From: Mauricio Fernandez <mfp@...>
Date: 2006-02-07T18:17:41+09:00
Subject: Re: Feeling sharp?  Sort an array subset in place.

On Tue, Feb 07, 2006 at 05:28:21PM +0900, Michael Judge wrote:
> Here's the situation: I have an array and I'd like to sort certain
> elements in place -- without touching the others.
> 
> Unsorted array contents:
>   3 <-- sort me!
>   x
>   1 <-- sort me!
>   y
>   2 <-- sort me!
> 
> Sorted array contents
>   1
>   x
>   2
>   y
>   3

class Array
  def sort_subset!
    els, indices = [], []
    each_with_index{|x,i| (els << x; indices << i) if yield(x) }
    els.sort.each_with_index{|x,i| self[indices[i]] = x}
  end
end

a = (0..10).sort_by{rand}
a                                                  # => [1, 8, 2, 10, 3, 0, 5, 4, 9, 7, 6]
a.sort_subset!{|x| x % 2 == 0}                     # => [0, 2, 4, 6, 8, 10]
a                                                  # => [1, 0, 2, 4, 3, 6, 5, 8, 9, 7, 10]
a.sort_subset!{|x| x % 2 == 1}                     # => [1, 3, 5, 7, 9]
a                                                  # => [1, 0, 2, 4, 3, 6, 5, 8, 7, 9, 10]

-- 
Mauricio Fernandez