Control Flow

Swift provides a variety of control flow statements including conditionals, loops, and early exits. This lesson covers if, guard, switch, for-in, while, and control transfer statements.

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If Statements

let temperature = 25

if temperature > 30 {
    print("It's hot!")
} else if temperature > 20 {
    print("It's pleasant.")
} else {
    print("It's cool.")
}

Note: Conditions in Swift must be Bool expressions. Unlike C or JavaScript, there is no implicit conversion from numbers to booleans.

If Expressions (Swift 5.9+)

let mood = if temperature > 25 { "warm" } else { "cool" }

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Guard Statements

guard provides an early exit when a condition is not met:

func greet(name: String?) {
    guard let name = name else {
        print("No name provided.")
        return
    }
    // 'name' is now a non-optional String
    print("Hello, \(name)!")
}

Best Practice: Use guard for preconditions at the top of a function. The happy path stays unindented, making code easier to read.

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Switch Statements

Swift's switch is powerful and exhaustive:

let grade = "A"

switch grade {
case "A":
    print("Excellent!")
case "B":
    print("Good job!")
case "C":
    print("Satisfactory")
case "D":
    print("Needs improvement")
case "F":
    print("Failing")
default:
    print("Invalid grade")
}

Note: Swift switch statements do not fall through by default. There is no need for break at the end of each case.

Interval Matching

let score = 85

switch score {
case 90...100:
    print("A")
case 80..<90:
    print("B")
case 70..<80:
    print("C")
case 60..<70:
    print("D")
default:
    print("F")
}

Tuple Matching

let point = (2, 0)

switch point {
case (0, 0):
    print("Origin")
case (_, 0):
    print("On the x-axis")
case (0, _):
    print("On the y-axis")
case (-2...2, -2...2):
    print("Inside the box")
default:
    print("Outside the box")
}

Value Bindings and Where Clauses

let anotherPoint = (1, -1)

switch anotherPoint {
case let (x, y) where x == y:
    print("On the line x == y")
case let (x, y) where x == -y:
    print("On the line x == -y")
case let (x, y):
    print("Somewhere else at (\(x), \(y))")
}

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For-In Loops

Iterating over Arrays

let fruits = ["Apple", "Banana", "Cherry"]

for fruit in fruits {
    print(fruit)
}

Iterating over Dictionaries

let ages = ["Alice": 30, "Bob": 25, "Charlie": 35]