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to Prime Factors in the Scala Track

Published at Sep 12 2019 · 0 comments
Instructions
Test suite
Solution

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

Example

What are the prime factors of 60?

  • Our first divisor is 2. 2 goes into 60, leaving 30.
  • 2 goes into 30, leaving 15.
    • 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
  • 3 goes cleanly into 15, leaving 5.
    • 3 does not go cleanly into 5. The next possible factor is 4.
    • 4 does not go cleanly into 5. The next possible factor is 5.
  • 5 does go cleanly into 5.
  • We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

  • 2 * 2 * 3 * 5
  • = 4 * 15
  • = 60
  • Success!

The Scala exercises assume an SBT project scheme. The exercise solution source should be placed within the exercise directory/src/main/scala. The exercise unit tests can be found within the exercise directory/src/test/scala.

To run the tests simply run the command sbt test in the exercise directory.

For more detailed info about the Scala track see the help page.

Source

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

PrimefactorsTest.scala

import org.scalatest.{Matchers, FunSuite}

/** @version 1.1.0 */
class PrimeFactorsTest extends FunSuite with Matchers {

  test("no factors") {
    PrimeFactors.factors(1) should be(List())
  }

  test("prime number") {
    pending
    PrimeFactors.factors(2) should be(List(2))
  }

  test("square of a prime") {
    pending
    PrimeFactors.factors(9) should be(List(3, 3))
  }

  test("cube of a prime") {
    pending
    PrimeFactors.factors(8) should be(List(2, 2, 2))
  }

  test("product of primes and non-primes") {
    pending
    PrimeFactors.factors(12) should be(List(2, 2, 3))
  }

  test("product of primes") {
    pending
    PrimeFactors.factors(901255) should be(List(5, 17, 23, 461))
  }

  test("factors include a large prime") {
    pending
    PrimeFactors.factors(93819012551l) should be(List(11, 9539, 894119))
  }
}
import scala.collection.mutable.ListBuffer

object PrimeFactors {

  def factors(input: Long): List[Int] = {
      primeFactor(2, input, ListBuffer.empty[Int])
  }

  def primeFactor(divisor: Int, input: Long, factors: ListBuffer[Int]): List[Int] = {
    input match {
      case input if(input > 1 && input % divisor == 0) => {
        factors+= divisor
        primeFactor(divisor, input/divisor, factors)
      }
      case input if(input > 1 && input % divisor != 0) => {
        primeFactor(divisor+1, input, factors)
      }
      case _ => factors.toList
    }
  }

}

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