On Sat, Nov 14, 2009 at 8:49 AM, Stephen Nelson-Smith <sanel...@gmail.com>wrote:
> Hi Wayne, > > > Just write your own merge: > > (simplified and probably inefficient and first thing off the top of my > head) > > newlist = [] > > for x, y, z in zip(list1, list2, list3): > > I think I need something like izip_longest don't I, since the list wil > be of varied length? > Yes, probably > > Also, where do these lists come from? They can't go in memory - > they're much too big. This is why I felt using some kind if generator > was the right way - I can produce 3 (or 12) sets of tuples... i just > need to work out how to merge them. > you can zip generators, too: In [10]: for x, y in zip(xrange(10), xrange(5)): ....: print x, y ....: ....: 0 0 1 1 2 2 3 3 4 4 In [38]: for x,y in itertools.izip_longest(xrange(10), xrange(5)): ....: print x,y ....: ....: 0 0 1 1 2 2 3 3 4 4 5 None 6 None 7 None 8 None 9 None > if y > x < z: > > newlist.append(x) > > elif x > y < z: > > newlist.append(y) > > elif x > z < y: > > newlist.append(z) > > I'm pretty sure that should work although it's untested. > > Well, no it won't work. The lists are in time order, but they won't > match up. One log may have entries at the same numerical position (ie > the 10th log entry) but earlier than the entries on the previous > lines. To give a simple example: > > List 1 List 2 List 3 > (1, cat) (2, fish) (1, cabbage) > (4, dog) (5, pig) (2, ferret) > (5, phone) (6, horse) (3, sausage) > > Won't this result in the lowest number *per row* being added to the > new list? Or am I misunderstanding how it works? I forgot to add the rest of them. But it appears that you'll have to come up with some better logic. I would use some type of priority queue-style implementation. I don't know if the heapq datatype would be sufficient, or if you'd need to roll your own. The way I would do it is check the top value of each, whichever has the smallest value, pop it off and add that to the new list (which could also be a generator[there's some other term for it but I forget ATM] writing out to a file) then check across the top of all 3 again. Make sense? HTH, Wayne
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