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paulfioravanti's solution

to Nth Prime in the Elixir Track

Published at Aug 22 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.

Running tests

Execute the tests with:

$ mix test

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

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

test_helper.exs

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
defmodule Prime do
  @doc """
  Generates the nth prime.
  """
  @spec nth(non_neg_integer) :: non_neg_integer
  def nth(count) when count > 0 do
    primes()
    |> Enum.at(count - 1)
  end

  defp primes do
    Stream.iterate(2, &(&1 + 1))
    |> Stream.reject(&composite?/1)
  end

  defp composite?(2), do: false

  defp composite?(number) do
    2..(number - 1)
    |> Enum.any?(&no_remainder?(number, &1))
  end

  defp no_remainder?(number1, number2), do: rem(number1, number2) == 0
end

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