The_Swift_Programming_Language

Statements

In Swift, there are two kinds of statements: simple statements and control flow statements. Simple statements are the most common and consist of either an expression or a declaration. Control flow statements are used to control the flow of execution in a program. There are three types of control flow statements in Swift: 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. Each type of control flow statement is described in detail below.

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

G R A M M A R O F A S T A T E M E N T

​statement → expression ;opt ​statement → declaration ;opt ​statement → loop-statement ;opt ​statement → branch-statement ;opt ​statement → labeled-statement

​statement → control-transfer-statement ;opt ​statements → statement statements opt

Loop Statements

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

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

G R A M M A R O F A L O O P S T A T E M E N T

​loop-statement → for-statement ​loop-statement → for-in-statement ​loop-statement → while-statement ​loop-statement → do-while-statement

A for-in statement has the following form:

for item in collection { statements

}

The generate method is called on the collection expression to obtain a value of a generator type—that is, a type that conforms to the Generator protocol. The program begins executing a loop by calling the next method on the stream. If the value returned is not None, it is assigned to the item pattern, the program executes the statements, and then continues execution at the beginning of the loop. Otherwise, the program does not perform assignment or execute the statements, and it is finished executing the for-in statement.

G R A M M A R O F A F O R- I N S T A T E M E N T

​for-in-statement → for pattern in expression 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 have a type that conforms to the LogicValue protocol. The condition can also be an optional binding declaration, as discussed in Optional Binding.

G R A M M A R O F A W H I L E S T A T E M E N T

​branch-statement → if-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 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 have a type that conforms to the LogicValue protocol. The condition can also be an optional binding declaration, as discussed in Optional Binding.

G R A M M A R O F A N I F S T A T E M E N T

​if-statement → if if-condition code-block else-clause opt ​if-condition → expression declaration

​else-clause → else code-block else if-statement

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 is very flexible. For instance, 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 Switch in the Control Flow chapter.

A switch case can optionally contain a guard expression after each pattern. A guard expression is introduced by the keyword where 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 guard expression 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 guard expression evaluates to true. For instance, a control expression matches the case in the example below only if it is a tuple that contains two elements of the same value, such as (1, 1).

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

As the above example shows, patterns in a case can also bind constants using the keyword let (they can also bind variables using the keyword var). These constants (or variables) can then be referenced in a corresponding guard expression and throughout the rest of the code within the scope of the case. That said, if the case contains multiple patterns that match the control expression, none of those patterns can contain constant or variable bindings.

A switch statement can also include a default case, introduced by the keyword default. 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 instance, when the control expression’s type is Int), you can include a default case to satisfy the requirement.

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 does not 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 keyword fallthrough, in the case from which you want execution to continue. For more information about the fallthrough statement, see Fallthrough Statement below.

G R A M M A R O F A S W I T C H S T A T E M E N T

​switch-statement → switch expression { switch-cases opt } ​switch-cases → switch-case switch-cases opt

​switch-case → case-label statements default-label statements ​switch-case → case-label ; default-label ;

​case-label → case case-item-list :

​case-item-list → pattern guard-clause opt pattern guard-clause opt , case-item-list ​default-label → default :

​guard-clause → where guard-expression ​guard-expression → expression

Labeled Statement

You can prefix a loop statement or a switch 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 Break Statement and 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 Labeled Statements in the Control Flow chapter.

G R A M M A R O F A L A B E L E D S T A T E M E N T

​labeled-statement → statement-label loop-statement statement-label switchstatement

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