All good computer books that are written to teach a programming language have a section that shows a user how to write a Hello World application. This book is no exception. In this section, we will show you how to write a Hello World application with Swift.

Let's begin by creating a new file named main.swift. The main.swift file is a special file in Swift and is the entry point for our application. It is the only file that can contain top-level code. Top-level code is the code that is not part of a function or type (enumeration, class, or structure). All of the code for our Hello World application is considered top-level code.

In Swift, to print a message to the console, we use the print() function. In its most basic form, we would use the print function to print out a single message as shown in the following code:

print("Hello World") 

Usually, when we use the print() function, we want to print more than just static text. We can include the value of variables and/or constants using a special sequence of characters, \( ), or by separating the values within the print() function with commas. The following code shows how to do this:

var name = "Jon" 
var language = "Swift" 
 
var message1 = "Welcome to the wonderful world of " 
var message2 = "\(name) Welcome to the wonderful world of \(language)!" 
 
print(name, message1, language, "!") 
print(message2) 

In order to compile this code, it will need to be in a file name main.swift. You can use any text editor, like emacs or VI, to create this file. Once the main.swift file is created, we will need to build our Hello World application. Type the following line in the same directory where the main.swift file is located:

    swiftc main.swift

Once the swift compiler finishes building the application, we will have an executable file named main in our directory. We can run the file using the following command:

    ./main 

If all goes well, you can see the following output:

Jon Welcome to the wonderful world of Swift!
Jon Welcome to the wonderful world of Swift!

We will look at the swift and swiftc commands later in this chapter.

We can also define two parameters in the print function that change how the message is displayed in the console. These parameters are the separator and terminator parameters. The separator parameter defines a string that is used to separate the values of the variable/constant in the print() function. By default, the print() function separates each variable/constant with a space. The terminator parameter defines which character is inserted at the end of the line. By default, the newline character is added at the end of the line.

The following code shows how we could create a comma-separated list that does not have a newline character at the end:

var name1 = "Jon" 
var name2 = "Kim" 
var name3 = "Kailey" 
var name4 = "Kara" 
 
print(name1, name2, name3, name4, separator:", ", terminator:"") 

There is one other parameter that we can add to our print() function. This is the toStream parameter. This parameter will let us redirect the output of the print() function. In the following example, we redirect the output to a variable named line:

var name1 = "Jon" 
var name2 = "Kim" 
var name3 = "Kailey" 
var name4 = "Kara" 
 
var line = "" 
 
print(name1, name2, name3, name4, separator:", ",  terminator:"", to:&line) 

The print() function was simply a useful tool for basic debugging, but now we can use the new enhanced print() function a lot more.