Control Flow

Introduction

Control flow statements decide which code runs, and how many times. Swift's conditionals and loops will feel familiar if you've used another C-family language, but Swift's switch statement in particular is far more powerful than its equivalents elsewhere, supporting ranges, tuples, and conditional pattern matching directly.

This lesson covers if/else, the full power of switch, for-in and while loops, and the statements you use to jump around inside them.

if / else

An if statement in Swift does not require parentheses around the condition, but the curly braces are always mandatory — even for a single statement.

let temperature = 72

if temperature > 85 {
    print("It's hot")
} else if temperature > 60 {
    print("It's pleasant")
} else {
    print("It's cold")
}

The switch Statement

Swift's switch must be exhaustive — it has to handle every possible case, either explicitly or with a default clause, which the compiler enforces. Unlike C, Java, or JavaScript, a matched case does not automatically fall through into the next one.

let grade = "B"

switch grade {
case "A":
    print("Excellent")
case "B", "C":
    print("Good")
default:
    print("Needs improvement")
}

Ranges and where Clauses

Swift's switch can match numeric ranges directly, and a where clause lets you attach an extra condition to a case for genuinely powerful pattern matching.

let score = 82

switch score {
case 90...100:
    print("A")
case 80..<90 where score % 2 == 0:
    print("B (even score)")
case 80..<90:
    print("B")
default:
    print("C or below")
}

The ... operator creates a closed range that includes both ends (90 through 100 inclusive), while ..< creates a half-open range that excludes the upper bound (80 up to, but not including, 90).

for-in Loops

The for-in loop iterates over sequences — arrays, ranges, dictionaries, and any other type conforming to Sequence.

let fruits = ["Apple", "Mango", "Banana"]
for fruit in fruits {
    print(fruit)
}

for i in 1...5 {
    print("Count: \(i)")
}

while and repeat-while

A while loop checks its condition before each iteration; repeat-while (Swift's version of do-while) always runs the body at least once, checking the condition afterward.

var count = 0
while count < 3 {
    print("count is \(count)")
    count += 1
}

var attempts = 0
repeat {
    attempts += 1
} while attempts < 1
print("attempts: \(attempts)")

Control Transfer Statements

Swift provides break and continue like most C-family languages, plus an explicit fallthrough keyword for the rare case where you actually want a switch case to fall into the next one.

for number in 1...10 {
    if number % 2 != 0 { continue }   // skip odd numbers
    if number > 6 { break }            // stop once we pass 6
    print(number)
}

A First Look at guard

Swift also has a guard statement for early exits: it checks a condition, and if it fails, the else block must exit the current scope (with return, break, continue, or throw). guard is especially powerful when combined with Optionals — you'll see that fully in the Optionals lesson.

func checkAge(_ age: Int) {
    guard age >= 18 else {
        print("Must be 18 or older")
        return
    }
    print("Access granted")
}
checkAge(15)
checkAge(20)

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

You can now direct the flow of a Swift program with conditionals, powerful pattern-matching switches, and loops. Next, you'll learn how to package logic into reusable functions and closures.