From: Robert Klemme <shortcutter@...>
Date: 2011-01-12T00:30:44+09:00
Subject: Re: Parallel Assignments and Elegance/Complexity Ratio.

On Tue, Jan 11, 2011 at 3:29 PM, Kedar Mhaswade
<kedar.mhaswade@gmail.com> wrote:
> In SICP, I read that "Programs should be written for people to read, and
> only incidentally for machines to execute".
>
> While reading David/Matz book, I stumbled upon parallel assignments and
> I thought the language was trying to be too flexible (adding complexity,
> at least for a newcomer). My head soon started
> spinning (when it reached a, (b, (c,d)) = 1, [2, [3, 4]] I was
> exhausted).
>
> My experience is recorded here:
> https://docs1.google.com/document/d/1zpHvfO4be3UvjaxU7L5gEPXFMENcMmEz9qqIcecEzOg/edit?hl=en#
>
> Summary is, I should only spend time learning
> - x, y, z = 1, 2, 3 (# => x=1, y=1, z=3), and
> - x, y = y, x (# => swap x and y)
>
> I gather that this might be a matter of taste and style, but are other
> variants used by community?

Just today I used

  def []=(*idx, val)
    index_check(idx)
    @data[idx] = val
  end

https://gist.github.com/772827

See also thread "Nooby question : multidimensional arrays.".

I find the pattern matching _very_ elegant.  This also comes in handy
in situations like this:

irb(main):001:0> a = Array.new(10) { [rand(10), rand(10)] }
=> [[2, 3], [7, 8], [4, 0], [4, 6], [5, 2], [9, 9], [4, 2], [8, 5],
[2, 6], [5, 9]]
irb(main):002:0> a.sort {|(a1,a2),(b1,b2)| x = a2 <=> b2; x == 0 ? a1
<=> b1 : x}
=> [[4, 0], [4, 2], [5, 2], [2, 3], [8, 5], [2, 6], [4, 6], [7, 8],
[5, 9], [9, 9]]

Although in this particular case you could also use

irb(main):004:0> a.sort_by {|arr| arr.reverse}
=> [[4, 0], [4, 2], [5, 2], [2, 3], [8, 5], [2, 6], [4, 6], [7, 8],
[5, 9], [9, 9]]
irb(main):005:0> a.sort_by {|x, y| [y, x]}
=> [[4, 0], [4, 2], [5, 2], [2, 3], [8, 5], [2, 6], [4, 6], [7, 8],
[5, 9], [9, 9]]

Generally I'd say I don't use it overly often but when I use it it is
really handy.  With #inject it's also useful if you iterate through a
collection of Arrays of known equal length:

irb(main):006:0> a.inject(0) {|s,(x,y)| s + x + y}
=> 100

Kind regards

robert

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