Demonstrates the importance of limiting the scope of variables to the smallest possible scope.

LimitingScopeOfVariables.kt

package dev.hbrown

/**
 * Demonstrates the importance of limiting the scope of a variable using the Sieve of Eratosthenes to find prime numbers
 * using a sequence builder. The algorithm is conceptually very easy:
 *
 * 1. take a list of numbers starting at 2.
 * 2. remove the first number, it is prime.
 * 3. from the rest of the numbers, remove the first number as well as all the numbers divisible by this prime number.
 *
 * In [limitedScope] we limit the scope of the `prime` variable to the loop where it is needed.
 * In [scopeNotLimited] we attempt to 'optimise' the need to create the variable in every loop away by moving the
 * variable outside of the loop.
 *
 */

class LimitingScopeOfVariables {

    fun limitedScope() {

        val primes: Sequence<Int> = sequence {
            var numbers = generateSequence(2) { it + 1 }

            while (true) {
                val prime = numbers.first()
                yield(prime)
                numbers = numbers.drop(1).filter { it % prime != 0 }
            }
        }

        println("limitedScope:    ${primes.take(10).toList()}")

    }

    fun scopeNotLimited() {
        val primes: Sequence<Int> = sequence {
            var numbers = generateSequence(2) { it + 1 }

            var prime: Int
            while (true) {
                prime = numbers.first()
                yield(prime)
                numbers = numbers.drop(1).filter { it % prime != 0 }
            }
        }

        println("scopeNotLimited: ${primes.take(10).toList()}")
    }

    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            val eg = LimitingScopeOfVariables()
            eg.limitedScope()    //[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
            eg.scopeNotLimited() //[2, 3, 5, 6, 7, 8, 9, 10, 11, 12]
        }
    }

}