In a pair of data, how do I determine which one is the key and which one is the value?
For instance, if I wanted to make a list of books and authors, do I put the author or the book as the key—and why?
Thanks! 
It’s usually contextual. The value very often tells you something about the key, but with a value alone, it’s worthless.
Imagine the number 14, then 29, then 45…
…
They mean nothing. But if you set them up as prices for three items: Book1, Book2TheSequel, Book3TheEpicFinale, it makes sense to match those values to their corresponding book.
However, you would rarely see a bookstore organize their books by price. (This is a very loose analogy, since online bookstores you can definitely filter things by price first if you really wanted to).
Also, a key may have a value that is an array of values. So one book title can hold much information (genre, author, price).
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When you phrase it like that, it’s very helpful! Thank you so much for going further into detail. I have a better understanding now, and will keep this in mind when doing more practice. Thanks again! 