Swift - Function Closures

Introduction

Swift closures are small, anonymous chunks of code that you can use like functions.

Closures can be passed to other functions to tell them how they should do a certain task.

The build-in sort function takes a closure, and uses that closure to determine how to compare two individual elements:

Demo

let my = [12, 5, 98, 2, 13] 
var a = my.sorted()
print(a)//from   www. j a  v  a2s  . c o  m
print(my)

Result

To sort an array so that small numbers go before large numbers:

Demo

let numbers = [12,1,56,312,120,13] 
var numbersSorted = numbers.sorted(by: { 
    (n1: Int, n2: Int) -> Bool in return n2 > n1 
}) //  ww  w .  ja  va 2 s .co  m
print(numbersSorted)

Result

The in keyword breaks up the closure from its definition and its implementation.

In the code above the definition was (n1: Int, n2: Int)->Bool, and the implementation of that closure came after the in keyword: return n2 > n1.

A closure, like a function, takes parameters.

In the preceding example, the closure specifies the name and type of the parameters that it works with.

The compiler can infer the type of the parameters for you, much like it can with variables.

You can re-write the code as follows:

Demo

let numbers = [12,1,56,32,120,13] 
let numbersSortedReverse = numbers.sorted(by: {n1, n2 in return n1 > n2}) 
print(numbersSortedReverse)

Result

If you omit the parameter names, you can refer to each parameter by number.

The first parameter is called $0, the second is called $1, etc.

If your closure only contains a single line of code, you can omit the return keyword:

Demo

let numbers = [12,1,56,32,120,13] 
var numbersSortedAgain = numbers.sorted(by: { $1 > $0 })
print(numbersSortedAgain)

Result

If a closure is the last parameter in a function call, you can put it outside the parentheses.

Demo

let numbers = [12,1,56,32,120,13] 
var numbersSortedReversedAgain = numbers.sorted { $0 > $1 } 
print(numbersSortedReversedAgain)

Result

Closures can be stored in variables. In that case, you can call them just like a function:

Demo

var comparator = {(a: Int, b:Int) in a < b} 
var a = comparator(1,2) // true 
print(a)

Result

Related Topics

• Using Closures
• Assigning Closures to Variables
• Writing Closures Inline
• Using Closures In Your Functions
• Type Inference in closure
• Closure Shorthand Argument Names
• Operator Function in closure
• Trailing Closures

Example

• Array map(): transform the elements inside an array into another array
• Map array element to new value with closure
• Array filter(): filter the elements inside an array and return a subset of the elements
• Extract values from an array
• Array reduce(): return the elements inside an array as a single item
• Extract all the prices from the array and create a single string listing all of them

Exercise

• Write program to convert number to English word
• Write program to extract only the odd numbers from the array
• Write program to to find out the largest number from the array
• Write program to to find out the average of all the numbers from the array

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