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范围

范围表达式是由“rangeTo”函数组成的，操作符的形式是..由in和!in补充。 范围被定义为任何可比类型,但是用于原生类型有更优化的实现。下面是使用范围的例子

if (i in 1..10) { // equivalent of 1 <= i && i <= 10
  println(i)
}

整型范围（IntRange, LongRange, CharRange）有一个额外的功能:他们可以遍历。 编译者需要关心的转换是简单模拟Java的索引for循环,不用担心越界。例如：

for (i in 1..4) print(i) // prints "1234"

for (i in 4..1) print(i) // prints nothing

你想要遍历数字颠倒顺序吗?这很简单。您可以使用标准库里面的downTo()函数

for (i in 4 downTo 1) print(i) // prints "4321"

是否可以任意进行数量的迭代,而不必每次的变化都是1呢?当然, step()函数可以实现

for (i in 1..4 step 2) print(i) // prints "13"

for (i in 4 downTo 1 step 2) print(i) // prints "42"

它是如何工作的

Ranges implement a common interface in the library: ClosedRange<T>.

ClosedRange<T> 在数学意义上表示一个间隔,是对比较类型的定义。 它有两个端点:‘start’和‘endInclusive’,这是包含在范围内。 主要的操作是contains,通常用in /!in {: .keyword }操作符内。

Integral type progressions (IntProgression, LongProgression, CharProgression) denote an arithmetic progression. Progressions are defined by the first element, the last element and a non-zero increment. The first element is first, subsequent elements are the previous element plus increment. The last element is always hit by iteration unless the progression is empty.

A progression is a subtype of Iterable<N>, where N is Int, Long or Char respectively, so it can be used in for-loops and functions like map, filter, etc. 迭代Progression与Java/JavaScript的基于索引的for循环等价:

for (int i = first; i != last; i += increment) {
  // ...
}

对于整型, ..操作符创建一个对象既实现了ClosedRange也实现了Progression。 For example, IntRange implements ClosedRange<Int> and extends IntProgression, thus all operations defined for IntProgression are available for IntRange as well. downTo()和 step()函数的结果一直是Progression。

Progressions are constructed with the fromClosedRange function defined in their companion objects:

  IntProgression.fromClosedRange(start, end, increment)

The last element of the progression is calculated to find maximum value not greater than the end value for positive increment or minimum value not less than the end value for negative increment such that (last - first) % increment == 0.

一些实用函数

rangeTo()

整型得rangeTo()运算符只要简单地调用构造函数*Range类,例如:

class Int {
  //...
  operator fun rangeTo(other: Long): LongRange = LongRange(this, other)
  //...
  operator fun rangeTo(other: Int): IntRange = IntRange(this, other)
  //...
}

Floating point numbers (Double, Float) do not define their rangeTo operator, and the one provided by the standard library for generic Comparable types is used instead:

  public operator fun <T: Comparable<T>> T.rangeTo(that: T): ClosedRange<T>

The range returned by this function cannot be used for iteration.

downTo()

downTo()的扩展函数可以为任何数字整型对定义,这里有两个例子:

fun Long.downTo(other: Int): LongProgression {
  return LongProgression.fromClosedRange(this, other, -1.0)
}

fun Byte.downTo(other: Int): IntProgression {
  return IntProgression.fromClosedRange(this, other, -1)
}

reversed()

定义reversed()扩展函数是为了每个 *Progression 类定义的,它们返回反向的级数。

fun IntProgression.reversed(): IntProgression {
  return IntProgression.fromClosedRange(last, first, -increment)
}

step()

step()扩展函数是为每个 *Progression 类定义的, 他们返回级数与都修改了step值(函数参数)。 注意,step值必需总是正的，因此这个函数从不改变的迭代方向。

fun IntProgression.step(step: Int): IntProgression {
  if (step <= 0) throw IllegalArgumentException("Step must be positive, was: $step")
  return IntProgression.fromClosedRange(first, last, if (increment > 0) step else -step)
}

fun CharProgression.step(step: Int): CharProgression {
  if (step <= 0) throw IllegalArgumentException("Step must be positive, was: $step")
  return CharProgression.fromClosedRange(first, last, step)
}

Note that the last value of the returned progression may become different from the last value of the original progression in order to preserve the invariant (last - first) % increment == 0. Here is an example:

  (1..12 step 2).last == 11  // progression with values [1, 3, 5, 7, 9, 11]
  (1..12 step 3).last == 10  // progression with values [1, 4, 7, 10]
  (1..12 step 4).last == 9   // progression with values [1, 5, 9]