[Tutor] Sorting more than one list

[Kent Johnson]

Max Noel wrote:
> 
> On Apr 1, 2005, at 09:59, Alan Gauld wrote:
> 
>> Since the data are obviously related (since you need to keep them
>> linked),
>> I'd be inclined to merge the lists into a list of tuples
>>
>> merged = [(a,b,c,d) for a in l1 for b in l2 for c in l3 for d in l4]
>>
>> Then you can sort 'merged' and it should just work.... The default
>> sort will order the tuples by the first member...
>>
>> Alan G.
> 
> 
>     What's the advantage of this compared to the "zip" method (which 
> yields the same result AFAICT)?

Yikes! Alan must have been up too late. They are not the same at all. Alan's code creates a list 
containing *every combination* of one element from each source list:

  >>> a=[1,2,3]
  >>> b=[4,5,6]
  >>> c=[7,8,9]
  >>> [ (aa,bb,cc) for aa in a for bb in b for cc in c ]
[(1, 4, 7), (1, 4, 8), (1, 4, 9), (1, 5, 7), (1, 5, 8), (1, 5, 9), (1, 6, 7), (1, 6, 8), (1, 6, 9), 
(2, 4, 7), (2, 4, 8), (2, 4, 9), (2, 5, 7), (2, 5,
  8), (2, 5, 9), (2, 6, 7), (2, 6, 8), (2, 6, 9), (3, 4, 7), (3, 4, 8), (3, 4, 9), (3, 5, 7), (3, 5, 
8), (3, 5, 9), (3, 6, 7), (3, 6, 8), (3, 6, 9)]

This is very different from zip(a,b,c) which makes a list of *corresponding* items from each list:

  >>> zip(a,b,c)
[(1, 4, 7), (2, 5, 8), (3, 6, 9)]

Kent