Published at Mar 25 2020
·
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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.

Go through the project setup instructions for Xcode using Swift:

http://exercism.io/languages/swift

http://exercism.io/languages/swift/tests

Notably from the source directory:

`swift test`

runs tests

`swift package generate-xcodeproj`

creates an Xcode project

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

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

```
import XCTest
@testable import NthPrimeTests
XCTMain([
testCase(NthPrimeTests.allTests),
])
```

```
import XCTest
@testable import NthPrime
class NthPrimeTests: XCTestCase {
func testFirst() {
XCTAssertEqual(2, Prime.nth(1))
}
func testSecond() {
XCTAssertEqual(3, Prime.nth(2))
}
func testSixthPrime() {
XCTAssertEqual(13, Prime.nth(6))
}
func testBigPrime() {
XCTAssertEqual(104_743, Prime.nth(10_001))
}
func testWeirdCase() {
XCTAssertNil(Prime.nth(0))
}
static var allTests: [(String, (NthPrimeTests) -> () throws -> Void)] {
return [
("testFirst", testFirst),
("testSecond", testSecond),
("testSixthPrime", testSixthPrime),
("testBigPrime", testBigPrime),
("testWeirdCase", testWeirdCase),
]
}
}
```

```
//Solution goes in Sources
struct Prime {
static func nth(_ nthNum:Int) -> Int??{
var count = 1
var num = 3
if nthNum < 1{
return nil
}
else if nthNum == 1 {
return 2
}
else{
while(count < nthNum){
if (isPrime(num)){
count += 1
}
num += 1
}
return num-1
}
}
static func isPrime(_ thisNum: Int) -> Bool{
for index in 2...Int((thisNum+1)/2){
if (thisNum % index == 0){
return false
} }
return true
}
}
```

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?

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