The in Keyword

The in keyword tests whether a value is within a range.

// InKeyword/MembershipInRange.kt

fun main() {
  val percent = 35
  println(percent in 1..100)
}
/* Output:
true
*/

In Booleans, you learned to check bounds explicitly:

// InKeyword/MembershipUsingBounds.kt

fun main() {
  val percent = 35
  println(0 <= percent && percent <= 100)
}
/* Output:
true
*/

0 <= x && x <= 100 is logically equivalent to x in 0..100. IntelliJ IDEA suggests automatically replacing the first form with the second, which is easier to read and understand.

The in keyword is used for both iteration and membership. An in inside the control expression of a for loop means iteration, otherwise in checks membership:

// InKeyword/IterationVsMembership.kt

fun main() {
  val values = 1..3
  for (v in values) {
    println("iteration $v")
  }
  val v = 2
  if (v in values)
    println("$v is a member of $values")
}
/* Output:
iteration 1
iteration 2
iteration 3
2 is a member of 1..3
*/

The in keyword is not limited to ranges. You can also check whether a character is a part of a String. The following example uses in instead of hasChar() from the previous atom:

// InKeyword/InString.kt

fun main() {
  println('t' in "kotlin")
  println('a' in "kotlin")
}
/* Output:
true
false
*/

Later in the book you’ll see that in works with other types, as well.

Here, in tests whether a character belongs to a range of characters:

// InKeyword/CharRange.kt

fun isDigit(ch: Char) = ch in '0'..'9'

fun notDigit(ch: Char) =
  ch !in '0'..'9'               // [1]

fun main() {
  println(isDigit('a'))
  println(isDigit('5'))
  println(notDigit('z'))
}
/* Output:
false
true
true
*/
  • [1] !in checks that a value doesn’t belong to a range.

You can create a Double range, but you can only use it to check for membership:

// InKeyword/FloatingPointRange.kt

fun inFloatRange(n: Double) {
  val r = 1.0..10.0
  println("$n in $r? ${n in r}")
}

fun main() {
  inFloatRange(0.999999)
  inFloatRange(5.0)
  inFloatRange(10.0)
  inFloatRange(10.0000001)
}
/* Output:
0.999999 in 1.0..10.0? false
5.0 in 1.0..10.0? true
10.0 in 1.0..10.0? true
10.0000001 in 1.0..10.0? false
*/

You can only use .. to define a floating-point range in Kotlin.

You can check whether a String is a member of a range of Strings:

// InKeyword/StringRange.kt

fun main() {
  println("ab" in "aa".."az")
  println("ba" in "aa".."az")
}
/* Output:
true
false
*/

Here Kotlin uses alphabetic comparison.

Exercises and solutions can be found at www.AtomicKotlin.com.

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Expressions & Statements