What does $0 and $1 mean in Swift Closures?

for each $0 swift
swift trailing closure
swift capture list
swift
swift closure reference type
swift callback
swift map
swift escaping
let sortedNumbers = numbers.sort { $0 > $1 }
print(sortedNumbers)

Can anyone explain, what $0 and $1 means in swift?

More Sample

array.forEach {
    actions.append($0)
}

$0 is the first parameter passed into the closure. $1 is the second parameter, etc. That closure you showed is shorthand for:

let sortedNumbers = numbers.sort { (firstObject, secondObject) in 
    return firstObject > secondObject
}

What does $0 mean in Swift?, Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other Nested functions, as introduced in Nested Functions, are a convenient means of Here, $0 and $1 refer to the closure's first and second String arguments. $0 is the first parameter passed into the closure. $1 is the second parameter, etc. That closure you showed is shorthand for: let sortedNumbers = numbers.sort { (firstObject, secondObject) in return firstObject > secondObject }

It represents shorthanded arguments sent into a closure, this example breaks it down:

Swift 4:

var add = { (arg1: Int, arg2: Int) -> Int in
    return arg1 + arg2
}
add = { (arg1, arg2) -> Int in
    return arg1 + arg2
}
add = { arg1, arg2 in
    arg1 + arg2
}
add = {
    $0 + $1
}

let result = add(20, 20) // 40

Closures, let sortedNumbers = numbers.sort { $0 > $1 } print(sortedNumbers) Can anyone explain, what $0 and $1 means in swift? More Sample array.forEach  These are placeholders for values passed into a closure. They are labeled by position instead of being named. $0 is the first argument to the closure, $1 the second, $2 the third, and so on. In this case sort takes two arguments. $0 is the current element of the spend array and $1 is the next. You would call sort like this.

Updated: April 21, 2020.

Swift 5.2

In your example $0 and $1 are Closure’s first and second String's arguments in a Shorthand Argument Names. The shorthand argument names are automatically provided by Swift. The first argument can be referenced by $0, the second argument can be referenced by $1, the third one by $2, and so on.

As you know, a Closure that's very close to a Lambda Function, or a Small Anonymous Function, is a self-contained block of functionality that can be passed around and used in your code. Closure has different names in other programming languages as well as slight differences in meaning – it's Lambda in Python and Kotlin or Block in C and Objective-C.

Pay attention to a great feature added in Swift 5.2 – Key Path Expressions (see below).

Let's see how this Closure can be shortened to the form called Shorthand Argument Names:

FIRST EXAMPLE :

let coffee: [String] = ["Cappuccino", "Espresso", "Latte", "Ristretto"]
1. Normal Function
func backward(_ n1: String, _ n2: String) -> Bool {
    return n1 > n2
}
var reverseOrder = coffee.sorted(by: backward)


/* RESULT: ["Ristretto", "Latte", "Espresso", "Cappuccino"] */
2. Inline Closure Expression
reverseOrder = coffee.sorted(by: { (n1: String, n2: String) -> Bool in return n1 > n2 } )
3. Inferring Type From Context
reverseOrder = coffee.sorted(by: { n1, n2 in return n1 > n2 } )
4. Implicit Returns from Single-Expression Closures
reverseOrder = coffee.sorted(by: { n1, n2 in n1 > n2 } )
5. Shorthand Argument Names
reverseOrder = coffee.sorted(by: { $0 > $1 } )

/* $0 and $1 are closure’s first and second String arguments. */
6. Operator Methods
reverseOrder = coffee.sorted(by: >)

/* RESULT: ["Ristretto", "Latte", "Espresso", "Cappuccino"] */

SECOND EXAMPLE :

let companies = ["bmw", "kfc", "ibm", "htc"]
Without a Shorthand Argument Name:
let uppercasedCompanies = companies.map { (item) -> String in item.uppercased() }

/* RESULT: ["BMW", "KFC", "IBM", "HTC"] */
With a Shorthand Argument Name:
let uppercasedCompanies = companies.map { $0.uppercased() }

/* RESULT: ["BMW", "KFC", "IBM", "HTC"] */

THIRD EXAMPLE :

Remainder Operator and Shorthand Argument Name in Higher Order Function filter:
let numbers: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

let filteredNumbers = numbers.filter { ($0 % 2) == 0 }

print(filteredNumbers)

/* RESULT: [2, 4, 6, 8, 10] */

FOURTH EXAMPLE :

Repeating $0 Shorthand Argument Name for Calculations:
let cubedNumber = { $0 * $0 * $0 } (25)

print(cubedNumber)

/* RESULT:  25^3 = 15625 */

FIFTH EXAMPLE :

Three Shorthand Argument Names – $0, $1, $2:
let math: (Int8, Int8, Int8) -> Int8 = { $0 + $1 - $2 }

func feedClosure() -> (Int8, Int8, Int8) -> Int8 {
    return math
}
feedClosure()(10, 20, 100)

/* RESULT:  (10 + 20 - 100) = -70 */

SIXTH EXAMPLE :

Five Shorthand Argument Names – $0, $1, $2, $3, $4:
let factorial = { $0 * $1 * $2 * $3 * $4 } (1, 2, 3, 4, 5)

print(factorial)

/* RESULT:  5! = 120 */

SEVENTH EXAMPLE :

Key Path Expression – array.HOF(\.property):

In Swift 5.2 you can access parameters of every instance via key path expression:

struct Lighter {
    let manufacturer: String
    let refillable: Bool
}

let zippo = Lighter(manufacturer: "Zippo", refillable: true)
let cricket = Lighter(manufacturer: "Cricket", refillable: false)

let lighters: [Lighter] = [zippo, cricket]

let refillableOnes = lighters.map(\.refillable)

print(refillableOnes)

/* RESULT:  [true, false] */

Of course, you can alternatively use a familiar syntax:

Regular syntax – $0.property:
let refillableOnes = lighters.map { $0.refillable }

print(refillableOnes)

/* RESULT:  [true, false] */

EIGHTH EXAMPLE :

Shorthand Argument Name with a subscript:
let arrays: [[String]] = [["Hello", "Hola"], ["world", "mundo"]]

let helloWorld = arrays.compactMap { $0[0] }

print(helloWorld)

/* RESULT:  ["Hello", "world"] */

NINETH EXAMPLESwift vs Kotlin vs Python :

Also, let's see how Kotlin's lambda is similar to Swift's closure:
Swift
let element: [String] = ["Argentum","Aurum","Platinum"]

let characterCount = element.map { $0.count }

print(characterCount)

/* RESULT:  [8, 5, 8] */ 
Kotlin

Often Kotlin's lambda expression has only one parameter with implicit name: it.

val element = listOf("Argentum","Aurum","Platinum")

val characterCount = element.map { it.length }

println(characterCount)

/* RESULT:  [8, 5, 8] */ 

But in Python there's no equivalent of Shorthand Argument Name.

Python
element = ["Argentum","Aurum","Platinum"]

characterCount = list(map(lambda x: len(x), element))

print(characterCount)

/* RESULT:  [8, 5, 8] */

What does $0 and $1 mean in Swift Closures?, You can refer to the parameters as $0 , $1 , $2 and so on. This means that when you assign a closure to more than one variable they will refer to the same  The in keyword can also be omitted, because the closure expression is made up entirely of its body: reversedNames = names.sorted(by: { $0 > $1 } ) Here, $0 and $1 refer to the closure’s first and second String arguments. 4.7k views · View 4 Upvoters. View more.

The refer to the first and second arguments of sort. Here, sort compares 2 elements and order them. You can look up Swift official documentation for more info:

Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the values of the closure’s arguments by the names $0, $1, $2, and so on.

Learn swift: What does $0 and $1 mean inside a closure?, Swift - Closures - Closures in Swift 4 are similar to that of self-contained Inline closures as shorthand argument names by representing $0, $1, $2 --- $n. Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the values of the closure’s arguments by the names $0, $1, $2, and so on.

In Addition with @Bobby's Answer I would like to Add an Example

var add: (Int,Int,Int)->Int
add = {
//So here the $0 is first argument $1 is second argument $3 is third argument
    return $0 + $1 + $2
//The above statement can also be written as $0 + $1 + $2 i.e is return is optional
}

let result = add(20, 30, 40) 
print(result) // Prints 90

Chapter 9: Closures, Closure arguments can be references by position ($0, $1, …) Functions and closures are first-class citizens in Swift because you can treat then Marking a closure with @escaping means you have to refer to self explicitly  However in Swift you can omit explicit names for the parameters of a closure. In this inside the closure you can refer that params using $0 for the first param, $1 for the second one and so on. So the previous snippet of code can be also be written like below. nums.forEach { print($0) }

Swift - Closures, The concept of Swift Closure is similar to blocks in C. Closures are nameless When we mention closures we generally mean the third form i.e. Closure to closure using shorthand argument names : $0, $1, $2 etc. for the first, second, third  Higher order functions in Swift: Filter, Map, Reduce, flatmap, compactMap Ever since I learned about functions and closures in detail, I wanted to know more about the advantages and usages of

Functional swift: All about Closures - Abhimuralidharan, When would you use closures with return values as parameters to a function? This syntax automatically numbers parameter names as $0 , $1 , $2 , and so $0 Give them actual names and their meaning becomes clearer. Swift Programming from Scratch The Swift Sandbox is integrated, making the exercises interactive. Read more about the book here. First Steps Conditionals Types Loops Strings Arrays Functions Recursion Closures Tuples & Enums Dictionaries Chapter 9: Closures Closures are self contained chunks of code that can be passed around and used in your code.

Swift Closure, U.S. energy trading experts said Monday's first-ever oil futures crash below $0 does not mean that Americans will suddenly be paid to fill up their cars' gas tanks.

Comments
  • Can i write my own logic in a closure using first parameter and second?
  • Yep, that's the entire purpose of the closure.
  • thanks mate, your answer helped me understand the better part of closures
  • Since the addition operator has the same function signature as that closure, you can break it down even further to just: add = (+)
  • It should specify the type with var add:((Int, Int) -> Int) = ... explicitly for the case 3 and 4, in my swift 5 test.
  • Andy-to-Andy - thanks! this is very helpful!
  • this cannot count as an answer. If you want to add an example, simply edit @bobby's answer