1. Basic Data Types

let age: Int = 25
let price: Double = 19.99
let isActive: Bool = true
let name: String = "Swift"

Collections

let numbers = [1, 2, 3]                  // Array
let uniqueNumbers = Set([1, 2, 3])       // Set
let user = ["id": 1, "name": "John"]     // Dictionary

---

2. Constants vs Variables

• let → Constant (cannot change)
• var → Variable (can change)

let maxLoginAttempts = 10
var currentLoginAttempt = 0
currentLoginAttempt += 1

---

3. Type Inference

Swift automatically detects types:

let count = 42        // Int
let pi = 3.14159      // Double

Explicit type declaration:

var message: String
message = "Hello"

---

4. Optionals (Handling Missing Values)

Optionals represent values that may or may not exist.

var possibleNumber: Int? = 123
possibleNumber = nil

Optional Binding

if let number = possibleNumber {
    print(number)
}

Default Value with ??

let name: String? = nil
let greeting = "Hello, " + (name ?? "Guest")

---

5. Tuples (Multiple Values)

let httpError = (404, "Not Found")
print(httpError.0)  // 404

Named tuple:

let status = (code: 200, message: "OK")
print(status.code)

---

6. Type Safety & Conversion

Swift prevents mixing incompatible types:

let number = 3
let decimal = 0.14
let result = Double(number) + decimal

Explicit conversion is required.

---

7. Assertions & Preconditions

Assertion (debug only):

let age = -1
assert(age >= 0, "Age cannot be negative")

Precondition (checked in production):

precondition(age >= 0, "Invalid age")

---

8. Error Handling

func makeSandwich() throws {
    // may throw error
}

do {
    try makeSandwich()
} catch {
    print("An error occurred")
}

---

Key Takeaways

• Use let by default.
• Swift is strongly typed and safe.
• Optionals prevent null-related crashes.
• Explicit type conversion avoids hidden bugs.
• Assertions help catch mistakes early.