Python - Groups Strings on Kth character

Last Updated : 28 Apr, 2023

Sometimes, while working with Python Strings, we can have a problem in which we need to perform Grouping of Python Strings on the basis of its Kth character. This kind of problem can come in day-day programming. Let's discuss certain ways in which this task can be performed.

Method #1: Using loop

This is one way in which this task can be performed. In this, we perform the task of grouping using a brute force approach. We iterate each string, and group the dictionary after a conditional check using a conditional statement.

Python3

# Python3 code to demonstrate working of
# Groups Strings on Kth character
# Using loop
from collections import defaultdict

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks"]

# printing original list
print("The original list is : " + str(test_list))

# initializing K
K = 2

# Groups Strings on Kth character
# Using loop
res = defaultdict(list)
for word in test_list:
    res[word[K - 1]].append(word)

# printing result
print("The strings grouping : " + str(dict(res)))

Output :

The original list is : ['gfg', 'is', 'best', 'for', 'geeks'] The strings grouping : {'f': ['gfg'], 's': ['is'], 'e': ['best', 'geeks'], 'o': ['for']}

Time complexity: O(n), where n is the number of elements in the test_list.
Auxiliary space: O(n), where n is the number of elements in the test_list.

Method #2: Using map() + loop

This is yet another way to solve this problem. In this variant, additional test of valid character is added using map().

Python3

# Python3 code to demonstrate working of
# Groups Strings on Kth character
# Using loop + map()

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks"]

# printing original list
print("The original list is : " + str(test_list))

# initializing K
K = 2

# Groups Strings on Kth character
# Using loop + map()
res = dict()
for char in map(chr, range(97, 123)):
    words = [idx for idx in test_list if idx[K - 1] == char]
    if words:
        res[char] = words

# printing result
print("The strings grouping : " + str(res))

Output :

The original list is : ['gfg', 'is', 'best', 'for', 'geeks'] The strings grouping : {'f': ['gfg'], 's': ['is'], 'e': ['best', 'geeks'], 'o': ['for']}

Time Complexity: O(N*N) where n is the number of elements in the dictionary. map() + loop is used to perform the task and it takes O(n*n) time.
Auxiliary Space: O(N) additional space of size n is created where n is the number of elements in the dictionary.

Method #3: Using list comprehension

Initializing list
Printing original list
Initializing K
Groups Strings on Kth character Using list comprehension
Printing result

Python3

# Python3 code to demonstrate working of 
# Groups Strings on Kth character
# Using list comprehension
from collections import defaultdict

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks"]

# printing original list
print("The original list is : " + str(test_list))

# initializing K 
K = 2

# Groups Strings on Kth character
# Using list comprehension
res = defaultdict(list)
[res[word[K-1]].append(word) for word in test_list]

# printing result 
print("The strings grouping : " + str(dict(res)))

Output

The original list is : ['gfg', 'is', 'best', 'for', 'geeks']
The strings grouping : {'f': ['gfg'], 's': ['is'], 'e': ['best', 'geeks'], 'o': ['for']}

Time complexity: O(n), where n is the number of words in the input list.
Auxiliary space: O(kn), where k is the number of unique Kth characters and n is the number of words in the input list.

Method 4: Using itertools.groupby() method

Python3

from itertools import groupby
from operator import itemgetter
from collections import defaultdict

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks"]

# printing original list
print("The original list is : " + str(test_list))

# initializing K 
K = 2

# Groups Strings on Kth character
# Using itertools.groupby()
test_list.sort(key=lambda x: x[K-1])
res = defaultdict(list)
for k, g in groupby(test_list, key=itemgetter(K-1)):
    res[k].extend(g)

# printing result 
print("The strings grouping : " + str(dict(res)))

Output

The original list is : ['gfg', 'is', 'best', 'for', 'geeks']
The strings grouping : {'e': ['best', 'geeks'], 'f': ['gfg'], 'o': ['for'], 's': ['is']}

Time complexity: O(NlogN), where N is the length of the input list.
Auxiliary space: O(N), due to the creation of a defaultdict and a sorted copy of the input list.