Python Program to Extract Rows of a matrix with Even frequency Elements

Given a Matrix, the task is to write a Python program to extract all the rows which have even frequencies of elements.

Examples:

Input: [[4, 5, 5, 2], [4, 4, 4, 4, 2, 2], [6, 5, 6, 5], [1, 2, 3, 4]]

Output: [[4, 4, 4, 4, 2, 2], [6, 5, 6, 5]]

Explanation: 

• frequency of 4-> 4 which is even
• frequency of 2-> 2 which is even
• frequency of 6-> 2 which is even
• frequency of 5-> 2 which is even

Method 1 : Using list comprehension, Counter() and all()

In this, count is maintained using Counter(), and all() is used to check if all frequencies are even, if not, row is not entered in result list.

Program:

Python3

from collections import Counter

# initializing list
test_list = [[4, 5, 5, 2], [4, 4, 4, 4, 2, 2],
             [6, 5, 6, 5], [1, 2, 3, 4]]

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

# Counter() gets the required frequency
res = [sub for sub in test_list if all(
    val % 2 == 0 for key, val in list(dict(Counter(sub)).items()))]

# printing result
print("Filtered Matrix ? : " + str(res))

Output:

The original list is : [[4, 5, 5, 2], [4, 4, 4, 4, 2, 2], [6, 5, 6, 5], [1, 2, 3, 4]]

Filtered Matrix ? : [[4, 4, 4, 4, 2, 2], [6, 5, 6, 5]]

Method 2 : Using filter(), Counter() and items()

Similar to above method, difference being filter() and lambda function is used for the task of filtering even frequency rows.

Program:

Python3

from collections import Counter

# initializing list
test_list = [[4, 5, 5, 2], [4, 4, 4, 4, 2, 2],
             [6, 5, 6, 5], [1, 2, 3, 4]]

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

# Counter() gets the required frequency
# filter() used to perform filtering
res = list(filter(lambda sub: all(val % 2 == 0 for key,
                                  val in list(dict(Counter(sub)).items())), test_list))

# printing result
print("Filtered Matrix ? : " + str(res))

Output:

The original list is : [[4, 5, 5, 2], [4, 4, 4, 4, 2, 2], [6, 5, 6, 5], [1, 2, 3, 4]]

Filtered Matrix ? : [[4, 4, 4, 4, 2, 2], [6, 5, 6, 5]]