For me, one of the worst experiences when coding along with books is when structures are left undefined like this. I feel it should at least be mentioned in the text that it will be defined later, and even then not left until after the tests. When this happens, there’s always the feeling in the back of my mind that the author has made a mistake (ever tried to read a packt book? 90% of the ones I’ve seen are unreadable. There are a few gems though ) Personally I would have defined it up front along with index, and explained it’s properties. That’s just my preference though.

Would like to chime in with @disknoir, while during the reading process it’s ok to see the data structure usage before it’s definition, but if you code along with the book it’s a super frustrating to see the tests, that actually you not able to run, because you missing some bits