Python – Group similar elements into Matrix

Sometimes, while working with Python Matrix, we can have a problem in which we need to perform grouping of all the elements with are same. This kind of problem can have application in data domains. Let’s discuss certain ways in which this task can be performed.

Input : test_list = [1, 3, 4, 4, 2, 3]
Output : [[1], [2], [3, 3], [4, 4]]

Input : test_list = [1, 3, 4, 2]
Output : [[1], [2], [3], [4]]

Method #1 : Using list comprehension + groupby()
The combination of above functions provide possible solution to this problem. In this, we perform task of grouping using groupby() and list comprehension assists in iteration.

# Python3 code to demonstrate working of
# Group similar elements into Matrix
# Using list comprehension + groupby()
from itertools import groupby

# initializing list
test_list = [1, 3, 5, 1, 3, 2, 5, 4, 2]

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

# Group similar elements into Matrix
# Using list comprehension + groupby()
res = [list(val) for key, val in groupby(sorted(test_list))]

# printing result
print("Matrix after grouping : " + str(res))

Output :

The original list : [1, 3, 5, 1, 3, 2, 5, 4, 2]
Matrix after grouping : [[1, 1], [2, 2], [3, 3], [4], [5, 5]]

Method #2 : Using list comprehension + Counter()
This is yet another approach to this problem. In this, we get the values along with it’s frequency using Counter() and then employ list comprehension to multiply each element with frequency to get duplicates

# Python3 code to demonstrate working of
# Group similar elements into Matrix
# Using list comprehension + Counter()
from collections import Counter

# initializing list
test_list = [1, 3, 5, 1, 3, 2, 5, 4, 2]

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

# Group similar elements into Matrix
# Using list comprehension + Counter()
temp = Counter(test_list)
res = [[key] * val for key, val in temp.items()]

# printing result
print("Matrix after grouping : " + str(res))

Output :

The original list : [1, 3, 5, 1, 3, 2, 5, 4, 2]
Matrix after grouping : [[1, 1], [2, 2], [3, 3], [4], [5, 5]]