Swift: Swift Conditionals

Q: How should I choose between if and switch?

Use if for 2-3 branches. Use switch when you have more than 3 branches or need range/pattern matching. switch enforces exhaustive handling of all possibilities, making it safer than if.

Q: What's the difference between ... and ..< ?

a...b includes both a and b (closed range). a..<b includes a but not b (half-open range). For example, 1...3 includes 1, 2, 3; 1..<3 includes 1, 2.

Q: Does the ternary operator hurt code readability?

For simple either-or choices (like assignments), the ternary is crisp and clear. When nesting ternary operators or when the logic gets complex, use if instead—a long one-liner becomes harder to read.

Q: Can the default branch be omitted?

No. Swift's switch must be exhaustive. If you've covered all cases (e.g. all cases of an enum), you can omit default.

Conditionals let your program react differently to different situations, just like a traffic light changes signals based on traffic. This lesson covers all conditional control mechanisms in Swift: if-else, switch, and more.

1. What You'll Learn

Using if / else if / else for branching logic
Using switch for multi-value matching and pattern matching
Using range operators ... and ..< to simplify range conditions
Using fallthrough for switch case fall-through
Using the where clause to add extra constraints to conditions

2. A True Story: A Backend Developer

(1) The Pain Point: Multi-Layer Role-Based Permissions Turned Code into Spaghetti

Bob is building a user permission system at a SaaS company. The system has 5 user roles (admin / editor / viewer / guest / banned), each with different access levels to over 20 API endpoints. Bob initially used 15 layers of nested if-else:

SWIFT

if role == "admin" {
    if action == "delete" {
        // allow
    } else if action == "edit" {
        // allow
    }
} else if role == "editor" {
    // 700 lines of repeated code...
}

The code quickly ballooned to over 700 lines. Adding a new role meant modifying 5 different places. Bob spent a full week fixing a security vulnerability caused by incorrect if-else ordering.

(2) The Solution: switch

Bob refactored the permission system using switch statements:

SWIFT

let role = "editor"
switch role {
case "admin":
    print("Full access granted")
case "editor":
    print("Read and write access")
case "viewer", "guest":
    print("Read-only access")
case "banned":
    print("Access denied")
default:
    print("Unknown role")
}

(3) The Result: Code Shrank 80%, Logic Became Crystal Clear

Dimension	15-Layer if-else	After switch Refactor
Lines of code	700+	150
Time to add a new role	30 minutes	3 minutes
Logic errors	3 per month	0 per month
Readability score	3/10	9/10

3. if/else Conditional Statements

if/else is Swift's most fundamental conditional control structure. It decides which code block to execute based on a Bool value.

graph TB
    A[Condition] -->|true| B[if block]
    A -->|false| C[else if / else block]
    B --> D[Continue]
    C --> D

Syntax	Description	Example
if condition { }	Execute when condition is true	if score >= 60 { }
if ... else { }	Execute else when condition is false	if ... else { }
if ... else if ... else { }	Check multiple conditions in order	if ... else if ... else { }

(1) Basic if and else

SWIFT

let temperature = 30
if temperature > 25 {
    print("It's hot outside")
} else {
    print("It's cool outside")
}

(2) Multiple Conditions with else if

SWIFT

let score = 85
if score >= 90 {
    print("Grade: A")
} else if score >= 80 {
    print("Grade: B")
} else if score >= 70 {
    print("Grade: C")
} else if score >= 60 {
    print("Grade: D")
} else {
    print("Grade: F")
}

SWIFT

// ============================================
// Display different messages based on login status
// ============================================
let isLoggedIn = true
let hasProfile = false
if isLoggedIn {
    print("Welcome back!")
    if hasProfile {
        print("Your profile is complete")
    } else {
        print("Please complete your profile")
    }
} else {
    print("Please log in first")
}

Output:
TEXT 📖 Display only
Welcome back!
Please complete your profile

4. switch Multi-Branch Matching

switch is a more powerful multi-branch matching tool than if-else. Swift's switch does not require break—execution automatically exits after a match.

graph TB
    A[Value] --> B[case 1]
    A --> C[case 2]
    A --> D[case 3]
    A --> E[default]
    B --> F[Execute and Exit]
    C --> F
    D --> F
    E --> F

Feature	Swift switch	C / Java switch
Implicit break	Automatic (no break needed)	Must write break
Range matching	Supports `...` and `..<`	Not supported
Compound matching	Comma-separated values	Relies on fallthrough
Exhaustive	Must cover all possibilities	Not enforced
Default branch	Uses default	Uses default

(1) Basic switch Syntax

SWIFT

let fruit = "apple"
switch fruit {
case "apple":
    print("It's an apple")
case "banana":
    print("It's a banana")
case "orange":
    print("It's an orange")
default:
    print("Unknown fruit")
}

(2) Range Matching and Compound Matching

SWIFT

let age = 25
switch age {
case 0..<13:
    print("Child")
case 13..<20:
    print("Teenager")
case 20..<65:
    print("Adult")
case 65...:
    print("Senior")
default:
    print("Invalid age")
}

▶ Example: HTTP Status Code Handling

SWIFT

// ============================================
// Handle HTTP response status codes with switch
// ============================================
let statusCode = 404
switch statusCode {
case 100..<200:
    print("Informational")
case 200..<300:
    print("Success")
case 300..<400:
    print("Redirection")
case 400..<500:
    print("Client error")
    if statusCode == 404 {
        print("Resource not found")
    }
case 500..<600:
    print("Server error")
default:
    print("Unknown status code")
}

Output:
TEXT 📖 Display only
Client error
Resource not found

5. Advanced Control: fallthrough, where, and the Ternary Operator

Swift provides additional tools to make conditional logic more flexible.

Tool	Purpose	Example
fallthrough	Fall through to the next case in switch	case "a": fallthrough
where	Add extra filtering to a condition	case let x where x > 10:
Ternary ? :	Concise either-or choice	let max = a > b ? a : b

(1) fallthrough

SWIFT

let number = 2
switch number {
case 1:
    print("One")
case 2:
    print("Two")
    fallthrough
case 3:
    print("Three or fell through from two")
default:
    print("Other")
}

(2) where Condition Filtering

SWIFT

let point = (x: 3, y: 4)
switch point {
case let (x, y) where x == y:
    print("On the diagonal")
case let (x, y) where x > y:
    print("X is larger")
case let (x, y) where x < y:
    print("Y is larger")
default:
    print("On an axis")
}

(3) The Ternary Conditional Operator

SWIFT

let isMember = true
let discount = isMember ? 0.2 : 0.0
print("Discount: \(discount * 100)%")

▶ Example: Order Discount Calculator

SWIFT

// ============================================
// Combine if/switch/ternary to calculate order discounts
// ============================================
let orderTotal = 250.0
let customerTier = "gold"
// Ternary operator: base discount
let baseDiscount = orderTotal > 100 ? 0.05 : 0.0
// switch: membership tier discount
let tierDiscount: Double
switch customerTier {
case "platinum":
    tierDiscount = 0.20
case "gold":
    tierDiscount = 0.15
case "silver":
    tierDiscount = 0.10
default:
    tierDiscount = 0.0
}
// if: cap the combined discount
let totalDiscount = baseDiscount + tierDiscount
let finalDiscount = totalDiscount > 0.3 ? 0.3 : totalDiscount
let finalPrice = orderTotal * (1 - finalDiscount)
print("Order total: $\(orderTotal)")
print("Tier: \(customerTier)")
print("Discount: \(Int(finalDiscount * 100))%")
print("Final price: $\(finalPrice)")

Output:
TEXT 📖 Display only
Order total: $250.0
Tier: gold
Discount: 20%
Final price: $200.0

6. Full Example: User Permission Management System

SWIFT

// ============================================
// User permission management system
// Integrates if/switch/where/ternary operators
// ============================================
import Foundation
enum UserRole {
    case admin, editor, viewer, guest, banned
}
enum ActionResult {
    case granted, denied(String)
}
func checkPermission(role: UserRole, action: String, isOwner: Bool) -> ActionResult {
    switch role {
    case .banned:
        return .denied("Account is banned")
    case .admin:
        return .granted
    case .editor:
        if action == "delete" && !isOwner {
            return .denied("Only owners can delete")
        }
        return .granted
    case .viewer:
        switch (action, isOwner) {
        case (_, false):
            return .denied("Viewers cannot modify content")
        case ("read", true):
            return .granted
        default:
            return .denied("Unknown action")
        }
    case .guest:
        return action == "read" ? .granted : .denied("Guests can only read")
    }
}
let testCases = [
    (UserRole.admin, "delete", false),
    (UserRole.editor, "delete", true),
    (UserRole.editor, "delete", false),
    (UserRole.viewer, "read", true),
    (UserRole.viewer, "write", false),
    (UserRole.guest, "read", false),
    (UserRole.banned, "read", false)
]
for (role, action, isOwner) in testCases {
    let result = checkPermission(role: role, action: action, isOwner: isOwner)
    switch result {
    case .granted:
        print("[GRANTED] \(role) can \(action)")
    case .denied(let reason):
        print("[DENIED] \(role) cannot \(action) -- \(reason)")
    }
}

Output:

TEXT 📖 Display only

[GRANTED] admin can delete
[GRANTED] editor can delete
[DENIED] editor cannot delete -- Only owners can delete
[GRANTED] viewer can read
[DENIED] viewer cannot write -- Viewers cannot modify content
[GRANTED] guest can read
[DENIED] banned cannot read -- Account is banned

❓ FAQ

Q Why doesn't Swift's switch need break?

A Swift's switch automatically exits after matching a case and won't fall through to the next case. To intentionally fall through, you must explicitly write fallthrough.

Q How should I choose between if and switch?

A Use if for 2-3 branches. Use switch when you have more than 3 branches or need range/pattern matching. switch enforces exhaustive handling of all possibilities, making it safer than if.

Q What's the difference between ... and ..<?

A a...b includes both a and b (closed range). a..<b includes a but not b (half-open range). For example, 1...3 includes 1, 2, 3; 1..<3 includes 1, 2.

Q Does the ternary operator hurt code readability?

A For simple either-or choices (like assignments), the ternary is crisp and clear. When nesting ternary operators or when the logic gets complex, use if instead—a long one-liner becomes harder to read.

Q Can the default branch be omitted?

A No. Swift's switch must be exhaustive. If you've covered all cases (e.g. all cases of an enum), you can omit default.

📖 Summary

if/else is the most fundamental conditional control, best for 2-3 branches
switch is best for 3+ branches and supports range matching, compound matching, and value binding
switch's fallthrough allows fall-through to the next case, useful for sequential matching scenarios
The where clause adds extra filtering conditions to switch cases and for loops
The ternary operator ? : is best for concise either-or assignments
Swift's switch requires exhaustive handling, enhancing code safety

📝 Exercises

Beginner: Use if/else to write a body temperature classifier: temp < 36.0 outputs "Low", 36.0...37.5 outputs "Normal", > 37.5 outputs "Fever".
Intermediate: Refactor the beginner exercise using switch to also support temp < 35.0 outputting "Dangerously Low". Use range matching syntax.
Challenge: Create a payment fee calculator. Fee rules: Credit card 2.5% (≥$100: 2.5% - 0.5%), Debit card 1.0% (capped at $5), PayPal 3.0% + $0.30. Implement with switch + where.

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Swift: Swift Conditionals

1. What You'll Learn

2. A True Story: A Backend Developer

(1) The Pain Point: Multi-Layer Role-Based Permissions Turned Code into Spaghetti

(2) The Solution: switch

(3) The Result: Code Shrank 80%, Logic Became Crystal Clear

3. if/else Conditional Statements

(1) Basic if and else

(2) Multiple Conditions with else if

▶ Example: Login Status Check

4. switch Multi-Branch Matching

(1) Basic switch Syntax

(2) Range Matching and Compound Matching

▶ Example: HTTP Status Code Handling

5. Advanced Control: fallthrough, where, and the Ternary Operator

(1) fallthrough

(2) where Condition Filtering

(3) The Ternary Conditional Operator

▶ Example: Order Discount Calculator

6. Full Example: User Permission Management System

❓ FAQ

📖 Summary

📝 Exercises

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