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
  }