array manipulation without for loops

[Tim Chase]

> I have two arrays that are of the same dimension but having 3 different
> values: 255, 1 or 2.
> I would like to set all the positions in both arrays having 255 to be
> equal, i.e., where one array has 255, I set the same elements in the
> other array to 255 and visa versa. Does anyone know how to do this
> without using for loops?

 >>> # make some sample data
 >>> c = 20
 >>> import random, itertools
 >>> a1 = [(1,2,255)[random.randint(0,2)] for x in xrange(c)]
 >>> a2 = [(1,2,255)[random.randint(0,2)] for x in xrange(c)]
 >>>
 >>> # actually do the work
 >>> all = [(x==255 or y==255) and (255, 255) or (x,y) for (x,y) 
in itertools.izip(a1,a2)]
 >>> b1 = [x[0] for x in all]
 >>> b2 = [x[1] for x in all]
 >>> a1, a2 = b1, b2 # if you want them to replace the originals

Seems to do what I understand that you're describing using "no" 
loops (other than those implied by list comprehension).

There may be some nice pythonic way to "unzip" a list of tuples 
created by zip() but I couldn't scare up such a method, and 
searching for "unzip" turned up a blizzard of hits for dealing 
with ZIP files, not for unzipping that which was previously 
zip()'ed.  My google-foo must be broken. :)  Otherwise, you could 
just do

 >>> b1,b2 = magic_unzip([(x==255 or y==255) and (255, 255) or 
(x,y) for (x,y) in itertools.izip(a1,a2)])

or

 >>> a1,a2 = magic_unzip([(x==255 or y==255) and (255, 255) or 
(x,y) for (x,y) in itertools.izip(a1,a2)])

if you just want to dispose of your originals.

-tkc