Eliminating Duplicates in Arrays

Arrays are an ordered random access collection which contains elements of a defined data type. These types can be anything from an Int to a defined model. An array is something that you will see very often within Swift projects when organizing your apps data.

Array Example

let numbers = [1, 3, 3, 5, 4, 4]
let names = ["Billy", "Sam", "Emily", "Bob", "Sam", "Billy"]

Lets check out the methods we can use to eliminate the duplicates in the arrays above.

Using Swift Algorithms

At WWDC 2021, Apple announced two new open source frameworks, one of them being Swift Algorithms. This is a stand-alone package that can be added to your project. Within the Swift Algorithm package contains a method called .uniqued() which removes duplicates from your array.

The only requirement to using this method, and much of Swift Algorithms is that your array elements must conform to Hashable. After adding the package to your project, you will simply use it in the following manner.

import Algorithms

let numbers = [1, 3, 3, 5, 4, 4]
let uniquedNumbers = numbers.uniqued()
// [1, 3, 5, 4]

let names = ["Billy", "Sam", "Emily", "Bob", "Sam", "Billy"]
let uniquedNames = names.uniqued()
// ["Billy", "Sam", "Emily", "Bob"]

This will return uniquedNumbers as a UniquedSequence which conforms to LazySequenceProtocol. You can then call that variable wherever you would call your array.

Using Array Extension

I often times don’t like adding dependencies unless absolutely needed. When adding them, I always way out the pros and cons, and if I can do it myself - I will. This method eliminates the need for the dependency and uses an array extension to filter the array and remove the duplicates. It will return a new array, without the duplicates.

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()
    }
}

Then to use the above extension, we’ll simply call the function on an array, much like we would do in the Swift Algorithms example.

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

let names = ["Billy", "Sam", "Emily", "Bob", "Sam", "Billy"]
let uniquedNames = names.removingDuplicates() 
// ["Billy", "Sam", "Emily", "Bob"]

Converting from Array to Set

A Set is an unordered collection with unique elements. When you convert an array over to a Set, you will automatically eliminate duplicates.

let numbers = [1, 3, 3, 5, 4, 4]
let uniquedNumbers = Set(numbers)
// [1, 3, 5, 4]

let names = ["Billy", "Sam", "Emily", "Bob", "Sam", "Billy"]
let uniquedNames = Set(names) 
// ["Billy", "Sam", "Emily", "Bob"]

You can avoid converting your new Set back to an Array if the following things are needed:

• Elements need to be unique.
• Order does not matter.
• The API you may use does NOT require the use of an array.
• You’re storing Hashable types.
• Speed matters as, using .contains() on a set takes the same amount of time if you have one element, as it does if you have one hundred items. This is a O(1) operation.