From: Robert Klemme <shortcutter@...>
Date: 2012-04-16T21:40:41+09:00
Subject: Re: Operator overloading of the subscript operator

On Sat, Apr 14, 2012 at 5:03 PM, Jan E. <lists@ruby-forum.com> wrote:

> You won't find this in the Ruby API, because it's a core feature of the
> language and not some special property of a method.

Err, what?

$ ri19 -T Array#[]=
Array#[]=

(from ruby core)
------------------------------------------------------------------------------
  ary[index]         = obj                      ->  obj
  ary[start, length] = obj or other_ary or nil  ->  obj or other_ary or nil
  ary[range]         = obj or other_ary or nil  ->  obj or other_ary or nil


------------------------------------------------------------------------------

Element Assignment---Sets the element at index, or replaces a subarray
starting at start and continuing for length elements, or
replaces a subarray specified by range.  If indices are greater than
the current capacity of the array, the array grows automatically. A negative
indices will count backward from the end of the array. Inserts elements if
length is zero. An IndexError is raised if a negative index
points past the beginning of the array. See also Array#push, and
Array#unshift.

  a = Array.new
  a[4] = "4";                 #=> [nil, nil, nil, nil, "4"]
  a[0, 3] = [ 'a', 'b', 'c' ] #=> ["a", "b", "c", nil, "4"]
  a[1..2] = [ 1, 2 ]          #=> ["a", 1, 2, nil, "4"]
  a[0, 2] = "?"               #=> ["?", 2, nil, "4"]
  a[0..2] = "A"               #=> ["A", "4"]
  a[-1]   = "Z"               #=> ["A", "Z"]
  a[1..-1] = nil              #=> ["A", nil]
  a[1..-1] = []               #=> ["A"]


> The Ruby interpreter evaluates every expression of the pattern
>
> IDENTIFIER[VAL_1, VAL_2, ..., VAL_n] = VAL
>
> to a call of the []= method:
>
> IDENTIFIER.[]=(VAL_1, VAL_2, ..., VAL_n, VAL)

Plus, the result of this expression is always VAL - regardless of the
return value of the method []=.

> This should be explained in every good book about Ruby basics.

Yes.  But this is only about the general syntax transformation.
Semantics of each implementation (e.g. Array#[]=, Hash#[]=) are part
of the class documentation (see above).

Kind regards

robert


-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/