If I have a list of [1, 7, 5] and then a separate List<Book> and book has a property called id...

# getting-started

[Colton Idle]

If I have a list of [1, 7, 5] and then a separate List<Book> and book has a property called id. How do I sort this list of books in the same order of my original list?

[ephemient]

do you mean

val booksById = books.associateBy { it.id }
ids.map { booksById[it] }

or

books.sortedWith(compareBy(compareBy(ids::indexOf), Book::id))

?

[Colton Idle]

so the firstList should be thought of as sortOrderList, and the second list is just random order books, that have an id.

I think the second one is what i want. let me try when i get home!

[Colton Idle]

Yep, the mapper variant worked for me. Hadn't thought about that route. thanks @ephemient

[nkiesel]

I though you asked for

data class Book(val id: Int, val title: String)
val lb = listOf(Book(5, "Five"), Book(1, "One"), Book(7, "Seven"))
val li = listOf(1, 7, 5)
lb.sortedBy { li.indexOf(it.id) }

[ephemient]

oh right, I was originally writing compareBy(compareBy(ids::indexOf, naturalOrder()), Book::id), but then I decided that keeping the original for unspecified items was probably a better choice, in which case sortedBy is sufficient

[ephemient]

if the list of IDs contains all the IDs, and all the IDs are unique, then all the methods here return the same value. but if the list of IDs is a subset, then the results are different

[nkiesel]

yeah, right. I assumed that the id list is just a re-ordered list of ids of the books list. If not, then all the corner cases become important to handle, and we would need more info on requirements.

[ephemient]

data class Book(val id: Int)
val ids = listOf(0, 2, 4)
val books = List(5) { Book(5 - it) }

ids.map(books.associateBy { it.id }::get) // => [null, Book(2), Book(4)]
books.sortedBy { ids.indexOf(it.id) } // => [Book(5), Book(3), Book(1), Book(2), Book(4)]
books.sortedBy { ids.indexOf(it.id) + Int.MIN_VALUE } // => [Book(2), Book(4), Book(5), Book(3), Book(1)]

[nkiesel]

if (ids.sorted() == books.map(Book::id).sorted()) books.sortedBy { ids.indexOf(it.id) } else books.shuffled()

🙂

[ephemient]

of course there's the classic

while (ids != books.map(Book::id)) books.shuffle()

😆

[nkiesel]

(it's Friday, after all...)

[nkiesel]

that's a good one as well

[nkiesel]

we need an Easter of Code!

[Colton Idle]

lmao. Thanks all for teaching!