Ruby - how to add an array to a hash key?

Question

I have this loop:

car_data = Hash.new
Car.all.each do |c|
  car_data[c.brand] = c.id
  car_data['NEW'] << c.id if c.new == 1
end

I have this snipper and trying to save all the new cars to car_data['NEW'], but this code keeps only one item in the hash (there should be 8).

I also tried to define that car_data['NEW'] as an array:

car_data = Hash.new
car_data['NEW'] = Hash.new
Car.all.each do |c|
  car_data[c.brand] = c.id
  car_data['NEW'] << c.id if c.new == 1
end

But the result was the same - just one item. How do I save the whole array to the hash key element?

Thank you.

Ruby convention is to use { } for a new Hash, that calling Hash.new is only necessary when supplying defaults like Hash.new(0). Less is more. — tadman
– tadman, Commented Jul 27, 2016 at 17:08

Simone Carletti · Accepted Answer · 2016-07-27 16:07:38Z

3

car_data['NEW'] has to be declared as Array.

car_data = Hash.new
car_data['NEW'] = []
Car.all.each do |c|
  car_data[c.brand] = c.id
  car_data['NEW'] << c.id if c.new == 1
end

You can also do it in a single step

car_data = { new: [] }
Car.all.each do |c|
  car_data[c.brand] = c.id
  car_data[:new] << c.id if c.new == 1
end

Frankly, it seems a little bit odd to me to use a Hash in that way. In particular, mixin different kind of information in Hash is a very bad approach inherited from other non object oriented languages.

I'd use at least two separate variables. But I don't know enough about the context to provide a meaningful example.

answered Jul 27, 2016 at 16:07

Simone Carletti

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Comments

maicher · Accepted Answer · 2016-07-27 16:20:38Z

3

You wrote, that you tried to define (initialize) car_data['NEW'] as an array, but what you did is... initialized it as a hash.

Change:

car_data['NEW'] = Hash.new

To:

car_data['NEW'] = []

The full code would look like:

car_data = Hash.new
car_data['NEW'] = []
Car.all.each do |c|
  car_data[c.brand] = c.id
  car_data['NEW'] << c.id if c.new == 1
end

edited Jul 27, 2016 at 16:20

answered Jul 27, 2016 at 16:07

maicher

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1 Comment

tadman Over a year ago

Better: car_data = { 'NEW' => [ ] }. Get it done in one shot.

Aleksei Matiushkin · Accepted Answer · 2016-07-27 17:32:43Z

1

car_data = Car.all.each_with_object(Hash.new { |h, k| h[k] = [] }) do |c, memo|
  memo[c.brand] = c.id
  memo['NEW'] << c.id if c.new == 1
end

or, simpler, let’s create it on the fly if needed:

car_data = Car.all.each_with_object({}) do |c, memo|
  memo[c.brand] = c.id
  (memo['NEW'] ||= []) << c.id if c.new == 1
end

Please also refer to comment by @tadman below, if the NEW key is to be existing in any case.

edited Jul 27, 2016 at 17:32

answered Jul 27, 2016 at 16:20

Aleksei Matiushkin

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3 Comments

tadman Over a year ago

Probably better with { 'NEW' => [ ] } than the cruft where it optionally initializes it.

Aleksei Matiushkin Over a year ago

@tadman This cruft contains a hint on how to deal with “maybe nil” objects. For the sophisticated initialization there is always variant 1 present.

tadman Over a year ago

I know what you're saying, but the difference here is in maybe not initializing the NEW key, which seems like a harmless thing to do by default.

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