Writing functions

Functions let us re-use code, which means we can write a function to do something interesting then run that function from lots of places. Repeating code is generally a bad idea, and functions help us avoid doing that.

To start with, we’re going to write a function that prints help information for users of our app. We might need this anywhere in our app, so having it as a function is a good idea.

Swift functions start with the func keyword, then your function name, then open and close parentheses. All the body of your function – the code that should be run when the function is requested – is placed inside braces.

Let’s write the printHelp() function now:

func printHelp() {
    let message = """
Welcome to MyApp!

Run this app inside a directory of images and
MyApp will resize them all into thumbnails
"""

    print(message)
}

We can now run that using printHelp() by itself:

printHelp()

Running a function is often referred to as calling a function.