Prime Products

Prime Products is a command-line tool which outputs the multiplication table of the first n prime numbers.

So, for instance, where n=5, you'd get a result like this:

+----+----+----+----+----+-----+
| *  | 2  | 3  | 5  | 7  | 11  |
| 2  | 4  | 6  | 10 | 14 | 22  |
| 3  | 6  | 9  | 15 | 21 | 33  |
| 5  | 10 | 15 | 25 | 35 | 55  |
| 7  | 14 | 21 | 35 | 49 | 77  |
| 11 | 22 | 33 | 55 | 77 | 121 |
+----+----+----+----+----+-----+

The first row and column of the table contains the first 5 primes. Each other cell contains the product of the primes for the corresponding row and column.

Installation

Install the gem by running gem install prime_products.

In development, you can install this gem by cloning the repository and running bundle.

Usage

To generate a table of prime products, run the prime_products executable, providing the size of the table as the first argument:

# If you have installed this gem from source, you will need to run
# `bundle exec prime_products 10`.
prime_products 10

If you do not provide an argument, we will use the first 5 by default.

Benchmarking

This gem uses a naive method of finding the first n primes which does not scale especially well. However, this seems reasonable given that the primes will be displayed on-screen, and so it is unlikely you will want to see a large number at once.

Despite being a naive solution, this is still moderately performant. On my machine:

• finding the first 10 takes ~0.00004s
• finding the first 100 takes ~0.001s
• finding the first 1000 takes ~0.2s
• finding the first 10000 takes ~31s

It is unlikely that you would want a table of more than 1000, which only takes ~0.2 seconds. If we wanted to use these numbers elsewhere, and needed more, then I would optimise this code.

You can see full benchmark results, comparing the Ruby standard library's implementation (Prime.first) to mine, by running ruby benchmarks/prime_numbers.rb.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. Run rake lint to check the code style with Rubocop.

You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.