Read these articles alongside reading Apple's Swift Programming Language Book.
The intended audience for this guide are people that don't know Swift and possibly don't know any other programming language.
We've covered the basics up to functions, and now I'll provide exercises that review topics from all of those chapters.
At this point, you should have read Subscripts in The Swift Programming Language. You should have a Playground page for this chapter with code in it that you generated while reading the book.