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liyuxuan5603's solution

to Nth Prime in the Swift Track

Published at Mar 25 2020 · 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 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

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.

LinuxMain.swift

import XCTest
@testable import NthPrimeTests

XCTMain([
    testCase(NthPrimeTests.allTests),
    ])

NthPrimeTests.swift

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
    }
}

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