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thekeele's solution

to Nucleotide Count in the Elixir Track

Published at Jul 13 2018 · 3 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given a single stranded DNA string, compute how many times each nucleotide occurs in the string.

The genetic language of every living thing on the planet is DNA. DNA is a large molecule that is built from an extremely long sequence of individual elements called nucleotides. 4 types exist in DNA and these differ only slightly and can be represented as the following symbols: 'A' for adenine, 'C' for cytosine, 'G' for guanine, and 'T' thymine.

Here is an analogy:

  • twigs are to birds nests as
  • nucleotides are to DNA as
  • legos are to lego houses as
  • words are to sentences as...

Running tests

Execute the tests with:

$ elixir nucleotide_count_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!"

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

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

For more detailed information about the Elixir track, please see the help page.


The Calculating DNA Nucleotides_problem at Rosalind http://rosalind.info/problems/dna/

Submitting Incomplete Solutions

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("nucleotide_count.exs", __DIR__)

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

defmodule NucleotideCountTest do
  use ExUnit.Case

  # @tag :pending
  test "empty dna string has no adenine" do
    assert NucleotideCount.count('', ?A) == 0

  @tag :pending
  test "repetitive cytosine gets counted" do
    assert NucleotideCount.count('CCCCC', ?C) == 5

  @tag :pending
  test "counts only thymine" do
    assert NucleotideCount.count('GGGGGTAACCCGG', ?T) == 1

  @tag :pending
  test "empty dna string has no nucleotides" do
    expected = %{?A => 0, ?T => 0, ?C => 0, ?G => 0}
    assert NucleotideCount.histogram('') == expected

  @tag :pending
  test "repetitive sequence has only guanine" do
    expected = %{?A => 0, ?T => 0, ?C => 0, ?G => 8}
    assert NucleotideCount.histogram('GGGGGGGG') == expected

  @tag :pending
  test "counts all nucleotides" do
    expected = %{?A => 20, ?T => 21, ?C => 12, ?G => 17}
    assert NucleotideCount.histogram(s) == expected
defmodule NucleotideCount do
  @nucleotides [?A, ?C, ?G, ?T]

  @doc """
  Counts individual nucleotides in a NucleotideCount strand.

  ## Examples

  iex> NucleotideCount.count('AATAA', ?A)

  iex> NucleotideCount.count('AATAA', ?T)
  @spec count([char], char) :: non_neg_integer
  def count(strand, nucleotide) do
    Enum.count(strand, &(&1 == nucleotide))

  @doc """
  Returns a summary of counts by nucleotide.

  ## Examples

  iex> NucleotideCount.histogram('AATAA')
  %{?A => 4, ?T => 1, ?C => 0, ?G => 0}
  @spec histogram([char]) :: map
  def histogram(strand) do
    Enum.into(@nucleotides, %{}, &({&1, NucleotideCount.count(strand, &1)}))

Community comments

Find this solution interesting? Ask the author a question to learn more.
Avatar of chadcoley

Nice Mark! I'm going to try and use this for my own education, so please bear with me.

I like the &(&1 vs the fn(x) -> x...that's handy! Is & syntax preferred (or perform better) than using the fn? Which is more common in XGPS?

Also, I used Map.new and I see Enum.into. Does one out perform or is preferred to another?

Avatar of sorliem

Nice! I had to wrap my mind around what you are doing with Enum.into/3... very clever.

Avatar of brzezinskip

Wow. My solution is so much worse, but this surely gives me some ideas. Thanks, good job!

What can you learn from this solution?

A huge amount can be learned from reading other peopleโ€™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?