From: David Brady <ruby_talk@...>
Date: 2005-08-28T11:18:53+09:00
Subject: Reopening with a bang: cannot assign to self

Hello,

I'm writing some code that benefits from extending Array.  One of the 
methods wants to be a bang method.  How do I reassign the contents of an 
Array from within it?  (I recognize that subclassing Array might be more 
proper, but I think the problem exists in the subclass as well.)

class Array
  # returns copy of array, with each value divided by the sum of
  # the array elements.  e.g. [1,2,2] => [0.2, 0.4, 0.4].
  # (this code works fine)
  def normalize
    total = Float( self.inject {|x,y| x+y} )
    self.collect {|x| x/total}
  end

  # returns modified array unless array.sum == 1.0, in which case
  # array is not modified and method returns nil
  # (this code does not work: syntax error - cannot assign to self)
  def normalize!
    total = Float( self.inject {|x,y| x+y} )
    if total.zero?
      nil
    else
      self = self.collect {|x| x/total}
    end
  end
end

I can work around the problem by iterating and using self.[]= but the 
collect is so much cleaner, I have to wonder if there's not a way to use it.

Hrm, I've just found "Array#collect!", which will do the trick I need, 
but I still wonder what the proper Ruby idiom is for extending a class 
with a bang method that essentially modifies or replaces the entire 
contents of the object, when there isn't an accessor method.  For 
example, how would you do "Fixnum#div!(x)", e.g. the equivalend of f /= x?

I recognize that this entire line of reasoning may be the result of 
wrongheaded thinking about Ruby.  If so, please enlighten me.

-dB

-- 
David Brady
ruby_talk@shinybit.com
I'm feeling really surreal today... OR AM I?