The `in` operator checks whether a value belongs to a range or collection; `!in` checks that it does not. The same syntax also supports iteration, while membership checks make validation and classification concise. section 2.4.4 section 2.6
Check characters with ranges
fun isLetter(c: Char) = c in 'a'..'z' || c in 'A'..'Z' fun isNotDigit(c: Char) = c !in '0'..'9' println(isLetter('q')) // true println(isNotDigit('x')) // true The range hides the underlying comparison between character codes. Membership checks can also appear directly in `when` branches, such as `in '0'..'9'` for digit recognition.
Membership works across several kinds of containers and ranges:
- Use comparable objects in ranges: `"Kotlin" in "Java".."Scala"` is true because strings are compared alphabetically. section 2.4.4
- Use open ranges such as `10..<20` when the ending value should be excluded; closed ranges such as `10..20` include both endpoints. section 9.3.2
Kotlin implements membership through an operator convention. In `element in container`, the right-hand object receives a `contains(element)` call. A class can therefore define its own membership behavior with an `operator fun contains(...)` function. section 9.3.2 section 9.3
