Swift Arrays

An array is an ordered list of values of the same type. Think of it like a numbered shelf in a warehouse — each slot holds one item, and each slot has a position number called an index. Arrays start counting from index 0.

Creating an Array

Array Literal

let fruits = ["Apple", "Banana", "Cherry"]
let scores = [95, 88, 72, 60]
let flags = [true, false, true]

Empty Array

var names: [String] = []
var counts = [Int]()

Array With a Default Value

var zeros = Array(repeating: 0, count: 5)
print(zeros)   // Output: [0, 0, 0, 0, 0]

Diagram: Array Index Layout

Array:   ["Apple", "Banana", "Cherry", "Date"]
Index:      0         1         2        3

fruits[0] → "Apple"
fruits[1] → "Banana"
fruits[3] → "Date"

Accessing Elements

let cities = ["Tokyo", "London", "Paris"]

print(cities[0])   // Output: Tokyo
print(cities[2])   // Output: Paris
print(cities.first ?? "None")   // Output: Tokyo
print(cities.last ?? "None")    // Output: Paris

Always use first and last properties (which return optionals) to avoid out-of-range crashes.

Array Properties

var items = ["Pen", "Book", "Bag"]

print(items.count)     // Output: 3
print(items.isEmpty)   // Output: false

Modifying an Array

Append — Add to the End

var colors = ["Red", "Green"]
colors.append("Blue")
print(colors)   // Output: ["Red", "Green", "Blue"]

Insert — Add at a Position

colors.insert("Yellow", at: 1)
print(colors)   // Output: ["Red", "Yellow", "Green", "Blue"]

Remove — Delete an Element

colors.remove(at: 0)
print(colors)   // Output: ["Yellow", "Green", "Blue"]

colors.removeLast()
print(colors)   // Output: ["Yellow", "Green"]

Update — Change an Element

var animals = ["Cat", "Dog", "Bird"]
animals[1] = "Fish"
print(animals)   // Output: ["Cat", "Fish", "Bird"]

Iterating Over an Array

for-in Loop

let planets = ["Mercury", "Venus", "Earth", "Mars"]

for planet in planets {
    print(planet)
}
// Output: Mercury Venus Earth Mars

Loop With Index Using enumerated()

for (index, planet) in planets.enumerated() {
    print("\(index + 1). \(planet)")
}
// Output:
// 1. Mercury
// 2. Venus
// 3. Earth
// 4. Mars

Useful Array Methods

contains

let numbers = [10, 20, 30, 40]
print(numbers.contains(20))   // Output: true
print(numbers.contains(99))   // Output: false

sorted

let unsorted = [5, 1, 4, 2, 3]
let sorted = unsorted.sorted()
print(sorted)   // Output: [1, 2, 3, 4, 5]

let descending = unsorted.sorted(by: >)
print(descending)   // Output: [5, 4, 3, 2, 1]

filter

let ages = [12, 19, 25, 14, 30]
let adults = ages.filter { $0 >= 18 }
print(adults)   // Output: [19, 25, 30]

map

let prices = [10.0, 20.0, 30.0]
let discounted = prices.map { $0 * 0.9 }
print(discounted)   // Output: [9.0, 18.0, 27.0]

reduce

let totals = [5, 10, 15]
let sum = totals.reduce(0) { $0 + $1 }
print(sum)   // Output: 30

Combining Arrays

let a = [1, 2, 3]
let b = [4, 5, 6]
let combined = a + b
print(combined)   // Output: [1, 2, 3, 4, 5, 6]

2D Arrays

An array of arrays creates a grid structure — useful for game boards, tables, and matrices.

var grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

print(grid[0][0])   // Output: 1 (row 0, column 0)
print(grid[1][2])   // Output: 6 (row 1, column 2)
print(grid[2][1])   // Output: 8 (row 2, column 1)

Diagram: 2D Array as a Grid

         Col 0   Col 1   Col 2
Row 0  [  1    ,   2   ,   3  ]
Row 1  [  4    ,   5   ,   6  ]
Row 2  [  7    ,   8   ,   9  ]

grid[1][2] → Row 1, Col 2 → 6

Summary

Arrays store ordered lists of same-type values accessed by zero-based index. Use append, insert, remove, and direct index assignment to modify them. Power methods like filter, map, and reduce transform arrays in one line. Arrays are the most common collection type in Swift.