swift小知识之Stride函数

stride函数，为可变步长类型值的序列

• from to

  其中from to，最后一个值将会小(大)于to的值

  示例一：
  

  for index in stride(from:0, to:3, by:1)  {
      print(index)
  }
  // print 0, 1, 2

  示例二：

  for index in stride(from:3, to:0, by:-1) {
      print(index)
  }
  // print 3，2，1

• from through

  而from through，最后一个值将会小(大)于等于through的值

  示例一：

  for index in stride(from:0, through:3, by:1)  {
      print(index)
  }
  // print 0, 1, 2, 3

  示例二：

  for index in stride(from:3, through:0, by:-1)  {
      print(index)
  }
  // print 3，2，1，0

• 调整by参数(步长)

  示例一：to，不包含末尾

  for index in stride(from: 0, to: 9, by: 3) {
      print(index)
  }
  // print 0, 3, 6

  示例二：through，包含末尾

  for index in stride(from: 0, through: 9, by: 3) {
    print(index)
  }
  // prints 0, 3, 6, 9

• 数组分割

  ps:步长与数组元素个数刚好整数倍，否则多出的元素会舍去

  示例一：

  var array = Array(1...10)
  let group1Arr = stride(from: 0, to: 8, by: 4).map {
      print(array[$0...$0+3])
  }
  //[1, 2, 3, 4]
  //[5, 6, 7, 8]

  示例二：

  let group2Arr = stride(from: 0, to: 8, by: 4).map {
        Array(array[$0...$0+3])
  }
  print(group2Arr) 
  // [[1, 2, 3, 4], [5, 6, 7, 8]]

  示例三：

  let step = 3
  let finalArray = stride(from: 0, to: array.endIndex - (array.endIndex % step), by: step).map {
      Array(array[$0...$0+(step - 1)])
  }
  // step = 2
  // [[1, 2], [3, 4], [5, 6], [7, 8], [9, 10]]
  // step = 3
  // [[1, 2, 3], [4, 5, 6], [7, 8, 9]]