Control Flow

Control Flow

Swift provides a variety of control flow statements. These include while loops to perform a task multiple times; if, guard, and switch statements to execute different branches of code based on certain conditions; and statements such as break and continue to transfer the flow of execution to another point in your code.

Swift also provides a for-in loop that makes it easy to iterate over arrays, dictionaries, ranges, strings, and other sequences.

Swift’s switch statement is considerably more powerful than its counterpart in many C-like languages. Cases can match many different patterns, including interval matches, tuples, and casts to a specific type. Matched values in a switch case can be bound to temporary constants or variables for use within the case’s body, and complex matching conditions can be expressed with a where clause for each case.

For-In Loops

You use the for-in loop to iterate over a sequence, such as items in an array, ranges of numbers, or characters in a string.

This example uses a for-in loop to iterate over the items in an array:

let names = ["Anna", "Alex", "Brian", "Jack"]
for name in names {
    print("Hello, \(name)!")
}
// Hello, Anna!
// Hello, Alex!
// Hello, Brian!
// Hello, Jack!

You can also iterate over a dictionary to access its key-value pairs. Each item in the dictionary is returned as a (key, value) tuple when the dictionary is iterated, and you can decompose the (key, value) tuple’s members as explicitly named constants for use within the body of the for-in loop. In the code example below, the dictionary’s keys are decomposed into a constant called animalName, and the dictionary’s values are decomposed into a constant called legCount.

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
    print("\(animalName)s have \(legCount) legs")
}
// cats have 4 legs
// ants have 6 legs
// spiders have 8 legs

The contents of a Dictionary are inherently unordered, and iterating over them doesn’t guarantee the order in which they will be retrieved. In particular, the order you insert items into a Dictionary doesn’t define the order they’re iterated. For more about arrays and dictionaries, see Collection Types.

You can also use for-in loops with numeric ranges. This example prints the first few entries in a five-times table:

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

The sequence being iterated over is a range of numbers from 1 to 5, inclusive, as indicated by the use of the closed range operator (...). The value of index is set to the first number in the range (1), and the statements inside the loop are executed. In this case, the loop contains only one statement, which prints an entry from the five-times table for the current value of index. After the statement is executed, the value of index is updated to contain the second value in the range (2), and the print(_:separator:terminator:) function is called again. This process continues until the end of the range is reached.

In the example above, index is a constant whose value is automatically set at the start of each iteration of the loop. As such, index doesn’t have to be declared before it’s used. It’s implicitly declared simply by its inclusion in the loop declaration, without the need for a let declaration keyword.

If you don’t need each value from a sequence, you can ignore the values by using an underscore in place of a variable name.

let base = 3
let power = 10
var answer = 1
for _ in 1...power {
    answer *= base
}
print("\(base) to the power of \(power) is \(answer)")
// Prints "3 to the power of 10 is 59049"

The example above calculates the value of one number to the power of another (in this case, 3 to the power of 10). It multiplies a starting value of 1 (that is, 3 to the power of 0) by 3, ten times, using a closed range that starts with 1 and ends with 10. For this calculation, the individual counter values each time through the loop are unnecessary—the code simply executes the loop the correct number of times. The underscore character (_) used in place of a loop variable causes the individual values to be ignored and doesn’t provide access to the current value during each iteration of the loop.

In some situations, you might not want to use closed ranges, which include both endpoints. Consider drawing the tick marks for every minute on a watch face. You want to draw 60 tick marks, starting with the 0 minute. Use the half-open range operator (..<) to include the lower bound but not the upper bound. For more about ranges, see Range Operators.

let minutes = 60
for tickMark in 0..<minutes {
    // render the tick mark each minute (60 times)
}

Some users might want fewer tick marks in their UI. They could prefer one mark every 5 minutes instead. Use the stride(from:to:by:) function to skip the unwanted marks.

let minuteInterval = 5
for tickMark in stride(from: 0, to: minutes, by: minuteInterval) {
    // render the tick mark every 5 minutes (0, 5, 10, 15 ... 45, 50, 55)
}

Closed ranges are also available, by using stride(from:through:by:) instead:

let hours = 12
let hourInterval = 3
for tickMark in stride(from: 3, through: hours, by: hourInterval) {
    // render the tick mark every 3 hours (3, 6, 9, 12)
}

The examples above use a for-in loop to iterate ranges, arrays, dictionaries, and strings. However, you can use this syntax to iterate any collection, including your own classes and collection types, as long as those types conform to the Sequence [https://developer.apple.com/documentation/swift/sequence\] protocol.

While Loops

A while loop performs a set of statements until a condition becomes false. These kinds of loops are best used when the number of iterations isn’t known before the first iteration begins. Swift provides two kinds of while loops:

• while evaluates its condition at the start of each pass through the loop.

• repeat-while evaluates its condition at the end of each pass through the loop.

While

A while loop starts by evaluating a single condition. If the condition is true, a set of statements is repeated until the condition becomes false.

Here’s the general form of a while loop:

while condition {
    statements
}

This example plays a simple game of Snakes and Ladders (also known as Chutes and Ladders):

The rules of the game are as follows:

• The board has 25 squares, and the aim is to land on or beyond square 25.

• The player’s starting square is “square zero”, which is just off the bottom-left corner of the board.

• Each turn, you roll a six-sided dice and move by that number of squares, following the horizontal path indicated by the dotted arrow above.

• If your turn ends at the bottom of a ladder, you move up that ladder.

• If your turn ends at the head of a snake, you move down that snake.

The game board is represented by an array of Int values. Its size is based on a constant called finalSquare, which is used to initialize the array and also to check for a win condition later in the example. Because the players start off the board, on “square zero”, the board is initialized with 26 zero Int values, not 25.

let finalSquare = 25
var board = [Int](repeating: 0, count: finalSquare + 1)

Some squares are then set to have more specific values for the snakes and ladders. Squares with a ladder base have a positive number to move you up the board, whereas squares with a snake head have a negative number to move you back down the board.

board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08

Square 3 contains the bottom of a ladder that moves you up to square 11. To represent this, board[03] is equal to +08, which is equivalent to an integer value of 8 (the difference between 3 and 11). To align the values and statements, the unary plus operator (+i) is explicitly used with the unary minus operator (-i) and numbers lower than 10 are padded with zeros. (Neither stylistic technique is strictly necessary, but they lead to neater code.)

var square = 0
var diceRoll = 0
while square < finalSquare {
    // roll the dice
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    // move by the rolled amount
    square += diceRoll
    if square < board.count {
        // if we're still on the board, move up or down for a snake or a ladder
        square += board[square]
    }
}
print("Game over!")

The example above uses a very simple approach to dice rolling. Instead of generating a random number, it starts with a diceRoll value of 0. Each time through the while loop, diceRoll is incremented by one and is then checked to see whether it has become too large. Whenever this return value equals 7, the dice roll has become too large and is reset to a value of 1. The result is a sequence of diceRoll values that’s always 1, 2, 3, 4, 5, 6, 1, 2 and so on.

After rolling the dice, the player moves forward by diceRoll squares. It’s possible that the dice roll may have moved the player beyond square 25, in which case the game is over. To cope with this scenario, the code checks that square is less than the board array’s count property. If square is valid, the value stored in board[square] is added to the current square value to move the player up or down any ladders or snakes.

Note

If this check isn’t performed, board[square] might try to access a value outside the bounds of the board array, which would trigger a runtime error.

The current while loop execution then ends, and the loop’s condition is checked to see if the loop should be executed again. If the player has moved on or beyond square number 25, the loop’s condition evaluates to false and the game ends.

A while loop is appropriate in this case, because the length of the game isn’t clear at the start of the while loop. Instead, the loop is executed until a particular condition is satisfied.

Repeat-While

The other variation of the while loop, known as the repeat-while loop, performs a single pass through the loop block first, before considering the loop’s condition. It then continues to repeat the loop until the condition is false.

Note

The repeat-while loop in Swift is analogous to a do-while loop in other languages.

Here’s the general form of a repeat-while loop:

repeat {
    statements
} while condition

Here’s the Snakes and Ladders example again, written as a repeat-while loop rather than a while loop. The values of finalSquare, board, square, and diceRoll are initialized in exactly the same way as with a while loop.

let finalSquare = 25
var board = [Int](repeating: 0, count: finalSquare + 1)
board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08
var square = 0
var diceRoll = 0

In this version of the game, the first action in the loop is to check for a ladder or a snake. No ladder on the board takes the player straight to square 25, and so it isn’t possible to win the game by moving up a ladder. Therefore, it’s safe to check for a snake or a ladder as the first action in the loop.

At the start of the game, the player is on “square zero”. board[0] always equals 0 and has no effect.

repeat {
    // move up or down for a snake or ladder
    square += board[square]
    // roll the dice
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    // move by the rolled amount
    square += diceRoll
} while square < finalSquare
print("Game over!")

After the code checks for snakes and ladders, the dice is rolled and the player is moved forward by diceRoll squares. The current loop execution then ends.

The loop’s condition (while square < finalSquare) is the same as before, but this time it’s not evaluated until the end of the first run through the loop. The structure of the repeat-while loop is better suited to this game than the while loop in the previous example. In the repeat-while loop above, square += board[square] is always executed immediately after the loop’s while condition confirms that square is still on the board. This behavior removes the need for the array bounds check seen in the while loop version of the game described earlier.

Conditional Statements

It’s often useful to execute different pieces of code based on certain conditions. You might want to run an extra piece of code when an error occurs, or to display a message when a value becomes too high or too low. To do this, you make parts of your code conditional.

Swift provides two ways to add conditional branches to your code: the if statement and the switch statement. Typically, you use the if statement to evaluate simple conditions with only a few possible outcomes. The switch statement is better suited to more complex conditions with multiple possible permutations and is useful in situations where pattern matching can help select an appropriate code branch to execute.

If

In its simplest form, the if statement has a single if condition. It executes a set of statements only if that condition is true.

var temperatureInFahrenheit = 30
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
}
// Prints "It's very cold. Consider wearing a scarf."

The example above checks whether the temperature is less than or equal to 32 degrees Fahrenheit (the freezing point of water). If it is, a message is printed. Otherwise, no message is printed, and code execution continues after the if statement’s closing brace.

The if statement can provide an alternative set of statements, known as an else clause, for situations when the if condition is false. These statements are indicated by the else keyword.

temperatureInFahrenheit = 40
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else {
    print("It's not that cold. Wear a t-shirt.")
}
// Prints "It's not that cold. Wear a t-shirt."

One of these two branches is always executed. Because the temperature has increased to 40 degrees Fahrenheit, it’s no longer cold enough to advise wearing a scarf and so the else branch is triggered instead.

You can chain multiple if statements together to consider additional clauses.

temperatureInFahrenheit = 90
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else if temperatureInFahrenheit >= 86 {
    print("It's really warm. Don't forget to wear sunscreen.")
} else {
    print("It's not that cold. Wear a t-shirt.")
}
// Prints "It's really warm. Don't forget to wear sunscreen."

Here, an additional if statement was added to respond to particularly warm temperatures. The final else clause remains, and it prints a response for any temperatures that are neither too warm nor too cold.

The final else clause is optional, however, and can be excluded if the set of conditions doesn’t need to be complete.

temperatureInFahrenheit = 72
if temperatureInFahrenheit <= 32 {
    print("It's very cold. Consider wearing a scarf.")
} else if temperatureInFahrenheit >= 86 {
    print("It's really warm. Don't forget to wear sunscreen.")
}

Because the temperature is neither too cold nor too warm to trigger the if or else if conditions, no message is printed.

Switch

A switch statement considers a value and compares it against several possible matching patterns. It then executes an appropriate block of code, based on the first pattern that matches successfully. A switch statement provides an alternative to the if statement for responding to multiple potential states.

In its simplest form, a switch statement compares a value against one or more values of the same type.

switch some value to consider {
case value 1:
    respond to value 1
case value 2,
     value 3:
    respond to value 2 or 3
default:
    otherwise, do something else
}

Every switch statement consists of multiple possible cases, each of which begins with the case keyword. In addition to comparing against specific values, Swift provides several ways for each case to specify more complex matching patterns. These options are described later in this chapter.

Like the body of an if statement, each case is a separate branch of code execution. The switch statement determines which branch should be selected. This procedure is known as switching on the value that’s being considered.

Every switch statement must be exhaustive. That is, every possible value of the type being considered must be matched by one of the switch cases. If it’s not appropriate to provide a case for every possible value, you can define a default case to cover any values that aren’t addressed explicitly. This default case is indicated by the default keyword, and must always appear last.

This example uses a switch statement to consider a single lowercase character called someCharacter:

let someCharacter: Character = "z"
switch someCharacter {
case "a":
    print("The first letter of the alphabet")
case "z":
    print("The last letter of the alphabet")
default:
    print("Some other character")
}
// Prints "The last letter of the alphabet"

The switch statement’s first case matches the first letter of the English alphabet, a, and its second case matches the last letter, z. Because the switch must have a case for every possible character, not just every alphabetic character, this switch statement uses a default case to match all characters other than a and z. This provision ensures that the switch statement is exhaustive.

No Implicit Fallthrough

In contrast with switch statements in C and Objective-C, switch statements in Swift don’t fall through the bottom of each case and into the next one by default. Instead, the entire switch statement finishes its execution as soon as the first matching switch case is completed, without requiring an explicit break statement. This makes the switch statement safer and easier to use than the one in C and avoids executing more than one switch case by mistake.

Note

Although break isn’t required in Swift, you can use a break statement to match and ignore a particular case or to break out of a matched case before that case has completed its execution. For details, see Break in a Switch Statement.

The body of each case must contain at least one executable statement. It isn’t valid to write the following code, because the first case is empty:

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a": // Invalid, the case has an empty body
case "A":
    print("The letter A")
default:
    print("Not the letter A")
}
// This will report a compile-time error.

Unlike a switch statement in C, this switch statement doesn’t match both "a" and "A". Rather, it reports a compile-time error that case "a": doesn’t contain any executable statements. This approach avoids accidental fallthrough from one case to another and makes for safer code that’s clearer in its intent.

To make a switch with a single case that matches both "a" and "A", combine the two values into a compound case, separating the values with commas.

let anotherCharacter: Character = "a"
switch anotherCharacter {
case "a", "A":
    print("The letter A")
default:
    print("Not the letter A")
}
// Prints "The letter A"

For readability, a compound case can also be written over multiple lines. For more information about compound cases, see Compound Cases.

Note

To explicitly fall through at the end of a particular switch case, use the fallthrough keyword, as described in Fallthrough.

Interval Matching

Values in switch cases can be checked for their inclusion in an interval. This example uses number intervals to provide a natural-language count for numbers of any size:

let approximateCount = 62
let countedThings = "moons orbiting Saturn"
let naturalCount: String
switch approximateCount {
case 0:
    naturalCount = "no"
case 1..<5:
    naturalCount = "a few"
case 5..<12:
    naturalCount = "several"
case 12..<100:
    naturalCount = "dozens of"
case 100..<1000:
    naturalCount = "hundreds of"
default:
    naturalCount = "many"
}
print("There are \(naturalCount) \(countedThings).")
// Prints "There are dozens of moons orbiting Saturn."

In the above example, approximateCount is evaluated in a switch statement. Each case compares that value to a number or interval. Because the value of approximateCount falls between 12 and 100, naturalCount is assigned the value "dozens of", and execution is transferred out of the switch statement.

Tuples

You can use tuples to test multiple values in the same switch statement. Each element of the tuple can be tested against a different value or interval of values. Alternatively, use the underscore character (_), also known as the wildcard pattern, to match any possible value.

The example below takes an (x, y) point, expressed as a simple tuple of type (Int, Int), and categorizes it on the graph that follows the example.

let somePoint = (1, 1)
switch somePoint {
case (0, 0):
    print("\(somePoint) is at the origin")
case (_, 0):
    print("\(somePoint) is on the x-axis")
case (0, _):
    print("\(somePoint) is on the y-axis")
case (-2...2, -2...2):
    print("\(somePoint) is inside the box")
default:
    print("\(somePoint) is outside of the box")
}
// Prints "(1, 1) is inside the box"

The switch statement determines whether the point is at the origin (0, 0), on the red x-axis, on the orange y-axis, inside the blue 4-by-4 box centered on the origin, or outside of the box.

Unlike C, Swift allows multiple switch cases to consider the same value or values. In fact, the point (0, 0) could match all four of the cases in this example. However, if multiple matches are possible, the first matching case is always used. The point (0, 0) would match case (0, 0) first, and so all other matching cases would be ignored.

Value Bindings

A switch case can name the value or values it matches to temporary constants or variables, for use in the body of the case. This behavior is known as value binding, because the values are bound to temporary constants or variables within the case’s body.

The example below takes an (x, y) point, expressed as a tuple of type (Int, Int), and categorizes it on the graph that follows:

let anotherPoint = (2, 0)
switch anotherPoint {
case (let x, 0):
    print("on the x-axis with an x value of \(x)")
case (0, let y):
    print("on the y-axis with a y value of \(y)")
case let (x, y):
    print("somewhere else at (\(x), \(y))")
}
// Prints "on the x-axis with an x value of 2"

The switch statement determines whether the point is on the red x-axis, on the orange y-axis, or elsewhere (on neither axis).

The three switch cases declare placeholder constants x and y, which temporarily take on one or both tuple values from anotherPoint. The first case, case (let x, 0), matches any point with a y value of 0 and assigns the point’s x value to the temporary constant x. Similarly, the second case, case (0, let y), matches any point with an x value of 0 and assigns the point’s y value to the temporary constant y.

After the temporary constants are declared, they can be used within the case’s code block. Here, they’re used to print the categorization of the point.

This switch statement doesn’t have a default case. The final case, case let (x, y), declares a tuple of two placeholder constants that can match any value. Because anotherPoint is always a tuple of two values, this case matches all possible remaining values, and a default case isn’t needed to make the switch statement exhaustive.

Where

A switch case can use a where clause to check for additional conditions.

The example below categorizes an (x, y) point on the following graph:

let yetAnotherPoint = (1, -1)
switch yetAnotherPoint {
case let (x, y) where x == y:
    print("(\(x), \(y)) is on the line x == y")
case let (x, y) where x == -y:
    print("(\(x), \(y)) is on the line x == -y")
case let (x, y):
    print("(\(x), \(y)) is just some arbitrary point")
}
// Prints "(1, -1) is on the line x == -y"

The switch statement determines whether the point is on the green diagonal line where x == y, on the purple diagonal line where x == -y, or neither.

The three switch cases declare placeholder constants x and y, which temporarily take on the two tuple values from yetAnotherPoint. These constants are used as part of a where clause, to create a dynamic filter. The switch case matches the current value of point only if the where clause’s condition evaluates to true for that value.

As in the previous example, the final case matches all possible remaining values, and so a default case isn’t needed to make the switch statement exhaustive.

Compound Cases

Multiple switch cases that share the same body can be combined by writing several patterns after case, with a comma between each of the patterns. If any of the patterns match, then the case is considered to match. The patterns can be written over multiple lines if the list is long. For example:

let someCharacter: Character = "e"
switch someCharacter {
case "a", "e", "i", "o", "u":
    print("\(someCharacter) is a vowel")
case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",
     "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":
    print("\(someCharacter) is a consonant")
default:
    print("\(someCharacter) isn't a vowel or a consonant")
}
// Prints "e is a vowel"

The switch statement’s first case matches all five lowercase vowels in the English language. Similarly, its second case matches all lowercase English consonants. Finally, the default case matches any other character.

Compound cases can also include value bindings. All of the patterns of a compound case have to include the same set of value bindings, and each binding has to get a value of the same type from all of the patterns in the compound case. This ensures that, no matter which part of the compound case matched, the code in the body of the case can always access a value for the bindings and that the value always has the same type.

let stillAnotherPoint = (9, 0)
switch stillAnotherPoint {
case (let distance, 0), (0, let distance):
    print("On an axis, \(distance) from the origin")
default:
    print("Not on an axis")
}
// Prints "On an axis, 9 from the origin"

The case above has two patterns: (let distance, 0) matches points on the x-axis and (0, let distance) matches points on the y-axis. Both patterns include a binding for distance and distance is an integer in both patterns—which means that the code in the body of the case can always access a value for distance.

Control Transfer Statements

Control transfer statements change the order in which your code is executed, by transferring control from one piece of code to another. Swift has five control transfer statements:

• continue

• break

• fallthrough

• return

• throw

The continue, break, and fallthrough statements are described below. The return statement is described in Functions, and the throw statement is described in Propagating Errors Using Throwing Functions.

Continue

The continue statement tells a loop to stop what it’s doing and start again at the beginning of the next iteration through the loop. It says “I am done with the current loop iteration” without leaving the loop altogether.

The following example removes all vowels and spaces from a lowercase string to create a cryptic puzzle phrase:

let puzzleInput = "great minds think alike"
var puzzleOutput = ""
let charactersToRemove: [Character] = ["a", "e", "i", "o", "u", " "]
for character in puzzleInput {
    if charactersToRemove.contains(character) {
        continue
    }
    puzzleOutput.append(character)
}
print(puzzleOutput)
// Prints "grtmndsthnklk"

The code above calls the continue keyword whenever it matches a vowel or a space, causing the current iteration of the loop to end immediately and to jump straight to the start of the next iteration.

Break

The break statement ends execution of an entire control flow statement immediately. The break statement can be used inside a switch or loop statement when you want to terminate the execution of the switch or loop statement earlier than would otherwise be the case.

Break in a Loop Statement

When used inside a loop statement, break ends the loop’s execution immediately and transfers control to the code after the loop’s closing brace (}). No further code from the current iteration of the loop is executed, and no further iterations of the loop are started.

Break in a Switch Statement

When used inside a switch statement, break causes the switch statement to end its execution immediately and to transfer control to the code after the switch statement’s closing brace (}).

This behavior can be used to match and ignore one or more cases in a switch statement. Because Swift’s switch statement is exhaustive and doesn’t allow empty cases, it’s sometimes necessary to deliberately match and ignore a case in order to make your intentions explicit. You do this by writing the break statement as the entire body of the case you want to ignore. When that case is matched by the switch statement, the break statement inside the case ends the switch statement’s execution immediately.

Note

A switch case that contains only a comment is reported as a compile-time error. Comments aren’t statements and don’t cause a switch case to be ignored. Always use a break statement to ignore a switch case.

The following example switches on a Character value and determines whether it represents a number symbol in one of four languages. For brevity, multiple values are covered in a single switch case.

let numberSymbol: Character = "三"  // Chinese symbol for the number 3
var possibleIntegerValue: Int?
switch numberSymbol {
case "1", "١", "一", "๑":
    possibleIntegerValue = 1
case "2", "٢", "二", "๒":
    possibleIntegerValue = 2
case "3", "٣", "三", "๓":
    possibleIntegerValue = 3
case "4", "٤", "四", "๔":
    possibleIntegerValue = 4
default:
    break
}
if let integerValue = possibleIntegerValue {
    print("The integer value of \(numberSymbol) is \(integerValue).")
} else {
    print("An integer value couldn't be found for \(numberSymbol).")
}
// Prints "The integer value of 三 is 3."

This example checks numberSymbol to determine whether it’s a Latin, Arabic, Chinese, or Thai symbol for the numbers 1 to 4. If a match is found, one of the switch statement’s cases sets an optional Int? variable called possibleIntegerValue to an appropriate integer value.

After the switch statement completes its execution, the example uses optional binding to determine whether a value was found. The possibleIntegerValue variable has an implicit initial value of nil by virtue of being an optional type, and so the optional binding will succeed only if possibleIntegerValue was set to an actual value by one of the switch statement’s first four cases.

Because it’s not practical to list every possible Character value in the example above, a default case handles any characters that aren’t matched. This default case doesn’t need to perform any action, and so it’s written with a single break statement as its body. As soon as the default case is matched, the break statement ends the switch statement’s execution, and code execution continues from the if let statement.

Fallthrough

In Swift, switch statements don’t fall through the bottom of each case and into the next one. That is, the entire switch statement completes its execution as soon as the first matching case is completed. By contrast, C requires you to insert an explicit break statement at the end of every switch case to prevent fallthrough. Avoiding default fallthrough means that Swift switch statements are much more concise and predictable than their counterparts in C, and thus they avoid executing multiple switch cases by mistake.

If you need C-style fallthrough behavior, you can opt in to this behavior on a case-by-case basis with the fallthrough keyword. The example below uses fallthrough to create a textual description of a number.

let integerToDescribe = 5
var description = "The number \(integerToDescribe) is"
switch integerToDescribe {
case 2, 3, 5, 7, 11, 13, 17, 19:
    description += " a prime number, and also"
    fallthrough
default:
    description += " an integer."
}
print(description)
// Prints "The number 5 is a prime number, and also an integer."

This example declares a new String variable called description and assigns it an initial value. The function then considers the value of integerToDescribe using a switch statement. If the value of integerToDescribe is one of the prime numbers in the list, the function appends text to the end of description, to note that the number is prime. It then uses the fallthrough keyword to “fall into” the default case as well. The default case adds some extra text to the end of the description, and the switch statement is complete.

Unless the value of integerToDescribe is in the list of known prime numbers, it isn’t matched by the first switch case at all. Because there are no other specific cases, integerToDescribe is matched by the default case.

After the switch statement has finished executing, the number’s description is printed using the print(_:separator:terminator:) function. In this example, the number 5 is correctly identified as a prime number.

Note

The fallthrough keyword doesn’t check the case conditions for the switch case that it causes execution to fall into. The fallthrough keyword simply causes code execution to move directly to the statements inside the next case (or default case) block, as in C’s standard switch statement behavior.

Labeled Statements

In Swift, you can nest loops and conditional statements inside other loops and conditional statements to create complex control flow structures. However, loops and conditional statements can both use the break statement to end their execution prematurely. Therefore, it’s sometimes useful to be explicit about which loop or conditional statement you want a break statement to terminate. Similarly, if you have multiple nested loops, it can be useful to be explicit about which loop the continue statement should affect.

To achieve these aims, you can mark a loop statement or conditional statement with a statement label. With a conditional statement, you can use a statement label with the break statement to end the execution of the labeled statement. With a loop statement, you can use a statement label with the break or continue statement to end or continue the execution of the labeled statement.

A labeled statement is indicated by placing a label on the same line as the statement’s introducer keyword, followed by a colon. Here’s an example of this syntax for a while loop, although the principle is the same for all loops and switch statements:

label name: while condition {
    statements
}

The following example uses the break and continue statements with a labeled while loop for an adapted version of the Snakes and Ladders game that you saw earlier in this chapter. This time around, the game has an extra rule:

• To win, you must land exactly on square 25.

If a particular dice roll would take you beyond square 25, you must roll again until you roll the exact number needed to land on square 25.

The game board is the same as before.

The values of finalSquare, board, square, and diceRoll are initialized in the same way as before:

let finalSquare = 25
var board = [Int](repeating: 0, count: finalSquare + 1)
board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08
var square = 0
var diceRoll = 0

This version of the game uses a while loop and a switch statement to implement the game’s logic. The while loop has a statement label called gameLoop to indicate that it’s the main game loop for the Snakes and Ladders game.

The while loop’s condition is while square != finalSquare, to reflect that you must land exactly on square 25.

gameLoop: while square != finalSquare {
    diceRoll += 1
    if diceRoll == 7 { diceRoll = 1 }
    switch square + diceRoll {
    case finalSquare:
        // diceRoll will move us to the final square, so the game is over
        break gameLoop
    case let newSquare where newSquare > finalSquare:
        // diceRoll will move us beyond the final square, so roll again
        continue gameLoop
    default:
        // this is a valid move, so find out its effect
        square += diceRoll
        square += board[square]
    }
}
print("Game over!")

The dice is rolled at the start of each loop. Rather than moving the player immediately, the loop uses a switch statement to consider the result of the move and to determine whether the move is allowed:

• If the dice roll will move the player onto the final square, the game is over. The break gameLoop statement transfers control to the first line of code outside of the while loop, which ends the game.

• If the dice roll will move the player beyond the final square, the move is invalid and the player needs to roll again. The continue gameLoop statement ends the current while loop iteration and begins the next iteration of the loop.

• In all other cases, the dice roll is a valid move. The player moves forward by diceRoll squares, and the game logic checks for any snakes and ladders. The loop then ends, and control returns to the while condition to decide whether another turn is required.

Note

If the break statement above didn’t use the gameLoop label, it would break out of the switch statement, not the while statement. Using the gameLoop label makes it clear which control statement should be terminated.

It isn’t strictly necessary to use the gameLoop label when calling continue gameLoop to jump to the next iteration of the loop. there’s only one loop in the game, and therefore no ambiguity as to which loop the continue statement will affect. However, there’s no harm in using the gameLoop label with the continue statement. Doing so is consistent with the label’s use alongside the break statement and helps make the game’s logic clearer to read and understand.

Early Exit

A guard statement, like an if statement, executes statements depending on the Boolean value of an expression. You use a guard statement to require that a condition must be true in order for the code after the guard statement to be executed. Unlike an if statement, a guard statement always has an else clause—the code inside the else clause is executed if the condition isn’t true.

func greet(person: [String: String]) {
    guard let name = person["name"] else {
        return
    }
    print("Hello \(name)!")
    guard let location = person["location"] else {
        print("I hope the weather is nice near you.")
        return
    }
    print("I hope the weather is nice in \(location).")
}
greet(person: ["name": "John"])
// Prints "Hello John!"
// Prints "I hope the weather is nice near you."
greet(person: ["name": "Jane", "location": "Cupertino"])
// Prints "Hello Jane!"
// Prints "I hope the weather is nice in Cupertino."

If the guard statement’s condition is met, code execution continues after the guard statement’s closing brace. Any variables or constants that were assigned values using an optional binding as part of the condition are available for the rest of the code block that the guard statement appears in.

If that condition isn’t met, the code inside the else branch is executed. That branch must transfer control to exit the code block in which the guard statement appears. It can do this with a control transfer statement such as return, break, continue, or throw, or it can call a function or method that doesn’t return, such as fatalError(_:file:line:).

Using a guard statement for requirements improves the readability of your code, compared to doing the same check with an if statement. It lets you write the code that’s typically executed without wrapping it in an else block, and it lets you keep the code that handles a violated requirement next to the requirement.

Checking API Availability

Swift has built-in support for checking API availability, which ensures that you don’t accidentally use APIs that are unavailable on a given deployment target.

The compiler uses availability information in the SDK to verify that all of the APIs used in your code are available on the deployment target specified by your project. Swift reports an error at compile time if you try to use an API that isn’t available.

You use an availability condition in an if or guard statement to conditionally execute a block of code, depending on whether the APIs you want to use are available at runtime. The compiler uses the information from the availability condition when it verifies that the APIs in that block of code are available.

if #available(iOS 10, macOS 10.12, *) {
    // Use iOS 10 APIs on iOS, and use macOS 10.12 APIs on macOS
} else {
    // Fall back to earlier iOS and macOS APIs
}

The availability condition above specifies that in iOS, the body of the if statement executes only in iOS 10 and later; in macOS, only in macOS 10.12 and later. The last argument, *, is required and specifies that on any other platform, the body of the if executes on the minimum deployment target specified by your target.

In its general form, the availability condition takes a list of platform names and versions. You use platform names such as iOS, macOS, watchOS, and tvOS—for the full list, see Declaration Attributes. In addition to specifying major version numbers like iOS 8 or macOS 10.10, you can specify minor versions numbers like iOS 11.2.6 and macOS 10.13.3.

if #available(platform name version, ..., *) {
    statements to execute if the APIs are available
} else {
    fallback statements to execute if the APIs are unavailable
}