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# LucaNicosia's solution

## to Nth Prime in the Kotlin Track

Published at Mar 20 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 {

private fun is_prime(num: Int): Boolean {
if(num == 3 || num == 2)
return true
if(num == 4 || num == 1) // special cases
return false
for(i in 3..num-2 step 2){
if(num%i == 0)
return false
}
return true
}

fun nth(n: Int): Int {
if(n <= 0)
throw IllegalArgumentException("There is no zeroth prime.")
var count = 0
var num = 2
while(true){
if(is_prime(num)){
count+=1
if(count == n)
return num
}
if(num!=2){
num+=2 // get only odd numbers
}else{
num+=1
}
}
}
}``````

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?