From: Dave Thomas <Dave@...>
Date: 2000-11-23T07:50:09+09:00
Subject: [ruby-talk:6534] Re: Question about the behavior of write attributes in blocks

"Christoph Rippel" <chr@subdimension.com> writes:

> Thank you for explanation - the output of "x".inspect() is
> 
> >>"x"
> 
> which made me assume that Strings are implemented as singletons  (which
> doesn't make much sense in the presence of a replace() method).
> 
> I was hoping to be able to write
> ***********
> x =  [1,2,3]
> x.each { |i|    i+= 1  }

You could write this as

  [1,2,3].collect! {|i| i+1}		# => [2, 3, 4]

This replaces each value with the result of the block.

> but there is no such method for Fixnum and the only way I can think about
> writing such a method always involves the use of the ``=()''-operator -
> which means I am effectively stuck. The String Class seems to be one of the
> few build in classes sporting a replace() method so I can write

There's a reason for this. Let's go back to Strings

  a = "cat"     # => "cat"
  b = a		# => "cat"
  a[0] = "b"	
  b		# => "bat"

We changed 'a', but 'b' got the value too. Why? Because both a and b
reference the same string object.

Now, let's look at what would happen if Fixnum had the same behavior:

  ## not real code ##
  a = 1        # => 1
  b = a        # => 1
  b.increment  # => 2
  a            # => 2

So we've just changed the value of '1' throughout the system. This
would be "a bad thing". So, Fixnums have value semantics, not
reference semantics. Variables hold the value, not a reference to the
value. That's why your first example doesn't work:

   [1,2,3].each { |i| i += 1 }

The variable 'i' receives successive Fixnum values, not
references. Changes to the value stored in the variable therefore
don't affect numbers globally.

> This is only a stop-cap measure since I really want to manipulate
> Arrays (or binary trees) of Fixnum or more general any Object.

Your collection can provide a #collect! method to change the values.


Regards


Dave