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Swift Academy · Lesson

In-out params & basic error handling (throws)

Mutate arguments with in-out parameters and handle recoverable failures using throws, try, and do/catch.

Lesson 3 of 38 steps

In-out params & basic error handling (throws) is a free Swift Academy lesson on CoddyKit. This is lesson 3 of 3. 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 3 lessons in total.

Intro

This lesson covers in-out parameters (mutate the caller's variable) and basic error handling with throws, try, and do/catch.

inout basics

inout lets a function modify the caller's variable. Call with &name.

func increment(_ value: inout Int) {
    value += 1   // mutate caller's variable
}
var count = 0
increment(&count)
print("count =", count)  // 1

When to use

Use inout sparingly:

  • Good for simple mutations (e.g., counters).
  • Prefer returning new values for clarity and testability.

throws + do/catch

Mark functions that can fail with throws. Use do/try/catch to handle errors.

enum FileError: Error { case notFound, unreadable }

func readFile(named: String) throws -> String {
    if named.isEmpty { throw FileError.notFound }
    return "contents of \\(named)"
}

// Calling a throwing function
do {
    let text = try readFile(named: "notes.txt")
    print(text)
} catch {
    print("Failed:", error)
}

try? vs try!

try? turns errors into nil (optional). try! will crash on error—avoid unless absolutely certain.

// try? converts errors to nil (optional)
let maybeText = try? readFile(named: "")
print(maybeText as Any)  // nil

// try! crashes on error—only use when you are sure
// let forcedText = try! readFile(named: "")  // avoid in production

Throwing helpers

You can throw from helpers. Prefer small enums for custom errors; avoid raw NSError unless needed.

func parseInt(_ s: String) throws -> Int {
    if let n = Int(s) { return n }
    else { throw NSError(domain: "Parse", code: 1) }
}

do {
    let n = try parseInt("10")
    print("n =", n)
} catch {
    print("parse error")
}

inout question

Quick check: What does inout mean for a parameter?

Answer it in the interactive lesson

Recap

Recap: Use inout for direct mutation (sparingly). Handle failures with throws, try, do/catch; prefer try? for optional results and avoid try! in production.

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Frequently Asked Questions

Is the “In-out params & basic error handling (throws)” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “In-out params & basic error handling (throws)”, 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 3 lessons in total.

What will I learn in “In-out params & basic error handling (throws)”?

Mutate arguments with in-out parameters and handle recoverable failures using throws, try, and do/catch. 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 3 of 3.

How long does the “In-out params & basic error handling (throws)” 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.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Defining functions, parameters, labels, default/variadic
  2. Return values & multiple returns via tuples
  3. In-out params & basic error handling (throws)
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