]

Common Mistakes:

• Trying to access property directly on optional causing error
• Using map instead of compactMap losing nil filtering
• Force unwrapping causing runtime crashes

5. Given a dictionary [String: Int?] representing item stock counts, how can you create a new dictionary with only items that have a non-nil stock count using compactMap or similar methods?

hard

A. Use filter { $0.value != nil } without unwrapping values.

B. Use mapValues { $0! } to force unwrap all values.

C. Use compactMapValues { $0 } to unwrap and filter out nil values.

D. Use compactMap { $0 } directly on the dictionary.

Solution

1. Step 1: Understand dictionary optional values

   The dictionary has optional Int values. We want to remove keys with nil values and unwrap the rest.

2. Step 2: Use compactMapValues to unwrap and filter

   compactMapValues { $0 } unwraps optionals in values and removes entries with nil values, producing a dictionary with non-optional Int values.

3. Final Answer:

   Use compactMapValues { $0 } to unwrap and filter out nil values. -> Option C

4. Quick Check:

   compactMapValues unwraps and filters dictionary values [OK]

Hint: Use compactMapValues to unwrap and remove nil dictionary values [OK]

Common Mistakes:

• Using mapValues with force unwrap risking crashes
• Filtering without unwrapping values
• Trying to use compactMap directly on dictionary