var arr = [2, 3, 4, 5]

func apply(for array: [Int], transform: (Int) -> Int) -> [Int] {
    var result: [Int] = []
    for element in array {
        result.append(transform(element))
    }
    return result
}

apply(for: arr) { $0 * 3 } //[6, 9, 12, 15]