Ranges Reloaded.md

title	date	author	tags	keywords	categories	reward	reward_title	reward_wechat	reward_alipay	source_url	translator	translator_url
Ranges Reloaded	2013-02-06 06:47:00 -0800	Evgeny Gerashchenko			官方动态	false	Have a nice Kotlin!			https://blog.jetbrains.com/kotlin/2013/02/ranges-reloaded/

In Kotlin M5 we have redesigned our ranges a little bit. Range expressions are formed with rangeTo functions that have the operator form of .. which are complemented by in and !in . Range is defined for any comparable type (subclass of Comparable), but for number primitives it is optimized. Here are examples of using ranges:

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if (i in 1..10) { // equivalent of 1 <= i && i <= 10
  println(i)
}
 
if (x !in 1.0..3.0) println(x)
 
if (str in "island".."isle") println(str)
// equivalent of "island" <= str && str <= "isle"

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Numerical ranges have extra feature: they can be iterated over. Compiler takes care about converting this in simple analogue of Java’s indexed for-loop, without extra overhead. Examples:

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for (i in 1..4) print(i) // prints "1234"  
 
for (i in 4..1) print(i) // prints nothing
 
for (x in 1.0..2.0) print("$x ") // prints "1.0 2.0 "

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What if you want to iterate over numbers in reversed order? It’s simple. You can use downTo() function defined in standard library:

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

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Is it possible to iterate over numbers with arbitrary step, not equal to 1? Sure, step() function will help you:

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for (i in 1..4 step 2) print(i) // prints "13"  
 
for (i in 4 downTo 1 step 2) print(i) // prints "42"  
 
for (i in 1.0..2.0 step 0.3) print("$x ") // prints "1.0 1.3 1.6 1.9 "

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How it works

There are two traits in the library: Range<T> and Progression<N>. Range<T> denotes an interval in the mathematical sense, defined for comparable types. It has two endpoints: start and end, which are included in the range. Main operation is contains(), usually used in the form of in/!in operators. Progression<N> denotes arithmetic progression , defined for number types. It has start, end and non-zero increment. Progression<N> is a subtype of Iterable<N>, so it can be used in for-loops and functions like map, filter, etc. First element is start, every next element equals previous plus increment. Iteration over Progression is equivalent to an indexed for-loop in Java/JavaScript:

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// if increment > 0
for (int i = start; i <= end; i += increment) {
  // ...
}
 
// if increment < 0
for (int i = start; i >= end; i += increment) {
  // ...
}

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For numbers, the “..” operator creates an object which is both Range and Progression. Result of downTo() and step() functions is always a Progression.