python list group by first character

Question

list1=['hello','hope','hate','hack','bit','basket','code','come','chess']

What I need is:

list2=[['hello','hope','hate','hack'],['bit','basket'],['code','come','chess']]

If the first character is the same and is the same group, then sublist it.

How can I solve this?

Answer 1

You can use itertools.groupby:

>>> from itertools import groupby
>>> list1 = ['hello','hope','hate','hack','bit','basket','code','come','chess']
>>> [list(g) for k, g in groupby(list1, key=lambda x: x[0])]
[['hello', 'hope', 'hate', 'hack'], ['bit', 'basket'], ['code', 'come', 'chess']]

Answer 2

Expanding on TerryA's answer:

To create a dict with the first letter as key and the matching elements as value, you can do

>>> list1=['hello','hope','hate','hack','bit','basket','code','come','chess', 'archetype', 'cheese']
... mydict={}
... for k, g in groupby(list1, key=lambda x: x[0]):
...    if k in mydict:
...        mydict[k] += g
...    else:
...        mydict[k]=list(g)
... print(mydict)
{'h': ['hello', 'hope', 'hate', 'hack'], 'b': ['bit', 'basket'], 'a': ['archetype'], 'c': ['code', 'come', 'chess', 'cheese']}

This also works if list1 is not sorted (as shown) and it can, of course, also be converted to a list of lists again with

>>> [v for k, v in mydict.items()]
[['hello', 'hope', 'hate', 'hack'], ['bit', 'basket'], ['archetype'], ['code', 'come', 'chess', 'cheese']]

Answer 3

You can do that with partition_by function from my funcy library:

from funcy import partition_by
list2 = partition_by(0, list1)

Note that this will only work if list1 is already sorted, as with itertools.groupby. If list1 is unsorted than sorting it and then partitioning would be inefficient, the better way would be to use group_by function:

from funcy import group_by
list2 = group_by(0, list1).values()

Answer 4

In Python 3.7+ (i.e. a version where dictionaries maintain insertion order) you could simply use a dict of lists keyed by the first character to group the words. This works with both sorted and unsorted input:

list1 = ['hello', 'hope', 'hate', 'bit', 'basket', 'code', 'come', 'chess', 'hack']
d = {}

for word in list1:
    d.setdefault(word[0], []).append(word)
list2 = list(d.values())
print(list2)
# [['hello', 'hope', 'hate', 'hack'], ['bit', 'basket'], ['code', 'come', 'chess']]