FREE: Read a new Swift article every day – click here! >>

How to remove duplicate items from an array

Paul Hudson       @twostraws

There are several ways of removing duplicate items from an array, but one of the easiest is with the following extension on Array:

extension Array where Element: Hashable {
    func removingDuplicates() -> [Element] {
        var addedDict = [Element: Bool]()

        return filter {
            addedDict.updateValue(true, forKey: $0) == nil
        }
    }

    mutating func removeDuplicates() {
        self = self.removingDuplicates()
    }
}

That provides two methods: one called removingDuplicates() that returns an array with duplicates removed, and one called removeDuplicates() that changes the array in place.

The method works using filter() and a dictionary: when you call updateValue() on a dictionary it returns nil if the key is new, so we can use that to figure out which items are unique.

For example:

let numbers = [1, 5, 3, 4, 5, 1, 3]
let unique = numbers.removingDuplicates()

Available from iOS 8.0

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Get six books for only $150

The Swift Power Pack includes my first six books for one low price, helping you jumpstart a new career in iOS development – check it out!

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Average rating: 5.0/5

Click here to visit the Hacking with Swift store >>