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:
To include color from the command line:
ruby -r minitest/pride nth_prime_test.rb
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.
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
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.