Swift Functions Tutorial

1- Create project

First of all, to practice, you need to create a project. Open XCode and create a new Project:

2- Defining Function

In Swift, a function is defined by the " func"  keyword, the specific function name, the function can have 0 or more parameters, and with or without a return type.
Syntax define a function:
// A function has return type

func functionName(parameters) -> returntype  
{
   // Statements.
   return value;
}


// The function has no return type
func functionName(parameters)
{
    // Statements.
}
Here's a function named sayHello, have a parameter of String type , and returns a String.
MyFirstFunction.swift
import Foundation


// Define a function
// Function name: sayHello
// Parameter: name, type of String
// Return: String

func sayHello(name:String) -> String {
    
    // If name is empty.
    if name.isEmpty  {
        // Ends the function
        return "Hello every body!"
    }
    // If name is not empty, this code will be executed.
    return "Hello " + name
    
}

// Define a function, no parameters, no returns type.

func test_sayHello()  {
    
    // Call sayHello function, pass empty string.
    var greeting1 = sayHello("")
    
    
    print("greeting1 = " + greeting1);
    
    // Call sayHello function, pass "Swift" as parameter.
    var greeting2 = sayHello("Swift")
    
    print("greeting2 = " + greeting2);
    
}
Edit the source file  main.swift to test the function that  you just created.
main.swift
import Foundation


// Call test_sayHello() function.

test_sayHello()
Running the example.

3- Rules call functions and methods

Rules call a function for Swift >= 2.1
  • First parameters to methods and functions should not have required argument labels.
  • Other parameters to methods and functions should have required argument labels.
  • All parameters to Constructors should have required argument labels.
RulesCallFunc.swift
import Foundation

// name1, name2 are Labels
func sayHello(name1: String, name2: String)   {
    
    print("Hello " + name1);
    print("Hello " + name2);
    
}



func test_sayHello()    {
    
    
    let tom = "Tom"
    let jerry = "Jerry"
    
    // Call function (Or method)
    // First parameter should not have required argument labels
    // Others parameter should have required argument labels.
    
    sayHello(tom, name2: jerry)

    
}
 

4- The function returns a value

The return statement stops the execution of a function and returns a value from that function.
FunctionExample1.swift
import Foundation


// Define a function to add 3 number type of Int, return Int.

func sum(a: Int, b: Int, c: Int) -> Int  {
    
    // Declare a variable type of Int
    var ret:Int = a + b + c
    
    return ret
    
}


// Define a function to find the largest number in 3 numbers.

func max(a: Int, b: Int, c: Int) -> Int {
    
    // Declare varriable m, assign value = a.
    var m = a
    
    // Check if m < b then assgin m = b.
    if m < b  {
        m = b
    }
    
    // If m > c then return m.
    if m >  c  {
        
        // End function.
        return m;
    }
    
    return c
    
}

5- Function returns multiple values

In Swift, a function can  returns multiple values, essentially, this function returns a tuple. Tuple is an abundance value is set in a pair of quotes (), and separated by commas . Each value has a name that can access it.
ReturningTupleExample.swift
import Foundation


// Defina a function named getMinMax
// Input is an array of Int.
// The function returns two values: the largest number in the array,
// and the smallest number in the array.

func getMinMax(arr: [Int]) -> (min: Int, max: Int)   {
    
    // If array has no elements, return (0,0).
    if arr.count == 0  {
        // End function
        return (0, 0)
    }
    
    // Declare 2 variables mi, ma.
    // assigned by the first element of the array.
    var mi: Int = arr[0]
    var ma: Int = arr[0]
 
    
    for a in arr  {
        
        if mi > a  {
            mi = a
        }
        if  ma  <  a  {
            ma = a
        }
    }
    
    return (mi, ma)
}

// Test getMinMax function.
func test_returningTupleExample()    {
    
    // An array of Int.
    var years: [Int] = [2000, 1989, 1975, 2005, 2016]
    
    // Call getMinMax function.
    
    var y = getMinMax(years)
    
    print("Max = \(y.max)")
    
    print("Min = \(y.min)")
    
    
}
Edit main.swift to test example.
main.swift
import Foundation

test_returningTupleExample()
Running the example.

6- Function with Variadic parameter

Swift uses  "variableName: DataType..." to mark a parameter is Variadic. See the example below which is about a function with  Variadic parameter and using this function.
VariadicParamsFunction.swift
import Foundation


// A function with Variadic parameter: nums
// Parameter nums: Like an array of Int.
func sum(nums: Int...) -> Int {
    
    var retNumber : Int = 0
    
 
    for n in nums {
        retNumber = retNumber + n
    }
    
    return retNumber
    
}

// Using function, with Variadic parameter:

func test_sum()    {
    
    // Call sum function, pass 3 parameters.
    var sum1 = sum(1, 2, 3)
    
    print("sum(1, 2, 3) = \(sum1)")
    
    // Call sum function, pass 4 parameters.
    var sum2 = sum(1,4, 5, 7)
    
    print("sum(1,4, 5, 7) = \(sum2)")
    
}
Running the example.

7- Function inside a function

Swift allows you to write a function within another function, this function is used internally of father function.
NestedFunction.swift
import Foundation

// Function return tax amount, base on country & salary.
func getTaxAmount(countryCode:String, salaryAmount:Int) -> Int  {
    
    // A function to caculate tax amount in United States.
    func getUsaTaxAmount( salaryAmount: Int)  -> Int {
        // 15%
        return 15*salaryAmount/100
    }
    
    // A function to caculate tax amount in Vietnam.
    func getVietnamTaxAmount( salaryAmount:Int)  -> Int {
        // 10%
        return 10 * salaryAmount/100
    }
    
    if countryCode == "US" {
        
        return getUsaTaxAmount(salaryAmount)
        
    } else if countryCode == "VN" {
        
        return getVietnamTaxAmount(salaryAmount)
        
    }
    
    // Other countries (5%)
    return  5 * salaryAmount / 100
    
}

8- Function Type

In Swift, each function has a type of function, see the following function:

func sum(a: Int,  b: Int)  -> Int   {
    return a + b;
}
Function type is:
(Int,Int) -> (Int)

9- The function returns a function

ReturningFunction.swift
import Foundation

// A function to caculate tax amount in United States.
// Function type is: (Int) -> (Int)
func getUsaTaxAmount(salary: Int)  -> Int {
    
    return 15 * salary / 100
}

// A function to caculate tax amount in Vietnam.
// Function type is: (Int) -> (Int)
func getVietnamTaxAmount(salary: Int) -> Int  {
    
    return 10 * salary / 100
}

// A default function to caculate tax amount.
// Function type is: (Int) -> (Int)
func getDefaultTaxAmount(salary: Int) -> Int {
    
    return 5 * salary / 100
}

// Function return a function.
func getTaxFunction(countryCode: String) ->  ( Int -> Int )  {
    
    if countryCode == "US"  {
        return getUsaTaxAmount
        
    } else if countryCode == "VN"  {
        
        return getVietnamTaxAmount
    }
    return getDefaultTaxAmount
    
}


// Test
func  test_returningFunction()  {
    
    var countryCode = "US"
    print("Country Code = " + countryCode)
    
    // Get function.
    var taxFunction =  getTaxFunction(countryCode)
    
    
    var salary = 10000
    
    print("Salary = \(salary)")
    
    var taxAmount = taxFunction(salary)
    
    print("Tax Amount = \(taxAmount)")
}
Edit main.swift to test example.
main.swift
import Foundation
 

test_returningFunction()
Running the example.