Avatar of davearonson

davearonson's solution

to RNA Transcription in the Ruby Track

Published at Jul 13 2018 · 2 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 DNA strand, return its RNA complement (per RNA transcription).

Both DNA and RNA strands are a sequence of nucleotides.

The four nucleotides found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T).

The four nucleotides found in RNA are adenine (A), cytosine (C), guanine (G) and uracil (U).

Given a DNA strand, its transcribed RNA strand is formed by replacing each nucleotide with its complement:

  • G -> C
  • C -> G
  • T -> A
  • A -> U

For installation and learning resources, refer to the exercism help 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 rna_transcription_test.rb

To include color from the command line:

ruby -r minitest/pride rna_transcription_test.rb


Rosalind http://rosalind.info/problems/rna

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


require 'minitest/autorun'
require_relative 'rna_transcription'

# Common test data version: 1.0.1 cb1fd3a
class RnaTranscriptionTest < Minitest::Test
  def test_rna_complement_of_cytosine_is_guanine
    # skip
    assert_equal 'G', Complement.of_dna('C')

  def test_rna_complement_of_guanine_is_cytosine
    assert_equal 'C', Complement.of_dna('G')

  def test_rna_complement_of_thymine_is_adenine
    assert_equal 'A', Complement.of_dna('T')

  def test_rna_complement_of_adenine_is_uracil
    assert_equal 'U', Complement.of_dna('A')

  def test_rna_complement
    assert_equal 'UGCACCAGAAUU', Complement.of_dna('ACGTGGTCTTAA')

  def test_correctly_handles_invalid_input_rna_instead_of_dna
    assert_equal '', Complement.of_dna('U')

  def test_correctly_handles_completely_invalid_dna_input
    assert_equal '', Complement.of_dna('XXX')

  def test_correctly_handles_partially_invalid_dna_input
    assert_equal '', Complement.of_dna('ACGTXXXCTTAA')

  # Problems in exercism evolve over time, as we find better ways to ask
  # questions.
  # The version number refers to the version of the problem you solved,
  # not your solution.
  # Define a constant named VERSION inside of the top level BookKeeping
  # module, which may be placed near the end of your file.
  # In your file, it will look like this:
  # module BookKeeping
  #   VERSION = 1 # Where the version number matches the one in the test.
  # end
  # If you are curious, read more about constants on RubyDoc:
  # http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

  def test_bookkeeping
    assert_equal 4, BookKeeping::VERSION
class Complement


  def self.of_dna(dna)
    complement_with_pairs(dna, DNA_PAIRS)

  def self.of_rna(rna)
    complement_with_pairs(rna, RNA_PAIRS)


    "A" => "U",
    "C" => "G",
    "G" => "C",
    "T" => "A",


  def self.complement_with_pairs(sequence, pairs)
    sequence.chars.map { |c|
      comp = pairs[c]
      comp ? comp : raise(ArgumentError)


Community comments

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Avatar of davearonson

The main tradeoff was making stuff private. In general I try to make anything that doesn't need to be public, private. If that gets too large or complicated, I take it as a sign that stuff needs to be extracted into another class, that the current one will use. If needed later, I could have made the pairs public.

What drove the ode structure in this direction was noticing that the two functions were doing essentially the same thing, indicating that I should extract their guts into complement_with_pairs, and that the two sets of pairs were the inversions of each other, simplifying the construction of one of them. What drove the data structure in this direction was the desire for very simple lookup. Not only would an array be slower, if the size were non-trivial, but the lookup procedure would not be as clear as a simple hash lookup.

Avatar of michaelkunc

I agree with the use of the hash to store the data. Using .invert is another nice touch. Good stuff.

What can you learn from this solution?

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?