more on scala Streams

Been a crazy week of Hackathon with no energy for standard in! Back today with some more fundamentals. The Scala Stream class is very handy for problem-solving like with Project Euler. I’ve been using it as an alternative to the imperative style looping, i.e. using a mutable counter and incrementing it until a condition is met.

Use case one: Stream all numbers from 1 until a condition is met

Example in the wild: Project Euler Problem 52

Turn a number into a canonical representation:

def getCanonical(n: Int): String = n.toString.split("").sorted.mkString

(e.getCanonical(125874) == e.getCanonical(251748)) should be(true)

Given n: Int, for i <- 2 to 6, return true if getCanonical(n * i) is equal in all cases

def allAreSame(n: Int): Boolean = {
    (2 to 6).map(_ * n).map(getCanonical).distinct.size == 1
}

Stream lazily until the condition is met

Stream.from(1).find(allAreSame)
// Option[142857]

–

Use case two: given a map function f = (Int) => Int and a value, max: Int, find all values of n where f(n) < max

Example in the wild: Project Euler Problem 42

Map function

def computeTriangle(n: Int): Int = ((0.5 * n) * (n + 1)).toInt

Return Stream where f(n) < max

def triNumsUnder(max: Int): Stream[Int] = {
    Stream.from(1).map(computeTriangle).takeWhile(_ < max)
}

e.triNumsUnder(56).toList should be(List(1, 3, 6, 10, 15, 21, 28, 36, 45, 55))

Rest of my solution here