[Tutor] Iterating through two lists at the same time with manipulation..

[Kent Johnson]

Iyer wrote:
> I have 2 lists:
> 
> List 1 has lists in it, such as
> 
> list1 = [[1,'A'],[2,'B'],[3,'C'],[4,'D']]
> 
> There is another list2 such as
> 
> list2 = [[1,'AA'],[3,'CC'], [4,'DD']]
> 
> For eg,
> 
> I wish to iterate over both the lists and produce the output
> 
> a = [[1,'A'],[1,'AA']]
> b = [[2,'B']]
> c = [[3,'C'],[3,'CC']]
> d = [[4,'D'],[4,'DD']]
> 
> Or [a,b,c,d] where a,b,c,d are defined above
> 
> What would be the best and quickest way to carry this out ?

If you want the result ordered by  the first element, you can combine 
the two lists, sort the combined list, and use itertools.groupby() to 
collect the groups:

In [1]: list1 = [[1,'A'],[2,'B'],[3,'C'],[4,'D']]
In [2]: list2 = [[1,'AA'],[3,'CC'], [4,'DD']]
In [3]: import itertools, operator
In [4]: l=list1+list2
In [5]: l.sort()
In [7]: [ list(g) for k, g in itertools.groupby(l, 
key=operator.itemgetter(0))]
Out[7]:
[[[1, 'A'], [1, 'AA']],
  [[2, 'B']],
  [[3, 'C'], [3, 'CC']],
  [[4, 'D'], [4, 'DD']]]


If you don't care about order, you can use a dict to collect like items:
In [8]: d={}
In [9]: for i in l:
    ...:     d.setdefault(i[0], []).append(i)
In [11]: d.values()
Out[11]:
[[[1, 'A'], [1, 'AA']],
  [[2, 'B']],
  [[3, 'C'], [3, 'CC']],
  [[4, 'D'], [4, 'DD']]]

These are in order but in general they will not be.

If you want to preserve the original order then I think you will have to 
  do it "by hand" as John suggests.

Kent