maxBy() multiple attributes of objects in List()

Question

Say I have a list of books

val books = List(Book)

where Book(bookID: String, bookName: String) and a map

val scores = Map(bookID -> score:float)

I would like to get the book with the highest score first, then alphabetically by bookName

Getting the book with the highest rated score is rather simple:

val bestBook = books.maxBy(x => score(x.bookID))

But in case of multiple books with the same score I would then like to sort by the bookName. I'm not sure how to add a second criterion to maxBy? I suppose I can maxBy() score first, then I retrieve the score of bestBook, and filter books with the max score, then minBy() the bookName?

But that seems really cumbersome, is there a more efficient way of doing this? I've seen previous threads that does something similar with the Ordering implicit: Scala Ordering by multiple values, but I couldn't figure out how to define Ordering in this context.

Gaël J · Accepted Answer · 2021-10-23 15:43:26Z

3

Using maxBy by both the higher score (thanks to the - on the score) and the name:

books
  .maxBy(b => (-score(b.bookId), b.name))

The same ordering on tuple can be used for sorting if needed.

edited Oct 23, 2021 at 15:43

answered Oct 23, 2021 at 9:31

Gaël J

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4 Comments

jeb2 Over a year ago

ah I didn't think of that, but in terms of performance, isn't sortBy going to be O(nlog n) while maxBy is O(n) (i think?)

Gaël J Over a year ago

Right, you can use the same approach with maxBy on a tuple.

Tim Over a year ago

Don't recommend sort for a simple max operation.

Gaël J Over a year ago

Sure, edited the post :)

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