Odd, even

How do we compute the parity of numbers—whether they are even or odd? With Scala 3.3 we use modulo division to see if 2 divides a number evenly (with no remainder).

With def, we define two small functions that return Booleans. With isEven we return true if a number is evenly divisible by 2. And with isOdd we do the opposite.

Example methods

Let us begin. With isEven we use a short syntax form in Scala to return whether the modulo division of a number by 2 equals 0. With isOdd we simply return "not isEven."

object Program {
    // These functions test for even and odd numbers.
    def isEven(number: Int) = number % 2 == 0
    def isOdd(number: Int) = !isEven(number)

    def main(args: Array[String]): Unit = {
        // Generate a range of numbers to test.
        val tests = List.range(-2, 10)

        // Loop over numbers and indicate whether they are even or odd.
        for (n <- tests) {
            print(n)
            if (isEven(n)) {
                println(" -> Even")
            }
            if (isOdd(n)) {
                println(" -> Odd")
            }
        }
    }
}
-2 -> Even
-1 -> Odd
0 -> Even
1 -> Odd
2 -> Even
3 -> Odd
4 -> Even
5 -> Odd
6 -> Even
7 -> Odd
8 -> Even
9 -> Odd

Filter, generate sequences

Let us add some complexity. We repeat our isEven and isOdd numbers. We use Seq.range and then filter() to generate sequences of even and odd numbers.

object Program {
    // Tests parity of numbers.
    def isEven(number: Int) = number % 2 == 0
    def isOdd(number: Int) = !isEven(number)

    def main(args: Array[String]): Unit = {
        // Generate even numbers.
        println("Filter even")
        val evens = Seq.range(0, 10).filter(isEven(_))
        println(evens)

        // Generate odd numbers.
        println("Filter odd")
        val odds = Seq.range(0, 10).filter(isOdd(_))
        println(odds)

        // Another way to get odd numbers.
        println("Filter not even")
        val odds2 = Seq.range(0, 10).filterNot(isEven(_))
        println(odds2)
    }
}
Filter even
Vector(0, 2, 4, 6, 8)
Filter odd
Vector(1, 3, 5, 7, 9)
Filter not even
Vector(1, 3, 5, 7, 9)

Testing a number's parity is simple. And with methods like filter and filterNot we can generate ranges of numbers based on their parity.