Swift 编程语言

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Welcome to Swift

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About Swift

Swift is a new programming language for iOS and OS X apps that builds on the best of C and Objective-C, without

the constraints of C compatibility. Swift adopts safe programming patterns and adds modern features to make

programming easier, more flexible, and more fun. Swift’s clean slate, backed by the mature and much-loved Cocoa

and Cocoa Touch frameworks, is an opportunity to reimagine how software development works.

Swift has been years in the making. Apple laid the foundation for Swift by advancing our existing compiler, debugger,

power of Objective-C’s dynamic object model. It provides seamless access to existing Cocoa frameworks and mix-

and-match interoperability with Objective-C code. Building from this common ground, Swift introduces many new

features and unifies the procedural and object-oriented portions of the language.

Swift is friendly to new programmers. It is the first industrial-quality systems programming language that is as

expressive and enjoyable as a scripting language. It supports playgrounds, an innovative feature that allows

programmers to experiment with Swift code and see the results immediately, without the overhead of building and

running an app.

Swift combines the best in modern language thinking with wisdom from the wider Apple engineering culture. The

compiler is optimized for performance, and the language is optimized for development, without compromising on

either. It’s designed to scale from “hello, world” to an entire operating system. All this makes Swift a sound future

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A Swift Tour

Swift, this can be done in a single line:

don’t need to write semicolons at the end of every statement.

programming tasks. Don’t worry if you don’t understand something—everything introduced in this tour is explained

in detail in the rest of this book.

For the best experience, open this chapter as a playground in Xcode. Playgrounds allow you to edit the

code listings and see the result immediately.

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A constant or variable must have the same type as the value you want to assign to it. However, you don’t always

have to write the type explicitly. Providing a value when you create a constant or variable lets the compiler infer its

type. In the example above, the compiler infers that myVariable is an integer because its initial value is a integer.

If the initial value doesn’t provide enough information (or if there is no initial value), specify the type by writing it after

the variable, separated by a colon.

1 let implicitInteger = 70

2 let implicitDouble = 70.0

3 let explicitDouble: Double = 70

Create a constant with an explicit type of Float and a value of 4.

Try removing the conversion to String from the last line. What error do you get?

before the parentheses. For example:

1 let apples = 3

2 let oranges = 5

3 let appleSummary = "I have \(apples) apples."

4 let fruitSummary = "I have \(apples + oranges) pieces of fruit."

Use \() to include a floating-point calculation in a string and to include someone’s name in a greeting.

brackets.

7 ]

8 occupations["Jayne"] = "Public Relations"

1 let emptyArray = String[]()

2 let emptyDictionary = Dictionary<String, Float>()

If type information can be inferred, you can write an empty array as [] and an empty dictionary as [:]—for

example, when you set a new value for a variable or pass an argument to a function.

shoppingList = [] // Went shopping and bought everything.

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around the condition or loop variable are optional. Braces around the body are required.

1 let individualScores = [75, 43, 103, 87, 12]

8 }

9 }

In an if statement, the conditional must be a Boolean expression—this means that code such as if score { ...

} is an error, not an implicit comparison to zero.

You can use if and let together to work with values that might be missing. These values are represented as

optionals. An optional value either contains a value or contains nil to indicate that the value is missing. Write a

question mark (?) after the type of a value to mark the value as optional.

1 var optionalString: String? = "Hello"

2 optionalString == nil