From: Josh Cheek <josh.cheek@...>
Date: 2011-01-12T04:58:16+09:00
Subject: Re: Parallel Assignments and Elegance/Complexity Ratio.

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On Tue, Jan 11, 2011 at 8:29 AM, 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?
>
> Thank you,
> Kedar
>
> --
> Posted via http://www.ruby-forum.com/.
>
>
I don't use it very often, but when I do, it usually makes an elegant
solution. I think part of the reason it doesn't seem that way is because you
are playing with it in too sterile of an environment. For example, you rate
"x, (y, (z, a))=[1, [2, [3, 4]]]" as lowest, suggesting it is equivalent to
"x=1;y=2;z=3;a=4" but this is not true. If you are actually assigning with
literals, you would, of course, use the equivalent way, but if your data
comes in as nested arrays, then you can't assign like that, instead you have
to do something like this:

def parallel(values)
  x, (y, (z, a))=values
  [x,y,z,a]
end
def alternative(values)
  x       = values.shift
  values  = values.shift
  y       = values.shift
  values  = values.shift
  z       = values.shift
  a       = values.shift
  [x,y,z,a]
end
parallel    [1, [2, [3, 4]]] # => [1, 2, 3, 4]
alternative [1, [2, [3, 4]]] # => [1, 2, 3, 4]




Now, I don't normally store data like that, so I haven't ever done anything
quite that fancy, but I use arrays on the RHS on occasion. It might look
something like this (though I don't normally store my data like this, either
-- it's really hard to think of a decent example!).

$stdin = DATA
while input = gets
  name , num = input.split
  puts "#{name.capitalize}'s favourite number is #{num}"
end
__END__
josh 12
bill 42
sally 13
ned 99
clara 1000000



The alternative of
name , num = input.split
is
values = input.split
name = values.shift
num = values

I consider the former to be much more elegant as it avoids a temporary
variable.

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