From: Martin DeMello <martindemello@...>
Date: 2012-05-02T13:38:25+09:00
Subject: Re: Overwriting one Ruby array or arrays with another

On Tue, May 1, 2012 at 9:14 PM, Craig Law <lists@ruby-forum.com> wrote:
> Martin
>
> Just as a matter of interest if I was forced to continue using an array
> of arrays such as [1, 1, "X", "20120502"] what would be the most
> efficient way of overwriting the array with another array as described
> in my second post (the only reason I ask is that my code at the moment
> is spitting out an array of arrays and I'm going to have to work out how
> to spit out the hashes in an ordered manner to use the more efficient
> has method you've described). Can I use a similar update method as
> you've done with hashes?

No, it's a bit more work if you want to use arrays that way, since you
don't have a trivial "find the cell with coordinates x, y" method.
Assuming that arrays a and b are sorted first by x and then by y, you
could write a helper method that did a binary search on array a (as a
side note, ruby does implement < and > for arrays, and the elements
are compared pairwise starting from the first one), and then say

def find_in_grid(array, x, y)
  # return the index of the cell starting [x, y, ...] if it is found,
otherwise nil
end

b.each do |cell|
  index = find_in_grid(a, cell[0], cell[1])
  if index
    a[index] = cell
  end
end

find_in_grid can be implemented using a binary search, or you could
maintain some sort of (e.g. pointers to the start of each grid row) if
you want to update it frequently. Also, note that if there is an (x,
y) in b that is *not* in a, inserting it will be an additional problem
since you have to maintain the sorted order and inserting into the
middle of an array is relatively expensive. Of course, all this might
just be premature optimisation - if the grid update is not slowing
your code down, do the simplest thing that could work, which is
convert a and b into hashes, update, and then convert back into an
array. (It might even be the fastest thing to do, if you have a dense
grid and a lot of updates).

To convert an array to a hash:

hash = {}
a.each do |cell|
  x, y, *rest = cell
  hash[[x, y]] = rest
end

and back:

hash.keys.sort.map {|k| [k] + hash[k]}

Where sorting the keys of the hash ensures that your array is spat out
in the proper order.

martin