Avatar of w1zeman1p

w1zeman1p's solution

to Nth Prime in the Ruby Track

Published at Feb 07 2019 · 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.


For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby nth_prime_test.rb

To include color from the command line:

ruby -r minitest/pride nth_prime_test.rb

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.

nth_prime_test.rb

require 'minitest/autorun'
require_relative 'nth_prime'

# Common test data version: 2.1.0 4a3ba76
class NthPrimeTest < Minitest::Test
  def test_first_prime
    # skip
    assert_equal 2, Prime.nth(1)
  end

  def test_second_prime
    skip
    assert_equal 3, Prime.nth(2)
  end

  def test_sixth_prime
    skip
    assert_equal 13, Prime.nth(6)
  end

  def test_big_prime
    skip
    assert_equal 104743, Prime.nth(10001)
  end

  def test_there_is_no_zeroth_prime
    skip
    assert_raises(ArgumentError) do
      Prime.nth(0)
    end
  end
end
class Prime
  def self.nth(n)
    raise ArgumentError if n <= 0
    primes = []
    i = 2
    while primes.length < n
      if Prime.is_prime?(primes, i)
        primes << i
      end
      i += 1
    end
    primes.last
  end

  def self.is_prime?(primes, x)
    primes.each do |prime|
      # Return early if the number in question is evenly divisible by a prime.
      return false if x != prime && x % prime == 0
    end
    true
  end
end

Community comments

Find this solution interesting? Ask the author a question to learn more.

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?