[Tutor] Sorting against another list

[Roeland Rengelink]

Sharriff Aina wrote:
> 
> Hi!
> 
> I have a list that is created dynamically, how do I sort this list so that
> it matches amother "mater template" list?
> 
> example:
> [ 'aa', 'qq', 'ttt', 'hh']  # master list
> 
> my dynamic list cotains all or only a few of the elements in the master
> list:
> 
> ['ttt', 'hh', 'aa'] ### i would like to sort it to produce ['aa''ttt',
> 'hh']  accoriding to the sequense in the master list
> 
> or [ 'hh', 'qq' 'ttt'] ### would like ['qq', 'ttt', 'hh']
> 
> I would like to sort these example list so that they at least follow the
> order of the master list.
> 
> Thanks for any ideas
> 
> Sharriff
> 
> _______________________________________________
> Tutor maillist  -  Tutor@python.org
> http://mail.python.org/mailman/listinfo/tutor


You can do a sort on a list with a custom comparison function

alist.sort(cmp_func)

The master list defines an order for it's elements, we can use this to
build a comparison function. Something like:

ordered_list = [ 'aa', 'qq', 'ttt', 'hh']

def cmp_func(a, b):
    '''A function returning
    -1 if a<b, 0  if a == b 1  if a>b
    ordering is based on the index of a and b in the global
'ordered_list'
    if a or b not in ordered_list, a ValueError is raised'''

    ord = ordered_list.index(a)-ordered_list.index(b)
    if ord < 0:
        return -1
    if ord > 0:
        return 1
    return 0

my_list = [ 'hh', 'qq', 'ttt']   
my_list.sort(cmp_func)
print mylist

will result in

['qq', 'ttt', 'hh']


Hope this helps.

Roeland
-- 
r.b.rigilink@chello.nl

"Half of what I say is nonsense. Unfortunately I don't know which half"