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PatrickMcSweeny's solution

to Nth Prime in the Ruby Track

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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.


For installation and learning resources, refer to the exercism help 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: 1.0.0 016d65b
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) { Prime.nth(0) }
  end

  # Problems in exercism evolve over time, as we find better ways to ask
  # questions.
  # The version number refers to the version of the problem you solved,
  # not your solution.
  #
  # Define a constant named VERSION inside of the top level BookKeeping
  # module, which may be placed near the end of your file.
  #
  # In your file, it will look like this:
  #
  # module BookKeeping
  #   VERSION = 1 # Where the version number matches the one in the test.
  # end
  #
  # If you are curious, read more about constants on RubyDoc:
  # http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

  def test_bookkeeping
    skip
    assert_equal 1, BookKeeping::VERSION
  end
end
module BookKeeping
  VERSION = 1
end

class Prime
  def self.nth(input)
    new(input).nth
  end

  def initialize(input)
    raise ArgumentError, 'There is no zeroth prime' if input.zero?
    @limit = input
  end

  def nth
    i = 2
    total = 0
    while total < limit
      if i.prime?
        prime = i
        total += 1
      end
      i += 1
    end
    prime
  end

  private

  attr_reader :limit
end

class Integer
  def prime?
    (2..(self / 2)).none? { |n| (self % n).zero? }
  end
end

What can you learn from this solution?

A huge amount can be learnt 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 I could read more about to develop my understanding?