How to find the difference between two arrays

Paul Hudson    February 14th 2018    @twostraws

If you have two arrays that contain similar items and want to find out their differences – i.e., which items exist in one or the other, but not both – the easiest thing to do is use a Set. Sets have a symmetricDifference() method that does exactly this, so you just need to convert both arrays to a set, then convert the result back into an array.

Here’s an extension to make it easier:

extension Array where Element: Hashable {
    func difference(from other: [Element]) -> [Element] {
        let thisSet = Set(self)
        let otherSet = Set(other)
        return Array(thisSet.symmetricDifference(otherSet))
    }
}

And here’s some example code you can use to try it out:

let names1 = ["John", "Paul", "Ringo"]
let names2 = ["Ringo", "Paul", "George"]
let difference = names1.difference(from: names2)

That will set difference to be ["George", "John"], because those are the two names that only appear once in either array.

Available from iOS 8.0

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