Functions 2

How to provide default values for parameters

Say we want to provide a default value for a parameter. Ex:

func printTimesTables(for number: Int, end: Int) {
    for i in 1...end {
        print("\(i) x \(number) is \(i * number)")
    }
}

printTimesTables(for: 5, end: 20)

We can do:

func printTimesTables(for number: Int, end: Int = 12) {
    for i in 1...end {
        print("\(i) x \(number) is \(i * number)")
    }
}

printTimesTables(for: 5, end: 20)
printTimesTables(for: 8)

Or:

func findDirections(from: String, to: String, route: String = "fastest", avoidHighways: Bool = false) {
    // code here
}

Now these are legal:

findDirections(from: "London", to: "Glasgow")
findDirections(from: "London", to: "Glasgow", route: "scenic")
findDirections(from: "London", to: "Glasgow", route: "scenic", avoidHighways: true)

Scored 9/12 on default parameters

How to handle errors in functions

studysubject

We need to handle errors gracefully or our code will crash.

Error handling in Swift looks like this:

• Telling swift about the possible errors that can happen.
• Write a function that can flag up errors if they heppen.
• Calling the function and handling the error.

Defining possible errors

enum PasswordError: Error {
    case short, obvious
}

Write a function that triggers the errors

func checkPassword(_ password: String) throws -> String {
    if password.count < 5 {
        throw PasswordError.short
    }

    if password == "12345" {
        throw PasswordError.obvious
    }

    if password.count < 8 {
        return "OK"
    } else if password.count < 10 {
        return "Good"
    } else {
        return "Excellent"
    }
}

Note: Being marked with throws doesn’t mean the function will throw, only that it can throw.

Handling errors look like this

let string = "12345"

do {
    let result = try checkPassword(string)
    print("Password rating: \(result)")
} catch {
    print("There was an error.")
}

The try keyword needs to be inside a do...catch block. You can also use !try which can work without the do...catch however the code will crash if an error is thrown.

You must have a default catch for every kind of error. You can also have specific catches for specific errors as well:

let string = "12345"

do {
    let result = try checkPassword(string)
    print("Password rating: \(result)")
} catch PasswordError.short {
    print("Please use a longer password.")
} catch PasswordError.obvious {
    print("I have the same combination on my luggage!")
} catch {
    print("There was an error.")
}

Note: try is used before functions that can throw.

do {
    try throwingFunction1()
    nonThrowingFunction1()
    try throwingFunction2()
    nonThrowingFunction2()
    try throwingFunction3()
} catch {
    // handle errors
}

Scored 7/12 on writing throwing functions. I could use a re-read of this subject. Though I will say out the first 6 questions, I only got 1 right. It also was 4AM. Stopped immediately. Woke up and finished questions 7-12 and got each of those correct.

Scored 6/6 on running throwing functions.

Summary: Functions

• Functions allow for code reuse
• Functions start with the word func followed by the name of the function, then curly braces.
• Adding parameters make the functions more flexible
• Parameter names can be controlled externally with the _. The parameter name is followed by the type ex: parameterName: type
• Provide a default for parameters you use repeatedly, this improves the conciseness of the function
• Use a tuple to return multiple pieces of data from a function
• If your function throws errors, define an enum with the error cases and throw within your functions as necessary. Use do, try... catch to handle them at the call site