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thekeele's solution

to Nth Prime in the Elixir Track

Published at May 30 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

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.

Running tests

Execute the tests with:

$ elixir nth_prime_test.exs

Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant @tag :pending with a # symbol.

For example:

# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end

Or, you can enable all the tests by commenting out the ExUnit.configure line in the test suite.

# ExUnit.configure exclude: :pending, trace: true

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

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.exs

if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("nth_prime.exs", __DIR__)
end

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)

defmodule NthPrimeTest do
  use ExUnit.Case

  # @tag :pending
  test "first prime" do
    assert Prime.nth(1) == 2
  end

  @tag :pending
  test "second prime" do
    assert Prime.nth(2) == 3
  end

  @tag :pending
  test "sixth prime" do
    assert Prime.nth(6) == 13
  end

  @tag :pending
  test "100th prime" do
    assert Prime.nth(100) == 541
  end

  @tag :pending
  test "weird case" do
    catch_error(Prime.nth(0))
  end
end
defmodule Prime do

  @spec nth(non_neg_integer) :: non_neg_integer
  def nth(count) when count < 1, do: raise ArithmeticError
  def nth(1), do: 2
  def nth(count) do
    Stream.iterate(2, &next_number/1)
    |> Enum.take(count)
    |> List.last()
  end

  defp next_number(number) do
    if is_prime?(number + 1) do
      number + 1
    else
      next_number(number + 1)
    end
  end

  defp is_prime?(n, factor \\ 2) do
    cond do
      factor > n / factor -> true
      rem(n, factor) == 0 -> false
      true -> is_prime?(n, factor + 1)
    end
  end
end

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