How to store method attributes in variable

Question

class Dates
  ATTRS = {date_from: '', date_to: ''}

  def set_dates(ATTRS)
    @date_from = date_from
    @date_to   = date_to
  end

  def show_dates(ATTRS)
    p date_from
    p date_to
  end
end

Dates.new.set_dates(date_from: Time.current, date_to: Time.current)
#-:4: formal argument cannot be a constant
#  def set_dates(ATTRS)
#                     ^
#-:9: formal argument cannot be a constant
#  def show_dates(ATTRS)

SO question is: Is it possible to store method attributes in variable ?

change to def set_dates(ATTRS) to def set_dates(**args). This is keyword arguments. — Arup Rakshit
– Arup Rakshit, Commented Mar 4, 2014 at 9:48
you right, i totally forgot about it, thanks, issue the answer. — nikolayp
– nikolayp, Commented Mar 4, 2014 at 9:52

Arie Xiao · Accepted Answer · 2014-03-04 10:03:45Z

4

You can use the new keyword arguments syntax in Ruby 2.0:

class Dates
  def set_dates(date_from: '', date_to: '')
    @date_from = date_from
    @date_to = date_to
  end
  def show_dates
    p @date_from
    p @date_to
  end
end

Or the hash arguments before Ruby 2.0:

class Dates
  ATTRS = {date_from: '', date_to: ''}
  def set_dates(attr)
    attr = ATTRS.merge(attr)
    @date_from = attr[:date_from]
    @date_to = attr[:date_to]
  end
  def show_dates
    p @date_from
    p @date_to
  end
end

For the show_dates method, I guess you meant showing the status of the Dates instance, so I made some modification to it.

Besides, in Ruby, variables started with capitalized letter are treated as constants, you cannot use it as formal arguments of the methods.

answered Mar 4, 2014 at 10:03

Arie Xiao

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Comments

Agis · Accepted Answer · 2014-03-04 10:27:19Z

2

I would write your code as below using Ruby (2.0.0 > =) new keyword rest argument(**) operator :

class Dates
  def set_dates(**attr)
    @date_from,@date_to = attr.values_at(:date_from,:date_to)
  end

  def show_dates
    p @date_from
    p @date_to
  end
end

edited Mar 4, 2014 at 10:27

Agis

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answered Mar 4, 2014 at 9:55

Arup Rakshit

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5 Comments

Agis Over a year ago

Where are the keyword arguments in your example?

Arie Xiao Over a year ago

The return value of Hash#values may be in arbitrary order. I think you shouldn't make assumptions that the value associated with :date_from comes first.

Arup Rakshit Over a year ago

@apneadiving I will down it.. wait

Arup Rakshit Over a year ago

@Agis What **attr is doing then ?

Agis Over a year ago

You are confusing the keywords arguments feature introduced in Ruby 2.0 with just expanding a hash argument. You're just accepting a hash no matter its contents are and assume that it has the wanted keys. This is not what keyword arguments is about. See @Arie Shaw answer, his method will raise an error if the proper arguments aren't passed.

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