Published at Jul 13 2018
·
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
```

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
```

```
module Prime
MINIMUM_PRIME = 1
module_function
def nth(number)
raise ArgumentError if number < MINIMUM_PRIME
primes.take(number).to_a.last
end
def primes
2.upto(Float::INFINITY).lazy.reject { |number| composite?(number) }
end
private_class_method :primes
def composite?(number)
2.upto(Math.sqrt(number)).any? { |n| (number % n).zero? }
end
private_class_method :composite?
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.

- What compromises have been made?
- Are there new concepts here that you could read more about to improve your understanding?

Level up your programming skills with 3,373 exercises across 50 languages, and insightful discussion with our volunteer team of welcoming mentors.
Exercism is
**100% free forever**.

## Community comments