What are the differences between functions and methods in Swift?

Question

I always thought functions and methods were the same, until I was learning Swift through the "Swift Programming Language" eBook. I found out that I cannot use greet("John", "Tuesday") to call a function that I declared inside a class, as shown in the eBook in the screen shot below:

I received a error saying that "Missing argument label 'day:' in call" as per this screen shot:

Here is the code:-

import Foundation
import UIKit

class ViewController2: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        //var dailyStatement = greet("John", "Tuesday")
        var dailyStatement = greet("John", day: "Tuesday")
        println(dailyStatement)
    }

    func greet(name: String, day: String) -> String {
        return "Hello (name), today is (day)."
    }
}

After some research, I found this post: Difference between a method and a function, and it seems to me that the function that I declared inside a class is actually called a method. So, the syntax that I use to call the method is different compared to the syntax that I use to call a function.

I never realized this difference when I was programming in Objective-C.

1. What are the differences between functions and methods in Swift?

2. When do we use functions and when do we use methods in Swift?

Answer 1

After a few hours of reading and experimenting, here are the things that I found out:-

Functions in Swift

Functions are self-contained chunks of code that perform a specific task. You give a function a name that identifies what it does, and this name is used to “call” the function to perform its task when needed.

Resource: Official Apple Documentation on Functions in Swift

Function Parameter Names

However, these parameter names are only used within the body of the function itself, and cannot be used when calling the function. These kinds of parameter names are known as local parameter names, because they are only available for use within the function’s body.

It means that by default, all the parameters for Function are local parameters.

But, sometimes we want to indicate the purpose of each parameter. So, we can actually define an external parameter name for each parameter. Example Code:

func someFunction(externalParameterName localParameterName: Int) {
    // function body goes here, and can use localParameterName
    // to refer to the argument value for that parameter
}

Another way to make the external parameter name is using hash symbol (#) to shorten the name.

func someFunction(#localParameterName: Int) {
    // function body goes here, and can use localParameterName
    // to refer to the argument value for that parameter
}

To call the above functions with external parameter, you may use

someFunction(localParameterName:10)

Methods in Swift

Methods are functions that are associated with a particular type. Classes, structures, and enumerations can all define instance methods, which encapsulate specific tasks and functionality for working with an instance of a given type.

Resource: Official Apple Documentation on Methods in Swift

However, the default behavior of local names and external names is different for functions and methods.

Specifically, Swift gives the first parameter name in a method a local parameter name by default, and gives the second and subsequent parameter names both local and external parameter names by default.

Code below shows the differences for default and non-default parameters for method in Swift.

import Foundation
import UIKit

class ViewController2: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        //Default methods calling
        var dailyStatement = greet("Rick", day: "Tuesday")
        println(dailyStatement)

        //First parameter is also an external parameter
        var dailyStatement2 = greet2(name:"John", day: "Sunday")
        println(dailyStatement2)
    }

    //Default: First Parameter is the local parameter, the rest are external parameters
    func greet (name: String, day: String) -> String {
        return "Hello (name), today is (day)."
    }

    //Use Hash symbol to make the First parameter as external parameter
    func greet2 (#name: String, day: String) -> String {
        return "Hello (name), today is (day)."
    }
}

I might miss some important details. Hope someone can provide a better answer.