Lesson 29Collections

Nested Collections

Collections Inside Collections

You can put collections inside other collections to create complex data structures like grids, tables, and hierarchies!

Types of Nested Collections

[[Int]]

Array of arrays (2D grid)

[[String: Any]]

Array of dictionaries

[String: [String]]

Dictionary of arrays

[String: [String: Int]]

Dictionary of dictionaries

Accessing Nested Data

Use multiple subscripts to dig deeper:

grid[0][1] - Row 0, Column 1
students[0]["name"] - First student's name
schedule["Monday"]![0] - First class Monday

Common Use Cases

Game boards - 2D arrays for tic-tac-toe, chess
Student records - Array of dictionaries
Weekly schedules - Dictionary of arrays
Organizational data - Nested dictionaries

main.swift

// NESTED ARRAYS (2D Arrays)
// Like a grid or table!
var grid: [[Int]] = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

// Accessing elements
print(grid[0])      // [1, 2, 3] - first row
print(grid[0][1])   // 2 - first row, second column
print(grid[2][2])   // 9 - third row, third column

// Modifying nested arrays
grid[1][1] = 50
print(grid[1])  // [4, 50, 6]

// Iterating nested arrays
for row in grid {
    for number in row {
        print(number, terminator: " ")
    }
    print()  // New line after each row
}

// ARRAY OF DICTIONARIES
let students: [[String: Any]] = [
    ["name": "Alice", "age": 12, "grade": "A"],
    ["name": "Bob", "age": 13, "grade": "B"],
    ["name": "Charlie", "age": 12, "grade": "A"]
]

// Access nested dictionary
print(students[0]["name"]!)  // Alice

for student in students {
    if let name = student["name"] as? String {
        print("Student: \(name)")
    }
}

// DICTIONARY OF ARRAYS
let schedule: [String: [String]] = [
    "Monday": ["Math", "English", "Science"],
    "Tuesday": ["Art", "Music", "PE"],
    "Wednesday": ["History", "Geography"]
]

// Access nested array
print(schedule["Monday"]!)  // ["Math", "English", "Science"]
print(schedule["Monday"]![0])  // Math

// DICTIONARY OF DICTIONARIES
let school: [String: [String: Int]] = [
    "ClassA": ["Alice": 95, "Bob": 87],
    "ClassB": ["Charlie": 92, "David": 88]
]

// Access nested value
print(school["ClassA"]!["Alice"]!)  // 95

// Flatten nested arrays
let nestedNumbers = [[1, 2], [3, 4], [5, 6]]
let flat = nestedNumbers.flatMap { $0 }
print(flat)  // [1, 2, 3, 4, 5, 6]

Try It Yourself!

Create a 3x3 tic-tac-toe board using a 2D array. Initialize it with empty strings, then place some X's and O's!