Swift 4 - Filtering Array with Array

Question

I have a list of entries, that have an id, and I want to filter them down to ones that have an entry.id matching one of the ids in selectedIDs. Is there a way to do this with filter or do I have to use a for loop?

struct Entry {
    let id: String
}
var allEntries = [Entry]()
var selectedIDs = [String]

e.g.

allEntries = [Entry(id: "1"), Entry(id:"2"), Entry(id:"3"), Entry(id:"4")]
selectedIDs = ["1", "3"]

// return selectedEntries
var selectedEntries = [Entry(id: "1"), Entry(id: "3")]

Code Different · Accepted Answer · 2018-10-15 13:34:06Z

7

There's nothing wrong with Rakesha Shastri's answer. For performance reasons, you may want to make selectedIDs a Set instead of an Array:

let allEntries = [Entry(id: "1"), Entry(id:"2"), Entry(id:"3"), Entry(id:"4")]
let selectedIDs: Set<String> = ["1", "3"]
let selectedEntries = allEntries.filter({ selectedIDs.contains($0.id) })

The reason is that searching an Array has a computational complexity of O(n) where n is the length of the array, while searching a Set (i.e. a hash table) is O(1) on average.

If you keep selectedIDs as array, the overall solution has a complexity of O(n * m) where n and m are the lengths of selectedIDs and allEntries, respectively.
If you use Set, the overall complexity reduces to O(m).

Having said that, your example is too trivial for either methods to make a difference.

edited Oct 15, 2018 at 13:34

answered Oct 15, 2018 at 11:08

Code Different

93.4k16 gold badges154 silver badges175 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rakesha Shastri · Accepted Answer · 2018-10-15 06:59:39Z

4

Filter allEntries based on whether selectedIDs contains id.

var allEntries = [Entry(id: "1"), Entry(id:"2"), Entry(id:"3"), Entry(id:"4")]
var selectedIDs = ["1", "3"]

var selectedEntries = allEntries.filter({ selectedIDs.contains($0.id) })

edited Oct 15, 2018 at 6:59

answered Oct 15, 2018 at 6:56

Rakesha Shastri

11.2k4 gold badges40 silver badges53 bronze badges

4 Comments

user3628240 Over a year ago

Thank you. Out of curiosity, if say selectedIDs were another type of struct with IDs as a property, how would this change the selectedIDs.contains($0.id) part?

Ashley Mills Over a year ago

map then filter

Rakesha Shastri Over a year ago

@user3628240 like Ashley said, you map the struct array to a string array and then do the filtering.

ielyamani Over a year ago

Contains(where:) would be more appropriate developer.apple.com/documentation/swift/array/2297359-contains

Baluta Eugen · Accepted Answer · 2018-10-17 09:48:08Z

1

You can use map(:) and filter(:) to achieve this

let matchingEntries = allEntries.map({$0.id}).filter({selectedIDs.contains($0)})

.map will return you just id's as an array, then .filter will filter them with the selectedIDs

answered Oct 17, 2018 at 9:48

Baluta Eugen

951 silver badge13 bronze badges

Comments

Bogdan Bystritskiy · Accepted Answer · 2022-02-10 11:35:17Z

1

Swift 5.5:

let filteredArray = array.filter { self.selectedIDs.map(\.id).contains($0.id) }

answered Feb 10, 2022 at 11:35

Bogdan Bystritskiy

1,34312 silver badges10 bronze badges

Collectives™ on Stack Overflow

Swift 4 - Filtering Array with Array

4 Answers 4

Comments

4 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related