Use reduce, in Swift 1.2, to filter a list of objects and create a array with objects with a specific property not nil. Based on an idea of Berik (https://github.com/berikv).

flatMapTwo.swift

let list: [String?] = [nil, nil, "ciao", "hello", nil]

struct Obj {
    var group: [String?]?
    let name: String?
}

let o1 = Obj(group: list, name: "first")
let o2 = Obj(group: nil, name: "second")

let list2 = [o1, o2]

list2.reduce([Obj]()) {
    if let e = $1.group {
        return $0 + [$1]
    } else {
        return $0
    }
}