Filter an array of objects by property, using an array of values in Swift

Question

What is the most efficient way of filtering an array of objects based on one of their properties, using an array of values? I could iterate through the items, but I can't help thinking there's a really efficient way using Array.filter and Array.contains - I'm just not proficient enough with Swift to be able to put the pieces together.

For example, if I have an array containing Book objects, each of which has a String author property, how would I filter it to show only books by John Smith, Arthur Price or David Jones?

Something along the lines of:

Class Book {
    var author : String = String()
}

var books : Array = [Book]()
//books added elsewhere

let authors = ["John Smith", "Arthur Price", "David Jones"]

let filteredBooks = books.filter({authors.contains({($0 as Book).author})})

Do you need the easiest way or the most effective? If the most effective then for example you need first prepare the data and convert the array to the dictionary with the key - the search field and value - the Book object. After this action your search will be the most effective — Alexey Pichukov
– Alexey Pichukov, Commented Oct 22, 2015 at 19:47
@Alex, how would you apply that method to the command I just added above? — Jon-Paul
– Jon-Paul, Commented Oct 22, 2015 at 19:56
you're working with an array, in this case, the search may not be as fast as possible, to make it as fast as possible you need to work with dictionary. If you need fast search see answer below from Arsen, there is an example of what I mean — Alexey Pichukov
– Alexey Pichukov, Commented Oct 22, 2015 at 20:03

Jon-Paul · Accepted Answer · 2015-10-22 20:20:20Z

20

This is what I have working in a playground, any reason why this is no good?

class Book {
    var author = String()

    init(author:String){
        self.author = author
    }
}

var allBooks: [Book] = []

allBooks.append(Book(author: "John Smith"))
allBooks.append(Book(author: "Arthur Price"))
allBooks.append(Book(author: "David Jones"))
allBooks.append(Book(author: "Somebody Else"))

let authors = ["Arthur Price", "David Jones"]

let filteredBooks = allBooks.filter({authors.contains($0.author)})

filteredBooks       // [{author "Arthur Price"}, {author "David Jones"}]

answered Oct 22, 2015 at 20:20

Jon-Paul

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Comments

raddevus · Accepted Answer · 2023-07-20 18:52:44Z

7

You could also use something like

let authorsAndBooks = authors.map { 
   (authorName) -> (String, [Book]) 
      in (authorName, 
      allBooks.filter({ $0.author == authorName })
   ) 
 }

This will array of tuples with the first element being the author name and the second element an array of his books, in case an author wrote more than one book.

edited Jul 20, 2023 at 18:52

raddevus

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answered Oct 23, 2015 at 9:37

vikingosegundo

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1 Comment

rd_ Over a year ago

u will get multiple times the same authorName with corresponding book array everytime.

Arsen · Accepted Answer · 2015-10-22 20:06:26Z

3

I'd recommend you make an index of books by author:

let book = Book(author: "Arsen")
let bookIndex = [book.author: [book]]

And now you have incredible fast access to your book filtered by author:

bookIndex["Arsen"] // => [Books]

For multiple authors:

var results = [Book]()
for author in authors {
    if let books = bookIndex[author] {
        results += books
    }
}

results

edited Oct 22, 2015 at 20:06

answered Oct 22, 2015 at 19:48

Arsen

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4 Comments

Jon-Paul Over a year ago

this looks interesting, but how would you use an index to filter by multiple values?

Arsen Over a year ago

@Jon-Paul check out the update. I think it can be written in one-line style with map and flatmap but still it very efficient way

Jon-Paul Over a year ago

Thanks - and this would be quicker than working with the array as I've suggested?

Jon-Paul Over a year ago

To be clear, I don't need it to be as fast as possible. I'm just interested in using the best design patterns as I learn Swift. :)

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