Swift Language

Arrays

Introduction#

Array is an ordered, random-access collection type.

Arrays are one of the most commonly used data types in an app. We use the Array type to hold elements of a single type, the array’s Element type. An array can store any kind of elements---from integers to strings to classes.

Syntax#

  • Array<Element> // The type of an array with elements of type Element
  • [Element] // Syntactic sugar for the type of an array with elements of type Element
  • [element0, element1, element2, … elementN] // An array literal
  • [Element]() // Creates a new empty array of type [Element]
  • Array(count:repeatedValue:) // Creates an array of count elements, each initialized to repeatedValue
  • Array(_:) // Creates an array from an arbitrary sequence

Remarks#

Arrays are an ordered collection of values. Values may repeat but must be of the same type.

Value Semantics

Copying an array will copy all of the items inside the original array.

Changing the new array will not change the original array.

var originalArray = ["Swift", "is", "great!"]
var newArray = originalArray
newArray[2] = "awesome!"
//originalArray = ["Swift", "is", "great!"]
//newArray = ["Swift", "is", "awesome!"]

Copied arrays will share the same space in memory as the original until they are changed. As a result of this there is a performance hit when the copied array is given its own space in memory as it is changed for the first time.

Basics of Arrays

Array is an ordered collection type in the Swift standard library. It provides O(1) random access and dynamic reallocation. Array is a generic type, so the type of values it contains are known at compile time.

As Array is a value type, its mutability is defined by whether it is annotated as a var (mutable) or let (immutable).

The type [Int] (meaning: an array containing Ints) is syntactic sugar for Array<T>.

Read more about arrays in The Swift Programming Language.

Empty arrays

The following three declarations are equivalent:

// A mutable array of Strings, initially empty.

var arrayOfStrings: [String] = []      // type annotation + array literal
var arrayOfStrings = [String]()        // invoking the [String] initializer
var arrayOfStrings = Array<String>()   // without syntactic sugar

Array literals

An array literal is written with square brackets surrounding comma-separated elements:

// Create an immutable array of type [Int] containing 2, 4, and 7
let arrayOfInts = [2, 4, 7]

The compiler can usually infer the type of an array based on the elements in the literal, but explicit type annotations can override the default:

let arrayOfUInt8s: [UInt8] = [2, 4, 7]   // type annotation on the variable
let arrayOfUInt8s = [2, 4, 7] as [UInt8] // type annotation on the initializer expression
let arrayOfUInt8s = [2 as UInt8, 4, 7]   // explicit for one element, inferred for the others

Arrays with repeated values

// An immutable array of type [String], containing ["Example", "Example", "Example"]
let arrayOfStrings = Array(repeating: "Example",count: 3)

Creating arrays from other sequences

let dictionary = ["foo" : 4, "bar" : 6]

// An immutable array of type [(String, Int)], containing [("bar", 6), ("foo", 4)]
let arrayOfKeyValuePairs = Array(dictionary)

Multi-dimensional arrays

In Swift, a multidimensional array is created by nesting arrays: a 2-dimensional array of Int is [[Int]] (or Array<Array<Int>>).

let array2x3 = [
    [1, 2, 3],
    [4, 5, 6]
]
// array2x3[0][1] is 2, and array2x3[1][2] is 6.

To create a multidimensional array of repeated values, use nested calls of the array initializer:

var array3x4x5 = Array(repeating: Array(repeating: Array(repeating: 0,count: 5),count: 4),count: 3)

Accessing Array Values

The following examples will use this array to demonstrate accessing values

var exampleArray:[Int] = [1,2,3,4,5]
//exampleArray = [1, 2, 3, 4, 5]

To access a value at a known index use the following syntax:

let exampleOne = exampleArray[2]
//exampleOne = 3

Note: The value at index two is the third value in the Array. Arrays use a zero based index which means the first element in the Array is at index 0.

let value0 = exampleArray[0]
let value1 = exampleArray[1]
let value2 = exampleArray[2]
let value3 = exampleArray[3]
let value4 = exampleArray[4]
//value0 = 1
//value1 = 2 
//value2 = 3
//value3 = 4
//value4 = 5

Access a subset of an Array using filter:

var filteredArray = exampleArray.filter({ $0 < 4 })
//filteredArray = [1, 2, 3]

Filters can have complex conditions like filtering only even numbers:

var evenArray = exampleArray.filter({ $0 % 2 == 0 })
//evenArray = [2, 4]

It is also possible to return the index of a given value, returning nil if the value wasn’t found.

exampleArray.indexOf(3) // Optional(2)

There are methods for the first, last, maximum or minimum value in an Array. These methods will return nil if the Array is empty.

exampleArray.first // Optional(1)
exampleArray.last // Optional(5)
exampleArray.maxElement() // Optional(5)
exampleArray.minElement() // Optional(1)

Useful Methods

Determine whether an array is empty or return its size

var exampleArray = [1,2,3,4,5]
exampleArray.isEmpty //false
exampleArray.count //5

Reverse an Array Note: The result is not performed on the array the method is called on and must be put into its own variable.

exampleArray = exampleArray.reverse()
//exampleArray = [9, 8, 7, 6, 5, 3, 2]

Modifying values in an array

There are multiple ways to append values onto an array

var exampleArray = [1,2,3,4,5]
exampleArray.append(6)
//exampleArray = [1, 2, 3, 4, 5, 6]
var sixOnwards = [7,8,9,10]
exampleArray += sixOnwards
//exampleArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

and remove values from an array

exampleArray.removeAtIndex(3)
//exampleArray = [1, 2, 3, 5, 6, 7, 8, 9, 10]
exampleArray.removeLast()
//exampleArray = [1, 2, 3, 5, 6, 7, 8, 9]
exampleArray.removeFirst()
//exampleArray = [2, 3, 5, 6, 7, 8, 9]

Sorting an Array

var array = [3, 2, 1]

Creating a new sorted array

As Array conforms to SequenceType, we can generate a new array of the sorted elements using a built in sort method.

In Swift 2, this is done with the sort() method.

let sorted = array.sort()  // [1, 2, 3]

As of Swift 3, it has been re-named to sorted().

let sorted = array.sorted()  // [1, 2, 3]

Sorting an existing array in place

As Array conforms to MutableCollectionType, we can sort its elements in place.

In Swift 2, this is done using the sortInPlace() method.

array.sortInPlace() // [1, 2, 3]

As of Swift 3, it has been renamed to sort().

array.sort() // [1, 2, 3]

Note: In order to use the above methods, the elements must conform to the Comparable protocol.

Sorting an array with a custom ordering

You may also sort an array using a closure to define whether one element should be ordered before another – which isn’t restricted to arrays where the elements must be Comparable. For example, it doesn’t make sense for a Landmark to be Comparable – but you can still sort an array of landmarks by height or name.

struct Landmark {
    let name : String
    let metersTall : Int
}

var landmarks = [Landmark(name: "Empire State Building", metersTall: 443),
                 Landmark(name: "Eifell Tower", metersTall: 300),
                 Landmark(name: "The Shard", metersTall: 310)]
// sort landmarks by height (ascending)
landmarks.sortInPlace {$0.metersTall < $1.metersTall}

print(landmarks) // [Landmark(name: "Eifell Tower", metersTall: 300), Landmark(name: "The Shard", metersTall: 310), Landmark(name: "Empire State Building", metersTall: 443)]

// create new array of landmarks sorted by name
let alphabeticalLandmarks = landmarks.sort {$0.name < $1.name}

print(alphabeticalLandmarks) // [Landmark(name: "Eifell Tower", metersTall: 300), Landmark(name: "Empire State Building", metersTall: 443), Landmark(name: "The Shard", metersTall: 310)]
// sort landmarks by height (ascending)
landmarks.sort {$0.metersTall < $1.metersTall}

// create new array of landmarks sorted by name
let alphabeticalLandmarks = landmarks.sorted {$0.name < $1.name}

Note: String comparison can yield unexpected results if the strings are inconsistent, see Sorting an Array of Strings.

Transforming the elements of an Array with map(_:)

As Array conforms to SequenceType, we can use map(_:) to transform an array of A into an array of B using a closure of type (A) throws -> B.

For example, we could use it to transform an array of Ints into an array of Strings like so:

let numbers = [1, 2, 3, 4, 5]
let words = numbers.map { String($0) }
print(words) // ["1", "2", "3", "4", "5"]

map(_:) will iterate through the array, applying the given closure to each element. The result of that closure will be used to populate a new array with the transformed elements.

Since String has an initialiser that receives an Int we can also use this clearer syntax:

let words = numbers.map(String.init)

A map(_:) transform need not change the type of the array – for example, it could also be used to multiply an array of Ints by two:

let numbers = [1, 2, 3, 4, 5]
let numbersTimes2 = numbers.map {$0 * 2}
print(numbersTimes2) // [2, 4, 6, 8, 10]

Extracting values of a given type from an Array with flatMap(_:)

The things Array contains values of Any type.

let things: [Any] = [1, "Hello", 2, true, false, "World", 3]

We can extract values of a given type and create a new Array of that specific type. Let’s say we want to extract all the Int(s) and put them into an Int Array in a safe way.

let numbers = things.flatMap { $0 as? Int }

Now numbers is defined as [Int]. The flatMap function discard all nil elements and the result thus contains only the following values:

[1, 2, 3]

Filtering an Array

You can use the filter(_:) method on SequenceType in order to create a new array containing the elements of the sequence that satisfy a given predicate, which can be provided as a closure.

For example, filtering even numbers from an [Int]:

let numbers = [22, 41, 23, 30]

let evenNumbers = numbers.filter { $0 % 2 == 0 }

print(evenNumbers)  // [22, 30]

Filtering a [Person], where their age is less than 30:

struct Person {
    var age : Int
}

let people = [Person(age: 22), Person(age: 41), Person(age: 23), Person(age: 30)]

let peopleYoungerThan30 = people.filter { $0.age < 30 }

print(peopleYoungerThan30) // [Person(age: 22), Person(age: 23)]

Filtering out nil from an Array transformation with flatMap(_:)

You can use flatMap(_:) in a similar manner to map(_:) in order to create an array by applying a transform to a sequence’s elements.

extension SequenceType {
    public func flatMap<T>(@noescape transform: (Self.Generator.Element) throws -> T?) rethrows -> [T]
}

The difference with this version of flatMap(_:) is that it expects the transform closure to return an Optional value T? for each of the elements. It will then safely unwrap each of these optional values, filtering out nil – resulting in an array of [T].

For example, you can this in order to transform a [String] into a [Int] using Int’s failable String initializer, filtering out any elements that cannot be converted:

let strings = ["1", "foo", "3", "4", "bar", "6"]

let numbersThatCanBeConverted = strings.flatMap { Int($0) }

print(numbersThatCanBeConverted) // [1, 3, 4, 6]

You can also use flatMap(_:)’s ability to filter out nil in order to simply convert an array of optionals into an array of non-optionals:

let optionalNumbers : [Int?] = [nil, 1, nil, 2, nil, 3]

let numbers = optionalNumbers.flatMap { $0 }

print(numbers) // [1, 2, 3]

Subscripting an Array with a Range

One can extract a series of consecutive elements from an Array using a Range.

let words = ["Hey", "Hello", "Bonjour", "Welcome", "Hi", "Hola"]
let range = 2...4
let slice = words[range] // ["Bonjour", "Welcome", "Hi"]

Subscripting an Array with a Range returns an ArraySlice. It’s a subsequence of the Array.

In our example, we have an Array of Strings, so we get back ArraySlice<String>.

Although an ArraySlice conforms to CollectionType and can be used with sort, filter, etc, its purpose is not for long-term storage but for transient computations: it should be converted back into an Array as soon as you’ve finished working with it.

For this, use the Array() initializer:

let result = Array(slice)

To sum up in a simple example without intermediary steps:

let words = ["Hey", "Hello", "Bonjour", "Welcome", "Hi", "Hola"]
let selectedWords = Array(words[2...4]) // ["Bonjour", "Welcome", "Hi"]

Grouping Array values

If we have a struct like this

struct Box {
    let name: String
    let thingsInside: Int
}

and an array of Box(es)

let boxes = [
    Box(name: "Box 0", thingsInside: 1),
    Box(name: "Box 1", thingsInside: 2),
    Box(name: "Box 2", thingsInside: 3),
    Box(name: "Box 3", thingsInside: 1),
    Box(name: "Box 4", thingsInside: 2),
    Box(name: "Box 5", thingsInside: 3),
    Box(name: "Box 6", thingsInside: 1)
]

we can group the boxes by the thingsInside property in order to get a Dictionary where the key is the number of things and the value is an array of boxes.

let grouped = boxes.reduce([Int:[Box]]()) { (res, box) -> [Int:[Box]] in
    var res = res
    res[box.thingsInside] = (res[box.thingsInside] ?? []) + [box]
    return res
}

Now grouped is a [Int:[Box]] and has the following content

[
    2: [Box(name: "Box 1", thingsInside: 2), Box(name: "Box 4", thingsInside: 2)], 
    3: [Box(name: "Box 2", thingsInside: 3), Box(name: "Box 5", thingsInside: 3)],
    1: [Box(name: "Box 0", thingsInside: 1), Box(name: "Box 3", thingsInside: 1), Box(name: "Box 6", thingsInside: 1)]
]

Flattening the result of an Array transformation with flatMap(_:)

As well as being able to create an array by filtering out nil from the transformed elements of a sequence, there is also a version of flatMap(_:) that expects the transformation closure to return a sequence S.

extension SequenceType {
    public func flatMap<S : SequenceType>(transform: (Self.Generator.Element) throws -> S) rethrows -> [S.Generator.Element]
}

Each sequence from the transformation will be concatenated, resulting in an array containing the combined elements of each sequence – [S.Generator.Element].

Combining the characters in an array of strings

For example, we can use it to take an array of prime strings and combine their characters into a single array:

let primes = ["2", "3", "5", "7", "11", "13", "17", "19"]
let allCharacters = primes.flatMap { $0.characters }
// => "["2", "3", "5", "7", "1", "1", "1", "3", "1", "7", "1", "9"]"

Breaking the above example down:

  1. primes is a [String] (As an array is a sequence, we can call flatMap(_:) on it).
  2. The transformation closure takes in one of the elements of primes, a String (Array<String>.Generator.Element).
  3. The closure then returns a sequence of type String.CharacterView.
  4. The result is then an array containing the combined elements of all the sequences from each of the transformation closure calls – [String.CharacterView.Generator.Element].

Flattening a multidimensional array

As flatMap(_:) will concatenate the sequences returned from the transformation closure calls, it can be used to flatten a multidimensional array – such as a 2D array into a 1D array, a 3D array into a 2D array etc.

This can simply be done by returning the given element $0 (a nested array) in the closure:

// A 2D array of type [[Int]]
let array2D = [[1, 3], [4], [6, 8, 10], [11]]

// A 1D array of type [Int]
let flattenedArray = array2D.flatMap { $0 }

print(flattenedArray) // [1, 3, 4, 6, 8, 10, 11]

Sorting an Array of Strings

The most simple way is to use sorted():

let words = ["Hello", "Bonjour", "Salute", "Ahola"]
let sortedWords = words.sorted()
print(sortedWords) // ["Ahola", "Bonjour", "Hello", "Salute"]

or sort()

var mutableWords = ["Hello", "Bonjour", "Salute", "Ahola"]
mutableWords.sort()
print(mutableWords) // ["Ahola", "Bonjour", "Hello", "Salute"]

You can pass a closure as an argument for sorting:

let words = ["Hello", "Bonjour", "Salute", "Ahola"]
let sortedWords = words.sorted(isOrderedBefore: { $0 > $1 })
print(sortedWords) // ["Salute", "Hello", "Bonjour", "Ahola"]

Alternative syntax with a trailing closure:

let words = ["Hello", "Bonjour", "Salute", "Ahola"]
let sortedWords = words.sorted() { $0 > $1 }
print(sortedWords) // ["Salute", "Hello", "Bonjour", "Ahola"]

But there will be unexpected results if the elements in the array are not consistent:

let words = ["Hello", "bonjour", "Salute", "ahola"]
let unexpected = words.sorted()
print(unexpected) // ["Hello", "Salute", "ahola", "bonjour"]

To address this issue, either sort on a lowercase version of the elements:

let words = ["Hello", "bonjour", "Salute", "ahola"]
let sortedWords = words.sorted { $0.lowercased() < $1.lowercased() }
print(sortedWords) // ["ahola", "bonjour", "Hello", "Salute"]

Or import Foundation and use NSString’s comparison methods like caseInsensitiveCompare:

let words = ["Hello", "bonjour", "Salute", "ahola"]
let sortedWords = words.sorted { $0.caseInsensitiveCompare($1) == .orderedAscending }
print(sortedWords) // ["ahola", "bonjour", "Hello", "Salute"]

Alternatively, use localizedCaseInsensitiveCompare, which can manage diacritics.

To properly sort Strings by the numeric value they contain, use compare with the .numeric option:

let files = ["File-42.txt", "File-01.txt", "File-5.txt", "File-007.txt", "File-10.txt"]
let sortedFiles = files.sorted() { $0.compare($1, options: .numeric) == .orderedAscending }
print(sortedFiles) // ["File-01.txt", "File-5.txt", "File-007.txt", "File-10.txt", "File-42.txt"]

Lazily flattening a multidimensional Array with flatten()

We can use flatten() in order to lazily reduce the nesting of a multi-dimensional sequence.

For example, lazy flattening a 2D array into a 1D array:

// A 2D array of type [[Int]]
let array2D = [[1, 3], [4], [6, 8, 10], [11]]

// A FlattenBidirectionalCollection<[[Int]]>
let lazilyFlattenedArray = array2D.flatten()

print(lazilyFlattenedArray.contains(4)) // true

In the above example, flatten() will return a FlattenBidirectionalCollection, which will lazily apply the flattening of the array. Therefore contains(_:) will only require the first two nested arrays of array2D to be flattened – as it will short-circuit upon finding the desired element.

Combining an Array’s elements with reduce(_:combine:)

reduce(_:combine:) can be used in order to combine the elements of a sequence into a single value. It takes an initial value for the result, as well as a closure to apply to each element – which will return the new accumulated value.

For example, we can use it to sum an array of numbers:

let numbers = [2, 5, 7, 8, 10, 4]

let sum = numbers.reduce(0) {accumulator, element in
    return accumulator + element
}

print(sum) // 36

We’re passing 0 into the initial value, as that’s the logical initial value for a summation. If we passed in a value of N, the resulting sum would be N + 36. The closure passed to reduce has two arguments. accumulator is the current accumulated value, which is assigned the value that the closure returns at each iteration. element is the current element in the iteration.

As in this example, we’re passing an (Int, Int) -> Int closure to reduce, which is simply outputting the addition of the two inputs – we can actually pass in the + operator directly, as operators are functions in Swift:

let sum = numbers.reduce(0, combine: +)

Removing element from an array without knowing it’s index

Generally, if we want to remove an element from an array, we need to know it’s index so that we can remove it easily using remove(at:) function.

But what if we don’t know the index but we know the value of element to be removed!

So here is the simple extension to an array which will allow us to remove an element from array easily without knowing it’s index:

Swift3

extension Array where Element: Equatable {

    mutating func remove(_ element: Element) {
        _ = index(of: element).flatMap {
            self.remove(at: $0)
        }
    }
}

e.g.

    var array = ["abc", "lmn", "pqr", "stu", "xyz"]
    array.remove("lmn")
    print("\(array)")    //["abc", "pqr", "stu", "xyz"]
    
    array.remove("nonexistent")
    print("\(array)")    //["abc", "pqr", "stu", "xyz"]
    //if provided element value is not present, then it will do nothing!

Also if, by mistake, we did something like this: array.remove(25) i.e. we provided value with different data type, compiler will throw an error mentioning-
cannot convert value to expected argument type

Finding the minimum or maximum element of an Array

You can use the minElement() and maxElement() methods to find the minimum or maximum element in a given sequence. For example, with an array of numbers:

let numbers = [2, 6, 1, 25, 13, 7, 9]

let minimumNumber = numbers.minElement() // Optional(1)
let maximumNumber = numbers.maxElement() // Optional(25)

As of Swift 3, the methods have been renamed to min() and max() respectively:

let minimumNumber = numbers.min() // Optional(1)
let maximumNumber = numbers.max() // Optional(25)

The returned values from these methods are Optional to reflect the fact that the array could be empty – if it is, nil will be returned.

Note: The above methods require the elements to conform to the Comparable protocol.

Finding the minimum or maximum element with a custom ordering

You may also use the above methods with a custom closure, defining whether one element should be ordered before another, allowing you to find the minimum or maximum element in an array where the elements aren’t necessarily Comparable.

For example, with an array of vectors:

struct Vector2 {
    let dx : Double
    let dy : Double
    
    var magnitude : Double {return sqrt(dx*dx+dy*dy)}
}

let vectors = [Vector2(dx: 3, dy: 2), Vector2(dx: 1, dy: 1), Vector2(dx: 2, dy: 2)]
// Vector2(dx: 1.0, dy: 1.0)
let lowestMagnitudeVec2 = vectors.minElement { $0.magnitude < $1.magnitude } 

// Vector2(dx: 3.0, dy: 2.0)
let highestMagnitudeVec2 = vectors.maxElement { $0.magnitude < $1.magnitude } 
let lowestMagnitudeVec2 = vectors.min { $0.magnitude < $1.magnitude }
let highestMagnitudeVec2 = vectors.max { $0.magnitude < $1.magnitude }

Accessing indices safely

By adding the following extension to array indices can be accessed without knowing if the index is inside bounds.

extension Array {
    subscript (safe index: Int) -> Element? {
        return indices ~= index ? self[index] : nil
    }
}

example:

if let thirdValue = array[safe: 2] {
    print(thirdValue)
}

Comparing 2 Arrays with zip

The zip function accepts 2 parameters of type SequenceType and returns a Zip2Sequence where each element contains a value from the first sequence and one from the second sequence.

Example

let nums = [1, 2, 3]
let animals = ["Dog", "Cat", "Tiger"]
let numsAndAnimals = zip(nums, animals)

nomsAndAnimals now contains the following values

sequence1 sequence1
1 "Dog"
2 "Cat"
3 "Tiger"

This is useful when you want to perform some kind of comparation between the n-th element of each Array.

Example

Given 2 Arrays of Int(s)

let list0 = [0, 2, 4]
let list1 = [0, 4, 8]

you want to check whether each value into list1 is the double of the related value in list0.

let list1HasDoubleOfList0 = !zip(list0, list1).filter { $0 != (2 * $1)}.isEmpty
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