Python – Rear character String categorization

Sometimes, we have a use case in which we need to perform the grouping of strings by various factors, like last letter or any other factor. These type of problems are typical to database queries and hence can occur in web development while programming. This article focuses on one such grouping by last letter of string. Let’s discuss certain ways in which this can be performed. 

Method #1: Using next() + lambda + loop

The combination of above 3 functions is used to solve this particular problem by the naive method. The lambda function performs the task of finding like rear character, and next function helps in forward iteration. 

Python3

# Python3 code to demonstrate
# Rear character String categorization
# using next() + lambda + loop

# initializing list
test_list = ['an', 'apple', 'geek', 'for', 'greek', 'free']

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

# using next() + lambda + loop
# Rear character String categorization
util_func = lambda x, y: x[len(x) - 1] == y[len(y) - 1]
res = []
for sub in test_list:
    ele = next((x for x in res if util_func(sub, x[len(x) - 1])), [])
    if ele == []:
        res.append(ele)
    ele.append(sub)

# print result
print("The list after Categorization : " + str(res))

Output : 

The original list : ['an', 'apple', 'geek', 'for', 'greek', 'free']
The list after Categorization : [['an'], ['apple', 'free'], ['geek', 'greek'], ['for']]

Time Complexity: O(n), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.

Method #2: Using sorted() + groupby()

This particular task can also be solved using the groupby function which offers a conventional method to solve this problem. The sorted function sorts the elements by rear character to be feed to groupby for the relevant grouping. 

Python3

# Python3 code to demonstrate
# Rear character String categorization
# using sorted() + groupby()
from itertools import groupby

# initializing list
test_list = ['an', 'apple', 'geek', 'for', 'greek', 'free']

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

# using sorted() + groupby()
# Rear character String categorization
util_func = lambda x: x[len(x) - 1]
temp = sorted(test_list, key = util_func)
res = [list(ele) for i, ele in groupby(temp, util_func)]

# print result
print("The list after Categorization : " + str(res))

Output : 

The original list : ['an', 'apple', 'geek', 'for', 'greek', 'free']
The list after Categorization : [['an'], ['apple', 'free'], ['geek', 'greek'], ['for']]

Time Complexity: O(n*nlogn), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.

Method #3: Using endswith() and for loops

In this method, we will create a new list with all the unique ending characters of the words from the given list, then we can group all the words based on their last characters using endswith() function.

Python3

# Python3 code to demonstrate
# Rear character String categorization

# initializing list
test_list = ['an', 'apple', 'geek', 'for', 'greek', 'free']

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

x = []
for i in test_list:
    if i[-1] not in x:
        x.append(i[-1])
res = []
for i in x:
    x = []
    for j in test_list:
        if(j.endswith(i)):
            x.append(j)
    res.append(x)

# print result
print("The list after Categorization : " + str(res))

Output

The original list : ['an', 'apple', 'geek', 'for', 'greek', 'free']
The list after Categorization : [['an'], ['apple', 'free'], ['geek', 'greek'], ['for']]

Method #4: Using defaultdict

• Import defaultdict from the collections module.
• Initialize the test_list.
• Create an empty defaultdict with a list as its default value. This means that if we try to access a key that doesn’t exist in the dictionary, it will automatically create a new empty list for that key.
• Iterate through the test_list, and for each string, append it to the list corresponding to its last character in the defaultdict.
• Finally, print out the values of the dictionary as a list.

Python3

# Python3 code to demonstrate
# Rear character String categorization
# using defaultdict

from collections import defaultdict

# initializing list
test_list = ['an', 'apple', 'geek', 'for', 'greek', 'free']

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

# using defaultdict
# Rear character String categorization
d = defaultdict(list)
for s in test_list:
    d[s[-1]].append(s)

# print result
print("The list after Categorization : " + str(list(d.values())))

Output

The original list : ['an', 'apple', 'geek', 'for', 'greek', 'free']
The list after Categorization : [['an'], ['apple', 'free'], ['geek', 'greek'], ['for']]

Time complexity: O(n)
Auxiliary space: O(n)