Swift Array map, filter 和 flatMap使用

用数组的reduce方法实现map的功能
let arr = [1, 3, 2]
let res = arr.reduce([]) {
(a: [Int], element: Int)->[Int] in
var t = Array(a)
t.append(element * 2)
return t
}

用reduce方法一次求出数组中奇数的和、以及偶数乘积
let arr = [1, 3, 2, 4]
let res = (Int, Int) = arr.reduce((0, 1)) {
(a: (Int, Int), element: Int)->(Int, Int) in
if element % 2 == 0 {
return (a.0, a.1 * element)
}else {
return (a.0 + element, a.1)
}
}
// res = (4, 8)

求一个数组中的偶数的平方和，链式调用
let arr = [1, 3, 2, 4]
let res = arr.filter {
$0 % 2 == 0
}.map {
$0 * $0
}.reduce(0) {
$0 + $1
}