Grouping items in a list based on their contents

Question

I have an array which looks like this:

array1 = [[4000,"Mark",5],[4100,"George",3],[4000,"Mark",2],[4200,"Steve",4],[4100,"George",2],[4000,"Mark",1]]

I'm wondering how I can reformat this array to look like this:

array2 = [[4000,"Mark",[5,2,1]],[4100,"George",[3,2]],[4200,"Steve",4]]

Answer 1

You can use an ordered dictionary (collections.OrderedDict) to store the first 2 items as key and the common numbers in a list as values:

>>> from collections import OrderedDict
>>> d=OrderedDict()
>>> for i,j,k in array1:
...    d.setdefault((i,j),[]).append(k)
... 
>>> [[i,j,k] for (i,j),k in d.items()]
[[4000, 'Mark', [5, 2, 1]], [4100, 'George', [3, 2]], [4200, 'Steve', [4]]] 

Answer 2

[a+[[c[-1] for c in b]] for a,b in itertools.groupby(operator.itemgetter(0,1),sorted(array1))]

I guess ... i doubt this is gonna help you learn anything ... a more appropriate place to ask for help if you dont even know where to start, is in class (ask your teacher or classmates) ...