Numeric Literals
Integer literals can be written as:
A decimal number, with no prefix
A binary number, with a 0bprefix
An octal number, with a 0oprefix
A hexadecimal number, with a 0xprefix
All of these integer literals have a decimal value of 17
:
let decimalInteger = 17
let binaryInteger = 0b10001 // 17 in binary notation
let octalInteger = 0o21 // 17 in octal notation
let hexadecimalInteger = 0x11 // 17 in hexadecimal notation
Integer Conversion
The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that will not fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:
let cannotBeNegative: UInt8 = -1
// UInt8 cannot store negative numbers, and so this will report an error
let tooBig: Int8 = Int8.max + 1
// Int8 cannot store a number larger than its maximum value,
// and so this will also report an error
Integer and Floating-Point Conversion
轉換型態才能相加
Conversions between integer and floating-point numeric types must be made explicit:
let three = 3
let pointOneFourOneFiveNine = 0.14159
let pi = Double(three) + pointOneFourOneFiveNine
// pi equals 3.14159, and is inferred to be of type Double
Floating-point to integer conversion must also be made explicit. An integer type can be initialized with a Double or Float value:
let integerPi = Int(pi)
// integerPi equals 3, and is inferred to be of type Int
Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.
Type Aliases
Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.
Type aliases are useful when you want to refer to an existing type by a name that is contextually more appropriate, such as when working with data of a specific size from an external source:
typealias AudioSample = UInt16
Once you define a type alias, you can use the alias anywhere you might use the original name:
var maxAmplitudeFound = AudioSample.min
// maxAmplitudeFound is now 0
Here, AudioSample is defined as an alias for UInt16. Because it is an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.
.min可以顯示該型態最小值
Booleans
Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:
let orangesAreOrange = true
let turnipsAreDelicious = false
The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don’t need to declare constants or variables as Bool if you set them to true or false as soon as you create them. Type inference helps make Swift code more concise and readable when it initializes constants or variables with other values whose type is already known.
Boolean values are particularly useful when you work with conditional statements such as the if statement:
if turnipsAreDelicious {
print("Mmm, tasty turnips!")
} else {
print("Eww, turnips are horrible.")
}
// Prints "Eww, turnips are horrible."
Conditional statements such as the if statement are covered in more detail in Control Flow.
Swift’s type safety prevents non-Boolean values from being substituted for Bool. The following example reports a compile-time error:
let i = 1
if i {
// this example will not compile, and will report an error
}
However, the alternative example below is valid:
let i = 1
if i == 1 {
// this example will compile successfully
}
The result of the i == 1 comparison is of type Bool, and so this second example passes the type-check. Comparisons like i == 1 are discussed in Basic Operators.
As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular
section of code is always clear.
跟C語言相似的布林
Tuples
Tuples group multiple values into a single compound value. The values within a tuple can be of any type and do not have to be of the same type as each other.
In this example, (404, "Not Found")
is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found
is returned if you request a webpage that doesn’t exist.
let http404Error = (404, "Not Found")
// http404Error is of type (Int, String), and equals (404, "Not Found")
The (404, "Not Found")
tuple groups together an Int
and a String
to give the HTTP status code two separate values: a number and a human-readable description. It can be described as “a tuple of type (Int, String)
”.
You can create tuples from any permutation of types, and they can contain as many different types as you like. There’s nothing stopping you from having a tuple of type (Int, Int, Int)
, or (String, Bool)
, or indeed any other permutation you require.
You can decompose a tuple’s contents into separate constants or variables, which you then access as usual:
let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)")
// Prints "The status code is 404"
print("The status message is \(statusMessage)")
// Prints "The status message is Not Found"
If you only need some of the tuple’s values, ignore parts of the tuple with an underscore (_
) when you decompose the tuple:
let (justTheStatusCode, _) = http404Error
print("The status code is \(justTheStatusCode)")
// Prints "The status code is 404"
Alternatively, access the individual element values in a tuple using index numbers starting at zero:
print("The status code is \(http404Error.0)")
// Prints "The status code is 404"
print("The status message is \(http404Error.1)")
// Prints "The status message is Not Found"
You can name the individual elements in a tuple when the tuple is defined:
let http200Status = (statusCode: 200, description: "OK")
If you name the elements in a tuple, you can use the element names to access the values of those elements:
print("The status code is \(http200Status.statusCode)")
// Prints "The status code is 200"
print("The status message is \(http200Status.description)")
// Prints "The status message is OK"
Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String)
tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see Functions with Multiple Return Values.
Tuples的宣告方式
Optionals
You use optionals in situations where a value may be absent. An optional represents two possibilities: Either there is a value, and you can unwrap the optional to access that value, or there isn’t a value at all.
有時候值可能有，也可能空值。使用？讓直變成可選。
Here’s an example of how optionals can be used to cope with the absence of a value. Swift’s Int type has an initializer which tries to convert a String value into an Int value. However, not every string can be converted into an integer. The string "123" can be converted into the numeric value 123 , but the string "hello, world" does not have an obvious numeric value to convert to. The example below uses the initializer to try to convert a String into an Int :
let possibleNumber = "123"
let convertedNumber = Int(possibleNumber)
// convertedNumber is inferred to be of type "Int?", or "optional Int"
nil
You set an optional variable to a valueless state by assigning it the special value nil:
var serverResponseCode: Int? = 404
// serverResponseCode contains an actual Int value of 404
serverResponseCode = nil
// serverResponseCode now contains no value
If you define an optional variable without providing a default value, the variable is automatically set to nil for you:
var surveyAnswer: String?
// surveyAnswer is automatically set to nil
If Statements and Forced Unwrapping
You can use an if statement to find out whether an optional contains a value by comparing the optional against nil. You perform this comparison with the “equal to” operator (==) or the “not equal to” operator (!=).If an optional has a value, it is considered to be “not equal to” nil:
if convertedNumber != nil {
print("convertedNumber contains some integer value.")
}
// Prints "convertedNumber contains some integer value."
== 等於
!= 不等於
Optional Binding
You use optional binding to find out whether an optional contains a value, and if so, to make that value available as a temporary constant or variable. Optional binding can be used with if and while statements to check for a value inside an optional, and to extract that value into a constant or variable, as part of a single action. if and while statements are described in more detail in Control Flow.
Write an optional binding for an if statement as follows:
if let *constantName* = *someOptional* {
*statements*
}
You can rewrite the possibleNumber
example from the Optionals section to use optional binding rather than forced unwrapping:
if let actualNumber = Int(possibleNumber) {
print("\"\(possibleNumber)\" has an integer value of \(actualNumber)")
} else {
print("\"\(possibleNumber)\" could not be converted to an integer")
}
// Prints ""123" has an integer value of 123"
You can include as many optional bindings and Boolean conditions in a single if statement as you need to, separated by commas. If any of the values in the optional bindings are nil or any Boolean condition evaluates to false, the whole if statement’s condition is considered to be false. The following if statements are equivalent:
if let firstNumber = Int("4"), let secondNumber = Int("42"), firstNumber < secondNumber && secondNumber < 100 {
print("\(firstNumber) < \(secondNumber) < 100")
}
// Prints "4 < 42 < 100"
if let firstNumber = Int("4") {
if let secondNumber = Int("42") {
if firstNumber < secondNumber && secondNumber < 100 {
print("\(firstNumber) < \(secondNumber) < 100")
}
}
}
// Prints "4 < 42 < 100"
單行if與多行if,&&連接條件
Implicitly Unwrapped Optionals
As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if
statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.
Sometimes it is clear from a program’s structure that an optional will always have a value, after that value is first set. In these cases, it is useful to remove the need to check and unwrap the optional’s value every time it is accessed, because it can be safely assumed to have a value all of the time.
These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation mark (String!) rather than a question mark (String?) after the type that you want to make optional.
Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties.
An implicitly unwrapped optional is a normal optional behind the scenes, but can also be used like a nonoptional value, without the need to unwrap the optional value each time it is accessed. The following example shows the difference in behavior between an optional string and an implicitly unwrapped optional string when accessing their wrapped value as an explicit String:
let possibleString: String? = "An optional string."
let forcedString: String = possibleString! // requires an exclamation mark
let assumedString: String! = "An implicitly unwrapped optional string."
let implicitString: String = assumedString // no need for an exclamation mark
第一條宣告顯示字串時要加驚嘆號，
第二條不用。
Error Handling
You use error handling to respond to error conditions your program may encounter during execution.
In contrast to optionals, which can use the presence or absence of a value to communicate success or failure of a function, error handling allows you to determine the underlying cause of failure, and, if necessary, propagate the error to another part of your program.
When a function encounters an error condition, it throws an error. That function’s caller can then catch the error and respond appropriately.
func canThrowAnError() throws {
// this function may or may not throw an error
}
A function indicates that it can throw an error by including the throws
keyword in its declaration. When you call a function that can throw an error, you prepend the try
keyword to the expression.
Swift automatically propagates errors out of their current scope until they are handled by a catch
clause.
do {
try canThrowAnError()
// no error was thrown
} catch {
// an error was thrown
}
先使用一個func...throws處理錯誤
再用do...try...catch...分辨錯誤處理方式
使用範例：
A do statement creates a new containing scope, which allows errors to be propagated to one or more catch clauses.
func makeASandwich() throws {
// ...
}
do {
try makeASandwich()
eatASandwich()
} catch SandwichError.outOfCleanDishes {
washDishes()
} catch SandwichError.missingIngredients(let ingredients) {
buyGroceries(ingredients)
}
Debugging with Assertions
An assertion is a runtime check that a Boolean condition definitely evaluates to true. Literally put, an assertion “asserts” that a condition is true. You use an assertion to make sure that an essential condition is satisfied before executing any further code. If the condition evaluates to true, code execution continues as usual; if the condition evaluates to false, code execution ends, and your app is terminated.
If your code triggers an assertion while running in a debug environment, such as when you build and run an app in Xcode, you can see exactly where the invalid state occurred and query the state of your app at the time that the assertion was triggered. An assertion also lets you provide a suitable debug message as to the nature of the assert.
You write an assertion by calling the Swift standard library global assert(::file:line:)
function. You pass this function an expression that evaluates to true or false and a message that should be displayed if the result of the condition is false:
let age = -3
assert(age >= 0, "A person's age cannot be less than zero")
// this causes the assertion to trigger, because age is not >= 0
In this example, code execution will continue only if age >= evaluates to true, that is, if the value of age is non-negative. If the value of age is negative, as in the code above, then age >= 0 evaluates to false, and the assertion is triggered, terminating the application.
The assertion message can be omitted if desired, as in the following example:
assert(age >= 0)
When to Use Assertions
Use an assertion whenever a condition has the potential to be false, but must definitely be true in order for your code to continue execution. Suitable scenarios for an assertion check include:
An integer subscript index is passed to a custom subscript implementation, but the subscript index value could be too low or too high.
A value is passed to a function, but an invalid value means that the function cannot fulfill its task.
An optional value is currently nil
, but a non-nil
value is essential for subsequent code to execute successfully.
See also Subscripts and Functions.
