Swift Introduction

• Swift Playgrounds (iPad and Mac): Apple's friendly app for learning, with interactive lessons and live results.
• Xcode (Mac): Apple's full development environment, used to build real apps. Choose File, New, Playground to experiment, or create an app project.
• Windows and Linux: install Swift from swift.org, then run files from a terminal with swift main.swift, or use VS Code with the Swift extension.
• Online: sites such as the SwiftFiddle playground run Swift in a browser with nothing to install.

Swift source files end in .swift. Every example in this course can run in any of these.

print("Hello, Swift!")
print("I'm learning to code.")

• print shows text in the console and then moves to a new line.
• In a playground or a script file, code at the top level runs from top to bottom, so there's no main function to write.
• Semicolons at the end of lines are optional, and almost nobody uses them.
• Swift is case-sensitive: print works, Print doesn't.

In a full app, code is organised into types and functions that the system calls, such as when a screen appears or a button is tapped, which you'll see in the SwiftUI lessons.

print can take several values separated by commas, and puts a space between them. Its separator and terminator options change what goes between values and at the end:

print("A", "B", "C")                   // A B C
print("A", "B", separator: "-")        // A-B
print("Loading", terminator: "")       // no new line afterwards

Comments are notes for people that Swift ignores. // starts a comment that runs to the end of the line, and /* ... */ can span several lines. Use them to explain why code does something, rather than repeating what it obviously does.

Swift is a compiled language: before your code runs, the compiler checks the whole file and turns it into machine code your computer can execute directly. That's why typing mistakes are reported before a single line runs, and why Swift programs are fast.

From a terminal you can work in two ways:

swift hello.swift              # compile in memory and run straight away
swiftc hello.swift -o hello    # build a program file called hello
./hello                        # run the program you built

The first form is handy while learning, because one command does everything. The second produces a standalone program you can run again without compiling, or give to someone with the same kind of computer. Xcode and Swift Playgrounds do the same job for you behind a Run button.

print is meant for people, so it shows text without its quotes. debugPrint shows values the way you'd write them in code, which helps when you're hunting a bug and need to see exactly what a value contains:

print("Hi")          // Hi
debugPrint("Hi")     // "Hi"
debugPrint(3, "a")   // 3 "a"

With debugPrint you can spot a stray space at the end of a string, or tell the number 3 apart from the text "3". In finished apps, console output isn't visible to users at all, so treat both as tools for you, the developer.