Ruby count array unique elements

Question

I have a statistics module for my library class that shows the most popular books and top readers

module Statistics
  def top_reader
    orders_grouped = orders.group_by(&:reader)
    tab = orders_grouped.max_by(3) { |_k, v| v.count }.map(&:first)
    puts "#{tab.map(&:name).join(', ')} - are the top readers"
  end

  def top_book
    orders_grouped = orders.group_by(&:book)
    tab = orders_grouped.max_by(2) { |_k, v| v.count }.map(&:first)
    puts "#{tab.map(&:title).join(', ')} - are the top books"
  end
end

That's how i get top reader and top book I need to get amount of unique readers of the top book now, i tried to do it this way but it's not working correctly and the code are not very clean

  def top_book_readers
    puts @orders.select { |order| books.include? order.book }.uniq(&:reader).size
  end

Expected result is just the amount of unique readers that purchased top book

Edit:

  def top_book_readers
    orders_grouped = orders.group_by(&:book)
    top_orders_grouped = orders_grouped.max_by(1) { |_k, v| v.count }

    top_orders_grouped.each do |book, orders|
      num_readers = orders.map(&:reader).uniq.count
      puts "#{book.title} has #{num_readers} readers"
    end
  end

This works now, number of readers of top books are also configurable

Do you want to get the readers of the top book, or the top two books as produced by your top_book method? — max pleaner
– max pleaner, Commented Apr 19, 2021 at 17:20
It appears you want something like @orders.group_by { ... }.count { |_,v| v.size == 1 }. — Cary Swoveland
– Cary Swoveland, Commented Apr 19, 2021 at 17:24
@maxpleaner i want to get readers of the top books i get. But as you can see the number of top books is configurable. Would appreciate if you could also help with 1 top book readers — Warden
– Warden, Commented Apr 19, 2021 at 17:44

stwienert · Accepted Answer · 2021-04-19 18:27:37Z

If understood correctly, it should work like this:


# same like yours
top_books = orders.
  group_by(&:book).
  max_by(3) { |_k, v| v.count }.
  map(&:first)

# Hash of { <Book 1> => 30, <Book 2> => 10 }
reader_count_per_book = top_books.
  map { |book| [book, book.readers.uniq.count] }.
  to_h

# Or: uniq readers, that read at least one of the top books
uniq_readers_of_top_books = top_books.flat_map(&:readers).uniq.count

In general, if you have a database attached, and using ActiveRecord (your question is tagged as Rails), then it might be better to do all the work in sql (without knowning about your models/relationships/column names):

top_books = Order.
  joins(:book).
  order('count(orders.id) desc').
  limit(3).
  group("books.id").
  count

uniq_readers_of_top_books = Book.
  joins(:readers).
  where(id: top_books.keys).
  count('distinct readers.id')

reader_count_per_book = Book.
  joins(:readers).
  where(id: top_books.keys).
  group('books.title').
  count('distinct readers.id')

max pleaner · Accepted Answer · 2021-04-19 20:12:02Z

2

You can re-use some of that group_by logic from the other methods.

orders_grouped = orders.group_by(&:book)
top_orders_grouped = orders_grouped.max_by(2) { |_k, v| v.count }

Then to get the readers from the top_orders_grouped, you just get the unique reader values:

top_orders_grouped.each do |book, orders|
  num_readers = orders.map(&:reader).uniq.count
  puts "#{book} has #{num_readers} readers"
end

I should also mention, it's usually a good idea to make your methods return data instead of only printing output. This enables you to re-use the methods more easily.

edited Apr 19, 2021 at 20:12

answered Apr 19, 2021 at 18:11

max pleaner

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3 Comments

3limin4t0r Over a year ago

Most of the time #map + #flatten = #flat_map (that is, if you expect to flatten 1 level)

Warden Over a year ago

But i just need to get the number, not an array. For example: "top book was purchased by #{top_book_readers} readers" And number would be an expected result. It's an application

engineersmnky Over a year ago

Seems like offloading the group and limit to the database would be a far better option IMO.

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