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.
Execute the tests with:
$ elixir nth_prime_test.exs
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
# @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.
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.
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
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.