How to count matching items in an array

Swift version: 5.1

Paul Hudson    @twostraws    May 28th 2019

If you want to count how many items in an array (or any collection) match a test you specify, the easiest thing to do is run the collection through a call to filter() then count the remainder.

For example, if you had an array of numbers and wanted to count how many were odd, you would write this:

let count1 = [1, 2, 3, 4, 5].filter { $0 % 2 == 1 }.count

Because this is something that all collections might want to do, you should consider wrapping it in an extension on Collection, like this:

extension Collection {
    func count(where test: (Element) throws -> Bool) rethrows -> Int {
        return try self.filter(test).count
    }
}

With that change, counting the odd numbers becomes this:

let count2 = [1, 2, 3, 4, 5].count { $0 % 2 == 1 }

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