Ranges

Imagine a situation where you need to check whether the integer variable c is greater than or equal to a and less than or equal to b. To do that you may write something like this:

val within = a <= c && c <= b

This code works well. However, Kotlin provides a more convenient way to do the same thing using ranges:

val within = c in a..b

Here, a..b is a range of numbers from a to b (including both border values), in is a special keyword that is used to check whether a value is within a range. Later you will see that this keyword can be used with other types as well. This is a closed-ended range.

Also, we have an open-ended range: a..<b is a range of numbers from a until b (excluding the border value, b).

The value of within is true if c belongs to the range inclusively; otherwise, it is false.

Here are some examples:

println(5 in 5..15)  // true
println(12 in 5..15) // true
println(15 in 5..15) // true
println(20 in 5..15) // false
println(5 in 5..<15)  // true
println(15 in 5..<15) // false

If you need to exclude the right border, you may subtract 1 from it or use ..< to get the open-ended range (the recommended way).

val withinExclRight = c in a..b - 1 // a <= c && c < b
val withinExclRight = c in a..<b // a <= c && c < b (the recommended way)

If you need to check that a value is not within a range, just add ! (not) before in.

val notWithin = 100 !in 10..99 // true

You may combine ranges using standard logical operators. The code below checks if c is within one of three ranges.

val within = c in 5..10 || c in 20..30 || c in 40..50 // true if c is within at least one range

You can assign a range to a variable and use it later.

val range = 100..200
println(300 in range) // false

In addition to integer ranges, you can also use ranges of characters and even strings (according to dictionary order).

println('b' in 'a'..'c') // true
println('k' in 'a'..'e') // false

println("hello" in "he".."hi") // true
println("abc" in "aab".."aac") // false

This is enough to understand ranges for integer numbers and characters. We won't cover other type ranges here.