In Python, How To Group Elements Together, Based On A Key (group Adjacent)?

In python, I'd like to group elements together based on a key (in example below, key is second element, or element[1]). initial_array = [[10, 0], [30, 0], [40, 2], [20, 2], [90, 0

Solution 1:

You can use itertools.groupby:

>>> from itertools import groupby
>>> from operator import itemgetter
>>> lis = [[10, 0], [30, 0], [40, 2], [20, 2], [90, 0], [80, 0]]
>>> [list(g) for k,g in groupby(lis, key=itemgetter(1))]
[[[10, 0], [30, 0]],
 [[40, 2], [20, 2]],
 [[90, 0], [80, 0]]]

For second one:

>>> ans = []
for k,g in groupby(lis, key=itemgetter(1)):
    l = list(g)
    ans.append(l)
    iflen(ans) > 1:
        ans[-2].append(l[0])
...         
>>> ans
[[[10, 0], [30, 0], [40, 2]],
 [[40, 2], [20, 2], [90, 0]],
 [[90, 0], [80, 0]]]

Update:

>>> from itertools import zip_longest
>>> lis = [[[10, 0], [30, 0]],
 [[40, 2], [20, 2]],
 [[90, 0], [80, 0]]]
>>> [x + ([y[0]] if y else []) for x,y in 
                                        zip_longest(lis,lis[1:])]
[[[10, 0], [30, 0], [40, 2]],
 [[40, 2], [20, 2], [90, 0]],
 [[90, 0], [80, 0]]]