Ruby and modifying self for a Float instance

Question

I would like to change the self value of a float instance.

I have the following method :

class Float
  def round_by(precision)
    (self * 10 ** precision).round.to_f / 10 ** precision
  end
end

And I would like to add the round_by! method which will modify the self value.

class Float
  def round_by!(precision)
    self = self.round_by(precision)
  end
end

But I got an error saying I can't change the value of self.

Any idea ?

Answer 1

You can't change the value of self. It always points to the current object, you can't make it point to something else.

When you want to mutate the value of an object, you either do this by calling other mutating methods or setting or changing the values of instance variables, not by trying to reassign self. However in this case, that won't help you, because Float doesn't have any mutating methods, and setting instance variables won't buy you anything, because none of the default float operations are affected by any instance variables.

So the bottom line is: you can't write mutating methods on floats, at least not in the way you want.