Loop Statements

Statements

Group expressions and control the flow of execution.

In Swift, there are three kinds of statements: simple statements, compiler control statements, and control flow statements. Simple statements are the most common and consist of either an expression or a declaration. Compiler control statements allow the program to change aspects of the compiler's behavior and include a conditional compilation block and a line control statement.

Control flow statements are used to control the flow of execution in a program. There are several types of control flow statements in Swift, including loop statements, branch statements, and control transfer statements. Loop statements allow a block of code to be executed repeatedly, branch statements allow a certain block of code to be executed only when certain conditions are met, and control transfer statements provide a way to alter the order in which code is executed. In addition, Swift provides a do statement to introduce scope, and catch and handle errors, and a defer statement for running cleanup actions just before the current scope exits.

A semicolon (;) can optionally appear after any statement and is used to separate multiple statements if they appear on the same line.

Grammar of a statement:

statement → expression ;?
statement → declaration ;?
statement → loop-statement ;?
statement → branch-statement ;?
statement → labeled-statement ;?
statement → control-transfer-statement ;?
statement → defer-statement ;?
statement → do-statement ;?
statement → compiler-control-statement
statements → statement statements?

Loop Statements

Loop statements allow a block of code to be executed repeatedly, depending on the conditions specified in the loop. Swift has three loop statements: a for-in statement, a while statement, and a repeat-while statement.

Control flow in a loop statement can be changed by a break statement and a continue statement and is discussed in doc:Statements#Break-Statement and doc:Statements#Continue-Statement below.

Grammar of a loop statement:

loop-statement → for-in-statement
loop-statement → while-statement
loop-statement → repeat-while-statement

For-In Statement

A for-in statement allows a block of code to be executed once for each item in a collection (or any type) that conforms to the Sequence protocol.

A for-in statement has the following form:

for <#item#> in <#collection#> {
   <#statements#>
}

The makeIterator() method is called on the collection expression to obtain a value of an iterator type --- that is, a type that conforms to the IteratorProtocol protocol. The program begins executing a loop by calling the next() method on the iterator. If the value returned isn't nil, it's assigned to the item pattern, the program executes the statements, and then continues execution at the beginning of the loop. Otherwise, the program doesn't perform assignment or execute the statements, and it's finished executing the for-in statement.

Grammar of a for-in statement:

for-in-statement → for case? pattern in expression where-clause? code-block

While Statement

A while statement allows a block of code to be executed repeatedly, as long as a condition remains true.

A while statement has the following form:

while <#condition#> {
   <#statements#>
}

A while statement is executed as follows:

1. The condition is evaluated.

   If true, execution continues to step 2. If false, the program is finished executing the while statement.

2. The program executes the statements, and execution returns to step 1.

Because the value of the condition is evaluated before the statements are executed, the statements in a while statement can be executed zero or more times.

The value of the condition must be of type Bool or a type bridged to Bool. The condition can also be an optional binding declaration, as discussed in doc:TheBasics#Optional-Binding.

Grammar of a while statement:

while-statement → while condition-list code-block

condition-list → condition | condition , condition-list
condition → expression | availability-condition | case-condition | optional-binding-condition

case-condition → case pattern initializer
optional-binding-condition → let pattern initializer? | var pattern initializer?

Repeat-While Statement

A repeat-while statement allows a block of code to be executed one or more times, as long as a condition remains true.

A repeat-while statement has the following form:

repeat {
   <#statements#>
} while <#condition#>

A repeat-while statement is executed as follows:

1. The program executes the statements, and execution continues to step 2.

2. The condition is evaluated.

   If true, execution returns to step 1. If false, the program is finished executing the repeat-while statement.

Because the value of the condition is evaluated after the statements are executed, the statements in a repeat-while statement are executed at least once.

The value of the condition must be of type Bool or a type bridged to Bool.

Grammar of a repeat-while statement:

repeat-while-statement → repeat code-block while expression

Branch Statements

Branch statements allow the program to execute certain parts of code depending on the value of one or more conditions. The values of the conditions specified in a branch statement control how the program branches and, therefore, what block of code is executed. Swift has three branch statements: an if statement, a guard statement, and a switch statement.

Control flow in an if statement or a switch statement can be changed by a break statement and is discussed in doc:Statements#Break-Statement below.

Grammar of a branch statement:

branch-statement → if-statement
branch-statement → guard-statement
branch-statement → switch-statement

If Statement

An if statement is used for executing code based on the evaluation of one or more conditions.

There are two basic forms of an if statement. In each form, the opening and closing braces are required.

The first form allows code to be executed only when a condition is true and has the following form:

if <#condition#> {
   <#statements#>
}

The second form of an if statement provides an additional else clause (introduced by the else keyword) and is used for executing one part of code when the condition is true and another part of code when the same condition is false. When a single else clause is present, an if statement has the following form:

if <#condition#> {
   <#statements to execute if condition is true#>
} else {
   <#statements to execute if condition is false#>
}

The else clause of an if statement can contain another if statement to test more than one condition. An if statement chained together in this way has the following form:

if <#condition 1#> {
   <#statements to execute if condition 1 is true#>
} else if <#condition 2#> {
   <#statements to execute if condition 2 is true#>
} else {
   <#statements to execute if both conditions are false#>
}

The value of any condition in an if statement must be of type Bool or a type bridged to Bool. The condition can also be an optional binding declaration, as discussed in doc:TheBasics#Optional-Binding.

Grammar of an if statement:

if-statement → if condition-list code-block else-clause?
else-clause → else code-block | else if-statement

Guard Statement

A guard statement is used to transfer program control out of a scope if one or more conditions aren't met.

A guard statement has the following form:

guard <#condition#> else {
   <#statements#>
}

The value of any condition in a guard statement must be of type Bool or a type bridged to Bool. The condition can also be an optional binding declaration, as discussed in doc:TheBasics#Optional-Binding.

Any constants or variables assigned a value from an optional binding declaration in a guard statement condition can be used for the rest of the guard statement's enclosing scope.

The else clause of a guard statement is required, and must either call a function with the Never return type or transfer program control outside the guard statement's enclosing scope using one of the following statements:

• return
• break
• continue
• throw

Control transfer statements are discussed in doc:Statements#Control-Transfer-Statements below. For more information on functions with the Never return type, see doc:Declarations#Functions-that-Never-Return.

Grammar of a guard statement:

guard-statement → guard condition-list else code-block

Switch Statement

A switch statement allows certain blocks of code to be executed depending on the value of a control expression.

A switch statement has the following form:

switch <#control expression#> {
case <#pattern 1#>:
    <#statements#>
case <#pattern 2#> where <#condition#>:
    <#statements#>
case <#pattern 3#> where <#condition#>,
    <#pattern 4#> where <#condition#>:
    <#statements#>
default:
    <#statements#>
}

The control expression of the switch statement is evaluated and then compared with the patterns specified in each case. If a match is found, the program executes the statements listed within the scope of that case. The scope of each case can't be empty. As a result, you must include at least one statement following the colon (:) of each case label. Use a single break statement if you don't intend to execute any code in the body of a matched case.

The values of expressions your code can branch on are very flexible. For example, in addition to the values of scalar types, such as integers and characters, your code can branch on the values of any type, including floating-point numbers, strings, tuples, instances of custom classes, and optionals. The value of the control expression can even be matched to the value of a case in an enumeration and checked for inclusion in a specified range of values. For examples of how to use these various types of values in switch statements, see doc:ControlFlow#Switch in doc:ControlFlow.

A switch case can optionally contain a where clause after each pattern. A where clause is introduced by the where keyword followed by an expression, and is used to provide an additional condition before a pattern in a case is considered matched to the control expression. If a where clause is present, the statements within the relevant case are executed only if the value of the control expression matches one of the patterns of the case and the expression of the where clause evaluates to true. For example, a control expression matches the case in the example below only if it's a tuple that contains two elements of the same value, such as (1, 1).

case let (x, y) where x == y:

As the above example shows, patterns in a case can also bind constants using the let keyword (they can also bind variables using the var keyword). These constants (or variables) can then be referenced in a corresponding where clause and throughout the rest of the code within the scope of the case. If the case contains multiple patterns that match the control expression, all of the patterns must contain the same constant or variable bindings, and each bound variable or constant must have the same type in all of the case's patterns.

A switch statement can also include a default case, introduced by the default keyword. The code within a default case is executed only if no other cases match the control expression. A switch statement can include only one default case, which must appear at the end of the switch statement.

Although the actual execution order of pattern-matching operations, and in particular the evaluation order of patterns in cases, is unspecified, pattern matching in a switch statement behaves as if the evaluation is performed in source order --- that is, the order in which they appear in source code. As a result, if multiple cases contain patterns that evaluate to the same value, and thus can match the value of the control expression, the program executes only the code within the first matching case in source order.

Switch Statements Must Be Exhaustive

In Swift, every possible value of the control expression’s type must match the value of at least one pattern of a case. When this simply isn’t feasible (for example, when the control expression’s type is Int), you can include a default case to satisfy the requirement.

Switching Over Future Enumeration Cases

A nonfrozen enumeration is a special kind of enumeration that may gain new enumeration cases in the future --- even after you compile and ship an app. Switching over a nonfrozen enumeration requires extra consideration. When a library's authors mark an enumeration as nonfrozen, they reserve the right to add new enumeration cases, and any code that interacts with that enumeration must be able to handle those future cases without being recompiled. Code that's compiled in library evolution mode, code in the Swift standard library, Swift overlays for Apple frameworks, and C and Objective-C code can declare nonfrozen enumerations. For information about frozen and nonfrozen enumerations, see doc:Attributes#frozen.

When switching over a nonfrozen enumeration value, you always need to include a default case, even if every case of the enumeration already has a corresponding switch case. You can apply the @unknown attribute to the default case, which indicates that the default case should match only enumeration cases that are added in the future. Swift produces a warning if the default case matches any enumeration case that's known at compile time. This future warning informs you that the library author added a new case to the enumeration that doesn't have a corresponding switch case.

The following example switches over all three existing cases of the Swift standard library's Mirror.AncestorRepresentation enumeration. If you add additional cases in the future, the compiler generates a warning to indicate that you need to update the switch statement to take the new cases into account.

let representation: Mirror.AncestorRepresentation = .generated
switch representation {
case .customized:
    print("Use the nearest ancestor’s implementation.")
case .generated:
    print("Generate a default mirror for all ancestor classes.")
case .suppressed:
    print("Suppress the representation of all ancestor classes.")
@unknown default:
    print("Use a representation that was unknown when this code was compiled.")
}
// Prints "Generate a default mirror for all ancestor classes."

Execution Does Not Fall Through Cases Implicitly

After the code within a matched case has finished executing, the program exits from the switch statement. Program execution doesn't continue or "fall through" to the next case or default case. That said, if you want execution to continue from one case to the next, explicitly include a fallthrough statement, which simply consists of the fallthrough keyword, in the case from which you want execution to continue. For more information about the fallthrough statement, see doc:Statements#Fallthrough-Statement below.

Grammar of a switch statement:

switch-statement → switch expression { switch-cases? }
switch-cases → switch-case switch-cases?
switch-case → case-label statements
switch-case → default-label statements
switch-case → conditional-switch-case

case-label → attributes? case case-item-list :
case-item-list → pattern where-clause? | pattern where-clause? , case-item-list
default-label → attributes? default :

where-clause → where where-expression
where-expression → expression

conditional-switch-case → switch-if-directive-clause switch-elseif-directive-clauses? switch-else-directive-clause? endif-directive
switch-if-directive-clause → if-directive compilation-condition switch-cases?
switch-elseif-directive-clauses → elseif-directive-clause switch-elseif-directive-clauses?
switch-elseif-directive-clause → elseif-directive compilation-condition switch-cases?
switch-else-directive-clause → else-directive switch-cases?

Labeled Statement

You can prefix a loop statement, an if statement, a switch statement, or a do statement with a statement label, which consists of the name of the label followed immediately by a colon (:). Use statement labels with break and continue statements to be explicit about how you want to change control flow in a loop statement or a switch statement, as discussed in doc:Statements#Break-Statement and doc:Statements#Continue-Statement below.

The scope of a labeled statement is the entire statement following the statement label. You can nest labeled statements, but the name of each statement label must be unique.

For more information and to see examples of how to use statement labels, see doc:ControlFlow#Labeled-Statements in doc:ControlFlow.

Grammar of a labeled statement:

labeled-statement → statement-label loop-statement
labeled-statement → statement-label if-statement
labeled-statement → statement-label switch-statement
labeled-statement → statement-label do-statement

statement-label → label-name :
label-name → identifier

Control Transfer Statements

Control transfer statements can change the order in which code in your program is executed by unconditionally transferring program control from one piece of code to another. Swift has five control transfer statements: a break statement, a continue statement, a fallthrough statement, a return statement, and a throw statement.

Grammar of a control transfer statement:

control-transfer-statement → break-statement
control-transfer-statement → continue-statement
control-transfer-statement → fallthrough-statement
control-transfer-statement → return-statement
control-transfer-statement → throw-statement

Break Statement

A break statement ends program execution of a loop, an if statement, or a switch statement. A break statement can consist of only the break keyword, or it can consist of the break keyword followed by the name of a statement label, as shown below.

break
break <#label name#>

When a break statement is followed by the name of a statement label, it ends program execution of the loop, if statement, or switch statement named by that label.

When a break statement isn't followed by the name of a statement label, it ends program execution of the switch statement or the innermost enclosing loop statement in which it occurs. You can't use an unlabeled break statement to break out of an if statement.

In both cases, program control is then transferred to the first line of code following the enclosing loop or switch statement, if any.

For examples of how to use a break statement, see doc:ControlFlow#Break and doc:ControlFlow#Labeled-Statements in doc:ControlFlow.

Grammar of a break statement:

break-statement → break label-name?

Continue Statement

A continue statement ends program execution of the current iteration of a loop statement but doesn't stop execution of the loop statement. A continue statement can consist of only the continue keyword, or it can consist of the continue keyword followed by the name of a statement label, as shown below.

continue
continue <#label name#>

When a continue statement is followed by the name of a statement label, it ends program execution of the current iteration of the loop statement named by that label.

When a continue statement isn't followed by the name of a statement label, it ends program execution of the current iteration of the innermost enclosing loop statement in which it occurs.

In both cases, program control is then transferred to the condition of the enclosing loop statement.

In a for statement, the increment expression is still evaluated after the continue statement is executed, because the increment expression is evaluated after the execution of the loop's body.

For examples of how to use a continue statement, see doc:ControlFlow#Continue and doc:ControlFlow#Labeled-Statements in doc:ControlFlow.

Grammar of a continue statement:

continue-statement → continue label-name?

Fallthrough Statement

A fallthrough statement consists of the fallthrough keyword and occurs only in a case block of a switch statement. A fallthrough statement causes program execution to continue from one case in a switch statement to the next case. Program execution continues to the next case even if the patterns of the case label don't match the value of the switch statement's control expression.

A fallthrough statement can appear anywhere inside a switch statement, not just as the last statement of a case block, but it can't be used in the final case block. It also can't transfer control into a case block whose pattern contains value binding patterns.

For an example of how to use a fallthrough statement in a switch statement, see doc:ControlFlow#Control-Transfer-Statements in doc:ControlFlow.

Grammar of a fallthrough statement:

fallthrough-statement → fallthrough

Return Statement

A return statement occurs in the body of a function or method definition and causes program execution to return to the calling function or method. Program execution continues at the point immediately following the function or method call.

A return statement can consist of only the return keyword, or it can consist of the return keyword followed by an expression, as shown below.

return
return <#expression#>

When a return statement is followed by an expression, the value of the expression is returned to the calling function or method. If the value of the expression doesn't match the value of the return type declared in the function or method declaration, the expression's value is converted to the return type before it's returned to the calling function or method.

Note: As described in doc:Declarations#Failable-Initializers, a special form of the return statement (return nil) can be used in a failable initializer to indicate initialization failure.

When a return statement isn't followed by an expression, it can be used only to return from a function or method that doesn't return a value (that is, when the return type of the function or method is Void or ()).

Grammar of a return statement:

return-statement → return expression?

Throw Statement

A throw statement occurs in the body of a throwing function or method, or in the body of a closure expression whose type is marked with the throws keyword.

A throw statement causes a program to end execution of the current scope and begin error propagation to its enclosing scope. The error that's thrown continues to propagate until it's handled by a catch clause of a do statement.

A throw statement consists of the throw keyword followed by an expression, as shown below.

throw <#expression#>

The value of the expression must have a type that conforms to the Error protocol. If the do statement or function that contains the throw statement declares the type of errors it throws, the value of the expression must be an instance of that type.

For an example of how to use a throw statement, see doc:ErrorHandling#Propagating-Errors-Using-Throwing-Functions in doc:ErrorHandling.

Grammar of a throw statement:

throw-statement → throw expression

Defer Statement

A defer statement is used for executing code just before transferring program control outside of the scope that the defer statement appears in.

A defer statement has the following form:

defer {
    <#statements#>
}

The statements within the defer statement are executed no matter how program control is transferred. This means that a defer statement can be used, for example, to perform manual resource management such as closing file descriptors, and to perform actions that need to happen even if an error is thrown.

The statements in the defer statement are executed at the end of the scope that encloses the defer statement.

func f(x: Int) {
  defer { print("First defer") }

  if x < 10 {
    defer { print("Second defer") }
    print("End of if")
  }

  print("End of function")
}
f(x: 5)
// Prints "End of if".
// Prints "Second defer".
// Prints "End of function".
// Prints "First defer".

In the code above, the defer in the if statement executes before the defer declared in the function f because the scope of the if statement ends before the scope of the function.

If multiple defer statements appear in the same scope, the order they appear is the reverse of the order they're executed. Executing the last defer statement in a given scope first means that statements inside that last defer statement can refer to resources that will be cleaned up by other defer statements.

func f() {
    defer { print("First defer") }
    defer { print("Second defer") }
    print("End of function")
}
f()
// Prints "End of function".
// Prints "Second defer".
// Prints "First defer".

The statements in the defer statement can't transfer program control outside of the defer statement.

Grammar of a defer statement:

defer-statement → defer code-block

Do Statement

The do statement is used to introduce a new scope and can optionally contain one or more catch clauses, which contain patterns that match against defined error conditions. Variables and constants declared in the scope of a do statement can be accessed only within that scope.

A do statement in Swift is similar to curly braces ({}) in C used to delimit a code block, and doesn't incur a performance cost at runtime.

A do statement has the following form:

do {
    try <#expression#>
    <#statements#>
} catch <#pattern 1#> {
    <#statements#>
} catch <#pattern 2#> where <#condition#> {
    <#statements#>
} catch <#pattern 3#>, <#pattern 4#> where <#condition#> {
    <#statements#>
} catch {
    <#statements#>
}

A do statement can optionally specify the type of error it throws, which has the following form:

do throws(<#type#>) {
    try <#expression#>
} catch <#pattern> {
    <#statements#>
} catch {
    <#statements#>
}

If the do statement includes a throws clause, the do block can throw errors of only the specified type. The type must be a concrete type that conforms to the Error protocol, an opaque type that conforms to the Error protocol, or the boxed protocol type any Error. If the do statement doesn't specify the type of error it throws, Swift infers the error type as follows:

• If every throws statement and try expression in the do code block is nested inside of an exhaustive error-handling mechanism, then Swift infers that the do statement is nonthrowing.

• If the do code block contains code that throws errors of only a single type outside of exhaustive error handling, other than throwing Never, then Swift infers that the do statement throws that concrete error type.

• If the do code block contains code that throws errors of more than a single type outside of exhaustive error handling, then Swift infers that the do statement throws any Error.

For more information about working with errors that have explicit types, see doc:ErrorHandling#Specifying-the-Error-Type.

If any statement in the do code block throws an error, program control is transferred to the first catch clause whose pattern matches the error. If none of the clauses match, the error propagates to the surrounding scope. If an error is unhandled at the top level, program execution stops with a runtime error.

Like a switch statement, the compiler attempts to infer whether catch clauses are exhaustive. If such a determination can be made, the error is considered handled. Otherwise, the error can propagate out of the containing scope, which means the error must be handled by an enclosing catch clause or the containing function must be declared with throws.

A catch clause that has multiple patterns matches the error if any of its patterns match the error. If a catch clause contains multiple patterns, all of the patterns must contain the same constant or variable bindings, and each bound variable or constant must have the same type in all of the catch clause's patterns.

To ensure that an error is handled, use a catch clause with a pattern that matches all errors, such as a wildcard pattern (_). If a catch clause doesn't specify a pattern, the catch clause matches and binds any error to a local constant named error. For more information about the patterns you can use in a catch clause, see doc:Patterns.

To see an example of how to use a do statement with several catch clauses, see doc:ErrorHandling#Handling-Errors.

Grammar of a do statement:

do-statement → do throws-clause? code-block catch-clauses?
catch-clauses → catch-clause catch-clauses?
catch-clause → catch catch-pattern-list? code-block
catch-pattern-list → catch-pattern | catch-pattern , catch-pattern-list
catch-pattern → pattern where-clause?

Compiler Control Statements

Compiler control statements allow the program to change aspects of the compiler's behavior. Swift has three compiler control statements: a conditional compilation block a line control statement, and a compile-time diagnostic statement.

Grammar of a compiler control statement:

compiler-control-statement → conditional-compilation-block
compiler-control-statement → line-control-statement
compiler-control-statement → diagnostic-statement

Conditional Compilation Block

A conditional compilation block allows code to be conditionally compiled depending on the value of one or more compilation conditions.

Every conditional compilation block begins with the #if compilation directive and ends with the #endif compilation directive. A simple conditional compilation block has the following form:

#if <#compilation condition#>
    <#statements#>
#endif

Unlike the condition of an if statement, the compilation condition is evaluated at compile time. As a result, the statements are compiled and executed only if the compilation condition evaluates to true at compile time.

The compilation condition can include the true and false Boolean literals, an identifier used with the -D command line flag, or any of the platform conditions listed in the table below.

The version number for the swift() and compiler() platform conditions consists of a major number, optional minor number, optional patch number, and so on, with a dot (.) separating each part of the version number. There must not be whitespace between the comparison operator and the version number. The version for compiler() is the compiler version, regardless of the Swift version setting passed to the compiler. The version for swift() is the language version currently being compiled. For example, if you compile your code using the Swift 5 compiler in Swift 4.2 mode, the compiler version is 5 and the language version is 4.2. With those settings, the following code prints all three messages:

#if compiler(>=5)
print("Compiled with the Swift 5 compiler or later")
#endif
#if swift(>=4.2)
print("Compiled in Swift 4.2 mode or later")
#endif
#if compiler(>=5) && swift(<5)
print("Compiled with the Swift 5 compiler or later in a Swift mode earlier than 5")
#endif
// Prints "Compiled with the Swift 5 compiler or later".
// Prints "Compiled in Swift 4.2 mode or later".
// Prints "Compiled with the Swift 5 compiler or later in a Swift mode earlier than 5".

The argument for the canImport() platform condition is the name of a module that may not be present on all platforms. The module can include periods (.) in its name. This condition tests whether it's possible to import the module, but doesn't actually import it. If the module is present, the platform condition returns true; otherwise, it returns false.

The targetEnvironment() platform condition returns true when code is being compiled for the specified environment; otherwise, it returns false.

Note: The arch(arm) platform condition doesn't return true for ARM 64 devices. The arch(i386) platform condition returns true when code is compiled for the 32–bit iOS simulator.