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alexwebgr's solution

to Resistor Color Trio in the Ruby Track

Published at Jan 12 2021 · 0 comments
Test suite

If you want to build something using a Raspberry Pi, you'll probably use resistors. For this exercise, you need to know only three things about them:

  • Each resistor has a resistance value.
  • Resistors are small - so small in fact that if you printed the resistance value on them, it would be hard to read. To get around this problem, manufacturers print color-coded bands onto the resistors to denote their resistance values. Each band has a position and a numeric value. For example, if they printed a brown band (value 1) followed by a green band (value 5), it would translate to the number 15.
  • In this exercise, you are going to create a helpful program so that you don't have to remember the values of the bands. The program will take 3 colors as input, and outputs the correct value, in ohms. The colors are mapped to the numbers from 0 to 9 in the sequence:

Black - Brown - Red - Orange - Yellow - Green - Blue - Violet - Grey - White

In resistor-color duo you decoded the first two colors. For instance: orange-orange got the main value 33. The third color stands for how many zeros need to be added to the main value. The main value plus the zeros gives us a value in ohms. For the exercise it doesn't matter what ohms really are. For example:

  • orange-orange-black would be 33 and no zeros, which becomes 33 ohms.
  • orange-orange-red would be 33 and 2 zeros, which becomes 3300 ohms.
  • orange-orange-orange would be 33 and 3 zeros, which becomes 33000 ohms.

(If Math is your thing, you may want to think of the zeros as exponents of 10. If Math is not your thing, go with the zeros. It really is the same thing, just in plain English instead of Math lingo.)

This exercise is about translating the colors into a label:

"... ohms"

So an input of "orange", "orange", "black" should return:

"33 ohms"

When we get more than a thousand ohms, we say "kiloohms". That's similar to saying "kilometer" for 1000 meters, and "kilograms" for 1000 grams. So an input of "orange", "orange", "orange" should return:

"33 kiloohms"

For installation and learning resources, refer to the Ruby resources 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 resistor_color_trio_test.rb

To include color from the command line:

ruby -r minitest/pride resistor_color_trio_test.rb


Maud de Vries, Erik Schierboom https://github.com/exercism/problem-specifications/issues/1549

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 'resistor_color_trio'

# Common test data version: 1.1.0 2c41a51
class ResistorColorTrioTest < Minitest::Test
  def test_orange_and_orange_and_black
    # skip
    assert_equal "Resistor value: 33 ohms", ResistorColorTrio.new(["orange", "orange", "black"]).label

  def test_blue_and_grey_and_brown
    assert_equal "Resistor value: 680 ohms", ResistorColorTrio.new(["blue", "grey", "brown"]).label

  def test_red_and_black_and_red
    assert_equal "Resistor value: 2 kiloohms", ResistorColorTrio.new(["red", "black", "red"]).label

  def test_green_and_brown_and_orange
    assert_equal "Resistor value: 51 kiloohms", ResistorColorTrio.new(["green", "brown", "orange"]).label

  def test_yellow_and_violet_and_yellow
    assert_equal "Resistor value: 470 kiloohms", ResistorColorTrio.new(["yellow", "violet", "yellow"]).label

  def test_invalid_color
    assert_raises(ArgumentError) do
      ResistorColorTrio.new(["yellow", "purple", "black"]).label
class ResistorColorTrio
  COLORS = %w(black brown red orange yellow green blue violet grey white)

  attr_reader :color_names
  attr_reader :zeros

  def initialize(color_names = [])
    @color_names = color_names
    @zeros = 0

  def ohms
    color_names.first(2).map do |v|
      raise ArgumentError if COLORS.index(v).nil?

  def zeros
    '0' * COLORS.index(color_names[2])

  def resistor_value
    (ohms + zeros).to_i

  def format
    (resistor_value >= 1000) ? "#{resistor_value / 1000} kiloohms" : "#{resistor_value} ohms"

  def label
    "Resistor value: #{format}"

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