Arrays

Arrays are ordered lists of values, all of the same data type and contained in the same structure. Swift, a very type-safe language, will throw compiler errors if values of different types are stored in a single array (e.g., a String in an Int array). However, values can be repeated; every occurrence of the same value gets stored in a different location.

Creating Arrays

Arrays can be created in several ways, each with a specific use case.

An empty array can be declared and initialized with the following syntax:

var arrayName = [Type]()

The above syntax is useful if elements are not supposed to be added yet, or the size of the array has not been determined.

Arrays can also be initialized with a given size and default values. For example, to store ten values for a Battleship game with a default location marker “U” (for “Unchecked”), an array can be initialized like this:

var alphaRow = [Character](repeating: "U", count: 10)
// Creates: ["U", "U", "U", "U", "U", "U", "U", "U", "U", "U"]

Swift also allows different values to be assigned within the array directly at initialization. It will even perform type inferencing, enabling the code to be shortened.

// Type declared
var statesVisited: [String] = ["NY", "CA", "TX", "FL"]
// Type inferenced
var statesToVisit = ["ME", "ID", "NV", "AK"]

Note: If an array is declared as a constant, using let, then it is immutable: its size or values of its members cannot change. This is especially useful if the data stored is to be protected from being manipulated later on in the program.

Accessing Arrays

Arrays can be accessed using subscript notation. Since all arrays are zero-indexed, the first index in it will be arrayName[0], and the last element will be one less than the .count property of the array.

var statesVisited: [String] = ["NY", "CA", "TX", "FL"]
print("Yesterday I went to \(statesVisited[0]).")
// Output: Yesterday I went to NY.

var statesToVisit = ["ME", "ID", "NV", "AK"]
print("Tomorrow I'm headed to \(statesToVisit[1])!")
// Output: Tomorrow I'm headed to NV!

Attempting to access an element that doesn’t exist through subscript notation will result in a runtime error.

print("After Nevada, I'm going to head to \(statesToVisit[4]).")
// Runtime Error: Index out of range

Values or elements of an array can be reassigned by accessing it through its index.

statesVisited[2] = "MI"
// ["NY", "CA", "MI", "FL"]

Iterating Over Arrays

To iterate over the entire contents of an array, a for-in loop is commonly used.

var topBabyNames = ["Liam", "Jackson"]

for name in topBabyNames {
  print("\(name)is the coolest name.")
}
// Output:
// Liam is the coolest name.
// Jackson is the coolest name.

Note: Alternatively, the .forEach() method can be used to iterate over an array.

Array Properties, Methods, and Testing

Built-in array properties in the Swift Standard Library include .first, .last, and .randomElement. These are used to access certain information within an array.

Note: If no values exist in the array, these properties will return nil.

print("The first and last states I've visited are \(statesVisited.first) and \(statesVisited.last), respectively.")
// Output: The first and last states I've visited are NY and FL, respectively.

print("To make life interesting, next week I'll fly to \(statesVisited.randomElement).")

Methods

There are many array manipulation methods available in the Swift Standard Library, including generic collection-based methods. Some of these include testing the contents of arrays (e.g. contains() and .isEmpty) while others can modify or manipulate the array entirely (e.g., map(), reduce(), and sort()).

Below are some methods available for arrays: