[Tutor] Iterable Understanding
waynejwerner at gmail.com
Sat Nov 14 16:19:25 CET 2009
On Sat, Nov 14, 2009 at 8:49 AM, Stephen Nelson-Smith <sanelson at gmail.com>wrote:
> Hi Wayne,
> > Just write your own merge:
> > (simplified and probably inefficient and first thing off the top of my
> > 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?
> 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 : for x, y in zip(xrange(10), xrange(5)):
....: print x, y
In : for x,y in itertools.izip_longest(xrange(10), xrange(5)):
....: print x,y
> 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.
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Tutor