CODEWITHSUNDEEP
ruby-on-railsrubyruby-on-rails-3
John Smith
John Smith

Reputation: 6259

Update hash value in class

Here I have a little demo-code:

class UpdateHash
  def initialize(value)
    @value = value
    save
  end

  def save
    @value = 10
  end
end


hash = {uno: "50"}

UpdateHash.new(hash[:uno])

puts hash[:uno]
 => "50"

What would I have to change so that the result of puts hash[:uno] is not "50" but 10. Is there the possibility to update the value of the hash in the class, without having to pass the whole Hash?

This would work: UpdateHash.new(hash). But am I able to only pass the value and update the value in the class?

Upvotes: 0

Views: 796

Answers (3)

Holger Just
Holger Just

Reputation: 55758

When passing objects around as arguments to methods, what happens is that Ruby copies the reference to the object. This is often called pass-by-object-reference which in turn is a variant of a pass-by-value system.

What this means is that while you can mutate the object pointed to by the passed reference or even update the reference to a point to a new object (i.e. setting a new object to the existing variable), you can't force other references to the same object point to a different object.

As such, on order to update the hash and set a new value, you need a reference to the hash itself. Just setting a new value to the @value variable won't update any other references like the one inside the hash.

Now, if the value would have been e.g. an Array you could have mutated it inside your class by e.g. adding new elements in your save method which would then be reflected in the hash too. This is possible since the reference to the object (i.e. the array in this case) is not changed here.

Upvotes: 3

tadman
tadman

Reputation: 211570

You'll need to capture a reference to the parent object if you want to manipulate any of the values and have them back-propagate to the caller's level:

class UpdateHash
  def initialize(hash)
    @hash = hash
    save
  end

  def save
    @hash[:uno] = 10
  end
end

hash = {uno: "50"}

UpdateHash.new(hash)

puts hash[:uno]
# => 10

Upvotes: 1

Sagar Pandya
Sagar Pandya

Reputation: 9497

Something like this perhaps:

class UpdateHash
  def initialize(hash,key)
    hash[key] = save
  end

  def save
    10
  end
end

hash = {uno: "50"}

UpdateHash.new(hash,:uno)
puts hash[:uno] #=> 10

Upvotes: 1

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