How to remove duplicate items from an array

Written by 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

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