Swift Array Examples, String ArraysUse int and string arrays. Append to arrays, loop over elements and modify arrays.
Array. Swift programs often use arrays to store elements. Arrays contain elements of one type. They expand automatically when new elements are added.
Array details. We can loop over elements in an array with for, and access elements with a subscript. Most Swift programs that are not trivial use arrays.
2D Array
First example. We create an empty String array. We then append() 3 string literals to the array. After the appends, the count of the array is now 3.
Var We use the var keyword to declare the array. This is a variable array, which means it can be changed.
Print This method prints the array count and the array's contents to the output. An array can be displayed in full this way.
Swift program that uses string array
// Create an empty array and add three elements to it. var languages = [String]() languages.append("Swift") languages.append("Python") languages.append("Java") // Display count of the array. print(languages.count) // Print the array contents. print(languages)
3 ["Swift", "Python", "Java"]
For-in loop. An array can be looped over with the for-in loop. This provides a clear syntax for getting each element in the array. Here we use a for-in loop on an Int array.
Tip A for-in loop is clearer when we do not need to access the indexes of an array.
Swift program that uses Int array, for-in loop
// Create an Int array. var ids = [Int]() ids.append(10) ids.append(20) ids.append(50) // Loop over and print all Ints. for id in ids { print(id) }
10 20 50
Initializer syntax. A shorter syntax form can be used to initialize an array. This requires only one line to declare (and place values in) the array.
Initialize Arrays
Swift program that uses array initializer
// Create variable Int array with three values. var sizes: [Int] = [5, 10, 15] // Append another Int. sizes.append(20) print("Sizes: \(sizes)")
Sizes: [5, 10, 15, 20]
Enumerated. Sometimes we want to get indexes and values in a single loop. The enumerated method is useful for this. Here we use enumerated() on a string array.
For Swift 3 does not support the C-style for loop syntax. We can use enumerated() to get indexes and values in an elegant way.
Tip With the index, we can access other elements in the array inside the loop body. Multiple elements can be tested in the logic.
Swift program that uses enumerated on string array
var birds = ["finch", "sparrow", "eagle"] // Use enumerated() on the string array. for (index, value) in birds.enumerated() { // Display indexes and values. print("\(index) = \(value)") }
0 = finch 1 = sparrow 2 = eagle
Let error. With "let" we declare a constant. When we create an array with let, we cannot call append (or other mutating methods) on the array.
Var This example would compile correctly if we were to specify "var" instead of let.
Tip With constant let arrays, we can enforce code correctness by preventing unexpected changes to the array.
Swift program that causes error with let, append
// With let an array is constant. // ... We cannot append to it. let codes: [String] = ["x", "r", "w"] codes.append("t")
/Users/.../main.swift:4:1: Immutable value of type '[String]' only has mutating members named 'append'
First, last. Often we want to get the first and last elements in an array. In Swift we can access the "first" and "last" properties of the array. These return optionals.
So We must check the result of first and last against nil to ensure they exist. They do not exist in an empty 0-element array.
Swift program that uses first, last optionals
let cars: [String] = ["truck", "coupe", "sedan"] // Access first and last optional elements. // ... Test them against nil before using them. if cars.first != nil { print(cars.first!) } if cars.last != nil { print(cars.last!) }
truck sedan
First, last indexes. Here we use indexes to get the first and last elements in an array. The first element is at index 0. And the last one is at the count minus one.
Swift program that uses first, last indexes
// Create a String array with three elements. let pets: [String] = ["dog", "cat", "bird"] // Get first element. print(pets[0]) // Get last element. print(pets[pets.count - 1])
dog bird
PopLast. This method removes the last element from an array and returns the element wrapped in an Optional type. We can use "if let" to safely get the popped elements.
Swift program that uses popLast
var names = ["bird", "fish"] // Get last element with popLast. if let last = names.popLast() { print("A", last) } if let last = names.popLast() { print("B", last) } if let last = names.popLast() { // This is not reached, but no error is caused. print("C", last) }
A fish B bird
Empty, isEmpty. An array with zero elements is empty. The isEmpty method will return true if the count is equal to 0. After an element is added, isEmpty will no longer return true.
Swift program that uses isEmpty
// Create an empty birds array. var birds: [String] = [] // The array is empty. if birds.isEmpty { print(true) } // Append an element so that the array is no longer empty. birds.append("parrot") if !birds.isEmpty { print(false) }
true false
Contains. Does an element exist within an array? To find this out, we use a for-in loop with an if-statement. We return true from contains() if a matching value is found in the array.
Tip The contains() func returns early if a match is found. This avoids processing the entire array.
Swift program that uses contains
var animals: [String] = ["bird", "frog", "elephant"] // Determine if this string exists in the array. if animals.contains("bird") { print(true) } // This string does not exist. if !animals.contains("diamond") { print(false) }
true false
Contains, where. The contains func can be used with a "where" func. The func receives an element of the array's element type, and returns a Bool that indicates matching.
Swift program that uses contains, where func
func colorStartsWithG(value: String) -> Bool { // Check for first letter. return value[value.startIndex] == "g" } func colorStartsWithX(value: String) -> Bool { return value[value.startIndex] == "x" } // A simple array. let colors = ["blue", "red", "green"] // Simple contains. if colors.contains("red") { print("CONTAINS RED") } // Use special funcs with contains. if colors.contains(where: colorStartsWithG) { print("CONTAINS COLOR STARTING WITH G") } if !colors.contains(where: colorStartsWithX) { print("DOES NOT CONTAIN COLOR STARTING WITH X") }
Error, out of range. When accessing elements by index, we must be careful to have a valid index. We can test the count before using an index. Here we see the "index out of range" error.
Swift program that causes index error
let plants: [String] = ["tree", "fern", "bush"] print(plants[999])
fatal error: Array index out of range (lldb)
Array repeating, count. An array can be created with repeating and count arguments. Repeating is the default fill value. The count is the total number of elements to create.
Swift program that uses count, repeatedValue
// Create an array with 10,000 elements. // ... Specify repeating and count. var x: [Int] = Array(repeating: 0, count: 10000) // Assign elements in array. x[0] = 1 x[9999] = 2 x[100] = 3 // Write array contents and size. print(x[0]) print(x[9999]) print(x[100]) print(x[1]) // Default is 0. print(x.count)
1 2 3 0 10000
Append arrays. Sometimes we need to append many values at once to an array. We can append new arrays in Swift to append each element individually.
Combine Arrays
Important This does not append sub-arrays, but the individual elements. It is an "append all" syntax form.
Swift program that appends arrays
// Create an Int array. var indexes = [10, 11, 12] // Append three Int elements at once. // ... The elements in the brackets are added individually. indexes += [20, 21, 22] // Display the result array. for element in indexes { print(element) }
10 11 12 20 21 22
Add two arrays. In Swift we can add two arrays together with the plus operator. The resulting array has all the individual elements from both arrays (including duplicates).
Tip With this style, we can use constant arrays with the let-keyword. We are not modifying array1 or array2 when we create a new array.
Swift program that adds two arrays together
// Two constant arrays with 2 elements each. let array1 = ["cat", "bird"] let array2 = ["fish", "plant"] // Combine the two arrays (add them together). let combined = array1 + array2 // Print the combined array count and elements. print(combined.count) print(combined)
4 ["cat", "bird", "fish", "plant"]
Remove at. We can remove elements from a Swift array by specifying an index. These are zero-based indexes, so 0 removes the first element, and 1 removes the second element.
Tip A Dictionary is a data structure that makes removal faster. In a linear array, removal often causes shifting or elements.
Here We remove the elements at index 2 and 0. They are highlighted in the output section before removal.
Swift program that uses remove
var ids = [10, 20, 300, 5000] print(ids) // Remove element at index 2 (the third element, 300). ids.remove(at: 2) print(ids) // Remove element at index 0 (the first element, 10). ids.remove(at: 0) print(ids)
[10, 20, 300, 5000] [10, 20, 5000] [20, 5000]
RemoveLast. In a linear collection, removing the last element is often fast—no elements need to be shifted forward after the removal. Swift provides an optimized removeLast.
Swift program that uses removeLast
var languages = ["Swift", "Java", "C#", "Go"] print(languages) // Remove the last element in the array. languages.removeLast() print(languages)
["Swift", "Java", "C#", "Go"] ["Swift", "Java", "C#"]
Assign range. In Swift we often use ranges to address collections. We can change a range of elements in an array. Here we change the elements at indexes 0 through 2 inclusive.
Tip The before and after parts do not need to have the same length. Elements are added individually, not as a sub-array.
Swift program that assigns array range
var years = [1999, 2000, 2001, 2015, 2017] // Replace elements at indexes 0, 1 and 2. // ... Replace 3 elements with 2 elements. years[0...2] = [2002, 2005] // Display result. print(years)
[2002, 2005, 2015, 2017]
Insert at. An element can be inserted into an array. This will cause the following elements to shift forward one to accommodate the new value. So this operation may be slow.
Argument The first argument to insert() is the element to insert. The second argument, specified with "at," is the target index.
Swift program that uses insert
var letters = ["A", "X", "Z"] print(letters) // Insert the letter C at index 1 (after the first element). letters.insert("C", at: 1) print(letters)
["A", "X", "Z"] ["A", "C", "X", "Z"]
Argument. An array can be passed as an argument to a func. We must specify the type of the array (its element type in square brackets). An array can also be returned.
Here We receive a String array in the validate func. We use a guard to ensure the first string element has a specific value.
Swift program that uses string array argument
func validate(animals: [String]) { // Test first element of string argument. guard animals.first == "ant" else { print("Error") return } print("OK") } let animals = ["ant", "fish", "hamster"] // Pass a string array as an argument. validate(animals: animals)
Copy array. An array that is received as a parameter cannot be modified. It is constant. We first copy the array and then we can modify an array parameter.
Swift program that uses array with func
func modify(values: [Int]) { // This func receives an array. // ... It adds to a copy of the array. var copy = Array(values) copy.append(10) copy.append(20) copy.append(30) print("Modify count: \(copy.count)") } // The array has 3 elements. let numbers = [0, 1, 2] print(numbers.count) // Call the func. modify(values: numbers) // The array still has only 3 elements. print("Numbers: \(numbers)")
3 Modify count: 6 Numbers: [0, 1, 2]
Index of. With this method we get the index of an element with a certain value. An Optional Int is returned. We can use optional binding (if let) to safely get the index.
Note Swift since version 3 uses "index" with a first argument labeled "of" instead of an actual indexOf method.
Swift program that uses indexOf
let letters = ["aa", "bb", "cc"] // Use index with an argument. let index1 = letters.index(of: "bb") let index2 = letters.index(of: "nonexistent") print(index1) print(index2) // See if the string exists in the array. // ... Use its index to print the value from the array. if let index3 = letters.index(of: "cc") { print(letters[index3]) }
Optional(1) nil cc
Indices. An array has indices—these are the values like 0, 1 and 2 that we use to index the first, second and third elements. With the indices property, we can loop over these values.
Swift program that uses indices
let sizes = ["small", "medium", "large"] // Get in dice so far ray. // ... Access each element. for i in sizes.indices { print("\(i) / \(sizes[i])") }
0 / small 1 / medium 2 / large
ElementsEqual. This method compares each element in one collection to another. Here we see that two of the arrays are equal, but the third one is not.
Swift program that uses elementsEqual
let numbers1 = [10, 100, 2000] let numbers2 = [10, 100, 2000] let numbers3 = [10, 0, 2000] // Use elementsEqual to compare entire arrays. if numbers1.elementsEqual(numbers2) { print("EQUAL") } if !numbers1.elementsEqual(numbers3) { print("NOT EQUAL") }
Reversed. The elements in an array come in a certain order. We can reverse this with the reversed() method. Here we use reversed() and then enumerated() to display our inverted array.
Swift program that uses reversed
var numbers = [10, 100, 2000] // Reverse the numbers. var inverted = numbers.reversed() // Display reversed numbers. for var pair in inverted.enumerated() { print(pair) }
(0, 2000) (1, 100) (2, 10)
FlatMap. This method transforms each element in a collection based on a mapping function. The func passed to flatMap must receive and return the element type.
Here We multiply each element in the array by 2. A copy is returned by flatMap, so the original "numbers" is unchanged.
Swift program that uses flatMap
func multiply (x: Int) -> Int { // Multiply argument by 2 and return. return x * 2 } let numbers = [10, 100, 2000] // Use flatMap on the array. let result = numbers.flatMap(multiply) // Before and after. print(numbers) print(result)
[10, 100, 2000] [20, 200, 4000]
Array of dictionaries. In Swift we can create arrays of dictionary elements. These dictionaries can have any key and value types. This is similar to the syntax for 2D arrays.
Array, Dictionaries
A summary. Arrays are a primary collection in Swift. An array has a dynamic size: it grows in capacity to accommodate elements. It is versatile and useful.
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