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jaklec's solution

to Nth Prime in the Kotlin Track

Published at Jan 06 2021 · 0 comments
Instructions
Test suite
Solution

Given a number n, determine what the nth prime is.

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

If your language provides methods in the standard library to deal with prime numbers, pretend they don't exist and implement them yourself.

Setup

Go through the setup instructions for Kotlin to install the necessary dependencies:

https://exercism.io/tracks/kotlin/installation

Making the test suite pass

Execute the tests with:

$ gradlew test

Use gradlew.bat if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore annotation.

Source

A variation on Problem 7 at Project Euler http://projecteuler.net/problem=7

Submitting Incomplete Solutions

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

PrimeTest.kt

import org.junit.Test
import org.junit.Ignore
import org.junit.Rule
import org.junit.rules.ExpectedException
import kotlin.test.assertEquals

class PrimeTest {

    @Rule
    @JvmField
    var expectedException: ExpectedException = ExpectedException.none()

    @Test
    fun firstPrime() {
        assertEquals(2, Prime.nth(1))
    }

    @Ignore
    @Test
    fun secondPrime() {
        assertEquals(3, Prime.nth(2))
    }

    @Ignore
    @Test
    fun sixthPrime() {
        assertEquals(13, Prime.nth(6))
    }

    @Ignore
    @Test
    fun bigPrime() {
        assertEquals(104743, Prime.nth(10001))
    }

    @Ignore
    @Test
    fun undefinedPrime() {
        expectedException.expect(IllegalArgumentException::class.java)
        expectedException.expectMessage("There is no zeroth prime.")

        Prime.nth(0)
    }

}
object Prime {

    fun nth(n: Int): Int =
        when (n) {
            0 -> throw IllegalArgumentException("There is no zeroth prime.")
            1 -> 2 // by definition
            else -> findNthOddPrime(n)
        }

    fun findNthOddPrime(n: Int): Int {
                // We know 2 is the first and only even prime.
                val primes = mutableListOf(2)

                // Continue from 3 and only check odd numbers.
                for (i in 3 until Int.MAX_VALUE step 2) {
                    // Create a sieve of the it*it primes to exhaust all possible candiates.
                    // We have a prime if current candiate is divisible by none of the others. 
                    val isPrime = primes.takeWhile { it * it <= i }.all { i % it != 0 }

                    if (isPrime) {
                        primes.add(i)
                        if (primes.size == n) {
                            return i
                        }
                    }
                }
                throw Exception("Unexpectedly reached max integer value.")
    }
}

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?