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## to Prime Factors in the Scala Track

Published at Aug 31 2019 · 1 comment
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.annotation.tailrec

object PrimeFactors {
def factors(n: Long): List[Long] = {
@tailrec
def fact(acc: List[Long], n: Long, candidate: Long): List[Long] = n match {
case _ if n < 2 => acc
case _ if n % candidate == 0 => fact(acc :+ candidate, n / candidate, candidate)
case _ => fact(acc, n, candidate + 1)
}

fact(List(), n, 2)
}
}`````` 