Avatar of davearonson

davearonson's solution

to Grains in the Ruby 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.

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

  • how many grains were on each square, and
  • the total number of grains

For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

  • Optimize for speed.
  • Optimize for readability.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

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 grains_test.rb

To include color from the command line:

ruby -r minitest/pride grains_test.rb


JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

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 'grains'

# Common test data version: 1.0.0 2e0e77e
class GrainsTest < Minitest::Test
  def test_1
    # skip
    assert_equal 1, Grains.square(1)

  def test_2
    assert_equal 2, Grains.square(2)

  def test_3
    assert_equal 4, Grains.square(3)

  def test_4
    assert_equal 8, Grains.square(4)

  def test_16
    assert_equal 32_768, Grains.square(16)

  def test_32
    assert_equal 2_147_483_648, Grains.square(32)

  def test_64
    assert_equal 9_223_372_036_854_775_808, Grains.square(64)

  def test_square_0_raises_an_exception
    assert_raises(ArgumentError) { Grains.square(0) }

  def test_negative_square_raises_an_exception
    assert_raises(ArgumentError) { Grains.square(-1) }

  def test_square_greater_than_64_raises_an_exception
    assert_raises(ArgumentError) { Grains.square(65) }

  def test_returns_the_total_number_of_grains_on_the_board
    assert_equal 18_446_744_073_709_551_615, Grains.total

  # 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 1, BookKeeping::VERSION
module Grains

  def self.square(i)
    2 ** (i - 1)

  def self.total
    2 ** SQUARES - 1


  SQUARES = 64


Community comments

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Avatar of remcopeereboom

I like that you made this into a module!

I personally prefer to not put spaces around exponentiation, just to make it more in line with algebraic notation.

Making SQUARES private is an excellent idea, unfortunately ruby doesn't have private constants. Definitely something to do in other languages though!

Avatar of bjmllr

I've never used it in anger, but since 1.9.3 or so we can make constants private with Module#private_constant: class Foo BAR = 7 private_constant :BAR end

Foo::BAR # => NameError: private constant Foo::BAR referenced

Avatar of remcopeereboom

@bjmllr Thanks! I didn't know you could do that.

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?