Simultaneous map and filter in Swift

gistfile1.swift

extension Array {
    // Similar to Lisp's map function that allows returning nil in the transform to omit the item from the resulting array.
    func mapAndFilter<U>(transform: (T) -> U?) -> U[] {
        var results = U[]()
        for item in self {
            if let transformedItem = transform(item) {
                results.append(transformedItem)
            }
        }
        return results
    }
}

// Examples

struct City {
    var name = ""
    var state = ""
    var inCalifornia: Bool {
        return state == "CA"
    }
}

var cities = [City(name: "San Francisco", state: "CA"), City(name: "Portland", state: "OR"), City(name: "Seattle", state: "WA")]

// 1. Say you wanted to get all the city names matching a certain condition with just map() and filter().

let californianCities = cities.filter {
    (city: City) -> Bool in
    return city.inCalifornia
}

let californianCityNames = californianCities.map {
    (city: City) -> String in
    return city.name
}

// 2. With mapAndFilter() it is more expressive as they're combined into one transformation:

let californianCityNames2 = cities.mapAndFilter {
    (city: City) -> String? in
    return city.inCalifornia ? city.name : nil
}