Swift Academy · Lesson

Higher-Order Functions: map, flatMap, compactMap

Transforming sequences without mutation using Swift's collection functions.

Lesson 2 of 412 steps

Higher-Order Functions: map, flatMap, compactMap is a free Swift Academy lesson on CoddyKit. This is lesson 2 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

Welcome

Higher-order functions take or return other functions. Swift's collection library is built on them: `map`, `flatMap`, `compactMap`, `filter`, `reduce`.

map — Transform Each Element

```swift let nums = [1,2,3,4] let squared = nums.map { $0 * $0 } // [1,4,9,16] let strs = nums.map(String.init) // ["1","2","3","4"] ```

flatMap — Transform and Flatten

```swift let words = ["hello world","foo bar"] let all = words.flatMap { $0.split(separator:" ").map(String.init) } // ["hello","world","foo","bar"] ```

compactMap — Transform and Drop Nils

```swift let mixed = ["1","two","3"] let nums = mixed.compactMap { Int($0) } // [1,3] ```

Passing Function References

```swift func double(_ n: Int) -> Int { n * 2 } let result = [1,2,3].map(double) // [2,4,6] ``` Function references can stand in for closure literals.

Chaining Higher-Order Functions

```swift let total = (1...100) .filter { $0 % 2 == 0 } .map { $0 * $0 } .reduce(0, +) // sum of squares of even numbers 1-100 ```

flatMap on Optionals

```swift let maybeStr: String? = "42" let maybeInt = maybeStr.flatMap { Int($0) } // Optional(42) let nil2: String? = nil let also = nil2.flatMap { Int($0) } // nil ```

Custom Higher-Order Function

```swift func applyTwice(_ f: (T) -> T, _ value: T) -> T { f(f(value)) } print(applyTwice({ $0 * 2 }, 3)) // 12 ```

sorted(by:) as HOF

```swift let words = ["banana","apple","cherry"] let sorted = words.sorted { $0.count < $1.count } // ["apple","banana","cherry"] ``` `sorted(by:)` accepts a comparator function.

Practical: Process User Data

```swift struct User { var name: String; var score: Int } let users = [User(name:"A",score:80),User(name:"B",score:45),User(name:"C",score:92)] let topNames = users.filter { $0.score >= 80 }.map { $0.name }.sorted() // ["A","C"] ```

Quick Check

What does `compactMap` do that regular `map` does not?

Answer it in the interactive lesson

Recap

Key takeaways: • `map` — transform every element • `flatMap` — transform + flatten nested results • `compactMap` — transform + drop nils • Pass function references directly: `.map(String.init)` • Chain for clean data pipelines Next: function composition and pipelines.

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Frequently Asked Questions

Is the “Higher-Order Functions: map, flatMap, compactMap” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Higher-Order Functions: map, flatMap, compactMap”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Higher-Order Functions: map, flatMap, compactMap”?

Transforming sequences without mutation using Swift's collection functions. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 2 of 4.

How long does the “Higher-Order Functions: map, flatMap, compactMap” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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All lessons in this course

Pure Functions and Side-Effect-Free Design
Higher-Order Functions: map, flatMap, compactMap
Function Composition and Pipelines
Lazy Sequences for Efficiency

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