Apple - The Swift programming language (pre-release)

Tradition suggests that the first program in a new language should print the words Hello, world on the screen. In Swift, this can be done in a single line:

If you have written code in C or Objective-C, this syntax looks familiar to youin Swift, this line of code is a complete program. You dont need to import a separate library for functionality like input/output or string handling. Code written at global scope is used as the entry point for the program, so you dont need a main function. You also dont need to write semicolons at the end of every statement.

This tour gives you enough information to start writing code in Swift by showing you how to accomplish a variety of programming tasks. Dont worry if you dont understand somethingeverything introduced in this tour is explained in detail in the rest of this book.

Note

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.

Use let to make a constant and var to make a variable. The value of a constant doesnt need to be known at compile time, but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places.

A constant or variable must have the same type as the value you want to assign to it. However, you dont 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 doesnt provide enough information (or if there is no initial value), specify the type by writing it after the variable, separated by a colon.

Experiment

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

Values are never implicitly converted to another type. If you need to convert a value to a different type, explicitly make an instance of the desired type.

Experiment

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

Theres an even simpler way to include values in strings: Write the value in parentheses, and write a backslash (\) before the parentheses. For example:

Experiment

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

Create arrays and dictionaries using brackets ([]), and access their elements by writing the index or key in brackets.

To create an empty array or dictionary, use the initializer syntax.

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.