Python Group List of Lists

Question

I have a list of lists in the following format:

[['Sarah', '12', 'Chocolate'],
 ['Anders', '11', 'Vanilla'],
 ['Sarah', '13', 'Strawberry'],
 ['John', '11', 'None'],
 # ...
]

And I want to group the sublists as follows:

[['Sarah', '12', 'Chocolate', '13', 'Strawberry'],
 ['Anders', '11', 'Vanilla'],
 ['John', '11', 'None'],
 # ...
]

Where I group by the first item of the sublists and order by the second (so Sarahs with age 12 come before Sarahs with age 13).

How to do this?

Answer 1

You didn't show any code, so I won't give a complete solution.

One good data structure would be to use a dict , with names as keys and a list of tuples as values:

data =[['Sarah', '12', 'Chocolate'],
    ['Anders', '11', 'Vanilla'],
    ['Sarah', '13', 'Strawberry'],
    ['John', '11', 'None']]

grouped = {}

for name, x, y in data:
    grouped.setdefault(name, []).append((x,y))

print(grouped)
# {'Sarah': [('12', 'Chocolate'), ('13', 'Strawberry')], 'Anders': [('11', 'Vanilla')], 'John': [('11', 'None')]}

You'd just need to sort the values.

Answer 2

A few possible solutions jump to mind, but here's one. Read through the comments and feel free to ask questions:

from collections import defaultdict

l = [
        ['Sarah', '12', 'Chocolate'],
        ['Anders', '11', 'Vanilla'],
        ['Sarah', '13', 'Strawberry'],
        ['John', '11', 'None'],
]

d = defaultdict(list)

for entry in l:
    d[entry[0]].append(entry[1:])

# Here d looks something like this:
# {'Sarah': [['12', 'Chocolate'], ['13', 'Strawberry']], ...}

result = [
    # 1. Sort the list by the first element of each sublist (parsed as an int).
    # 2. Flatten the result as in https://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python/952952#952952.
    # 3. Prepend the key (name).
    [k] + [item for sublist in sorted(v, key=lambda x: int(x[0])) for item in sublist]
    for k,v in d.items()
]

print(result)

# Output: [['Sarah', '12', 'Chocolate', '13', 'Strawberry'], ['Anders', '11', 'Vanilla'], ['John', '11', 'None']]

Answer 3

You can try this :

import itertools


list_1=[['Sarah', '12', 'Chocolate'],
       ['Anders', '11', 'Vanilla'],
       ['Sarah', '13', 'Strawberry'],
       ['John', '11', 'None']]

repeated_list=[]
unique_list=[]
for item_1,item_2 in itertools.combinations(list_1,2):
    if item_1[0]==item_2[0]:
        repeated_list.append(item_1+item_2)

    else:

        for item_3 in repeated_list:
            if item_2[0] not in item_3:
                if item_2 not in unique_list:
                    unique_list.append(item_2)

            elif item_1[0] not in item_3:
                if item_1 not in unique_list:
                    unique_list.append(item_1)


print(unique_list+repeated_list)

Output:

[['John', '11', 'None'], ['Anders', '11', 'Vanilla'], ['Sarah', '12', 'Chocolate', 'Sarah', '13', 'Strawberry']]