Arrays in

About Arrays

Arrays are one of Swift's three primary collection types. Arrays are ordered lists of elements where the elements can be of any type, however, all elements of any given list must have the same type.

Arrays literals are written as a series of elements, each separated by commas, enclosed in square brackets. Empty arrays are just a pair of square brackets. Type names for arrays are written in one of two ways: Array<T> or [T] where T is the type of the elements in thee array. When creating an empty array, the type must be specified.

let evenInts: Array<Int> = [2, 4, 6, 8, 10, 12]
var oddInts: [Int] = [1, 3, 5, 7, 9, 11, 13]
let greetings = ["Hello!", "Hi!", "¡Hola!"]
var myStringArray: [String] = []

Elements of an array can be accessed individually by supplying the index of the element inside square brackets following the array; array indices are Ints and start with 0 for the first (leftmost) element. This subscript notation can be used to get the element at that index as well as to set the element at that index, provided the array was defined as a variable (i.e. using var).

Trying to access elements at indices outside the valid range of indices will result in a runtime error that crashes the program. Since any invalid array index access will crash a program, it is common to test arrays to see if the are empty before working with them by checking the isEmpty property or checking if an index is valid by ensuring that it is greater than or equal to 0 and less than the array's count property.

guard !evenInts.isEmpty, !oddInts.isEmpty else { return }
evenInts[2]
// => 6
oddInts[0] = 27
// oddInts is now [27, 3, 5, 7, 9, 11, 13]

// these operations are not allowed
greetings[3]
// error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).
evenInts[1] = 0
// Cannot assign through subscript: 'evenInts' is a 'let' constant

Arrays in Swift are not fixed size (though constant arrays, defined using let cannot be modified, including adding and removing elements). Elements can quickly be appended or dropped from the end of an array, and elements can be inserted or removed at any other location, though these operations are slower. The entire contents of another array can also be inserted at a given position in the original array.

The elements of an array can be stepped through one at a time using a for-in loop. This type of loop takes each element of the array, in order, and binds the element to a specified name for further processing inside the loop body. For example, to print out all of the odd integers in an array one can write:

let ints = [1, 3, 6, 14, 17, 8, 23, 5, 18, 11]

for int in ints {
  if !int.isMultiple(of: 2) {
    print(int)
  }
}

// prints out:
// 1
// 3
// 17
// 23
// 5
// 11