Python – Count the frequency of matrix row length

Given a Matrix, the task is to write a Python program to get the count frequency of its rows lengths.

Input : test_list = [[6, 3, 1], [8, 9], [2], [10, 12, 7], [4, 11]]

Output : {3: 2, 2: 2, 1: 1}

Explanation : 2 lists of length 3 are present, 2 lists of size 2 and 1 of 1 length is present.

Input : test_list = [[6, 3, 1], [8, 9], [10, 12, 7], [4, 11]]

Output : {3: 2, 2: 2}

Explanation : 2 lists of length 3 are present, 2 lists of size 2.

Method #1 : Using dictionary + loop

In this we check for each row length, if length has occurred in the memorize dictionary, then the result is incremented or if a new size occurs, the element is registered as new.

Python3

# Python3 code to demonstrate working of
# Row lengths counts
# Using dictionary + loop

# initializing list
test_list = [[6, 3, 1], [8, 9], [2],
             [10, 12, 7], [4, 11]]

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

res = dict()
for sub in test_list:

    # initializing incase of new length
    if len(sub) not in res:
        res[len(sub)] = 1

    # increment in case of length present
    else:
        res[len(sub)] += 1

# printing result
print("Row length frequencies : " + str(res))

Output

The original list is : [[6, 3, 1], [8, 9], [2], [10, 12, 7], [4, 11]]
Row length frequencies : {3: 2, 2: 2, 1: 1}

Time complexity: O(n), where n is the total number of sublists in the input list.
Auxiliary space complexity: O(n), where n is the total number of sublists in the input list. The dictionary used to store the frequency of each length takes up O(n) space.

Method #2 : Using Counter() + map() + len()

In this, map() and len() get the lengths of each sublist in the matrix, Counter is used to keep the frequency of each of the lengths.

Python3

# Python3 code to demonstrate working of
# Row lengths counts
# Using Counter() + map() + len()
from collections import Counter

# initializing list
test_list = [[6, 3, 1], [8, 9], [2],
             [10, 12, 7], [4, 11]]

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

# Counter gets the frequencies of counts
# map and len gets lengths of sublist
res = dict(Counter(map(len, test_list)))

# printing result
print("Row length frequencies : " + str(res))

Output:

The original list is : [[6, 3, 1], [8, 9], [2], [10, 12, 7], [4, 11]]
Row length frequencies : {3: 2, 2: 2, 1: 1}

Time complexity: O(n), where n is the total number of elements in the input list.
Auxiliary space: O(k), where k is the number of unique row lengths in the input list.

Method #3 : Using count() method

Python3

# Python3 code to demonstrate working of
# Row lengths counts
# Using dictionary + loop

# initializing list
test_list = [[6, 3, 1], [8, 9], [2],
             [10, 12, 7], [4, 11]]

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

res = dict()
x = []
for sub in test_list:
    x.append(len(sub))
y = set(x)
for i in y:
    res[i] = x.count(i)

# printing result
print("Row length frequencies : " + str(res))

Output

The original list is : [[6, 3, 1], [8, 9], [2], [10, 12, 7], [4, 11]]
Row length frequencies : {1: 1, 2: 2, 3: 2}

Time complexity: O(n^2) where n is the length of the input list. 
Auxiliary space: O(n) where n is the length of the input list. 

Method #4 : Using operator.countOf() method

Python3

# Python3 code to demonstrate working of
# Row lengths counts
# Using dictionary + loop
import operator as op
# initializing list
test_list = [[6, 3, 1], [8, 9], [2],
             [10, 12, 7], [4, 11]]

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

res = dict()
x = []
for sub in test_list:
    x.append(len(sub))
y = set(x)
for i in y:
    res[i] = op.countOf(x,i)

# printing result
print("Row length frequencies : " + str(res))

Output

The original list is : [[6, 3, 1], [8, 9], [2], [10, 12, 7], [4, 11]]
Row length frequencies : {1: 1, 2: 2, 3: 2}

Time Complexity: O(N*N)
Auxiliary Space:  O(N*N)

Method #6: Using list comprehension and dictionary

1. Initialize a list of sublists test_list.
2. Print the original list using the print() function.
3. Use a list comprehension to create a new list lengths containing the lengths of all sublists in test_list. The len() function is used to get the length of each sublist.
4. Use a dictionary comprehension to count the frequency of each length in the lengths list obtained in step 3. The key of each key-value pair in the resulting dictionary is a length, and the value is the number of times that length appears in the lengths list. The count() method is used to count the frequency of each length.
5. Store the resulting dictionary in the variable res.
6. Print the resulting dictionary using the print() function.

Python3

# Python3 code to demonstrate working of
# Row lengths counts
# Using list comprehension and dictionary

# initializing list
test_list = [[6, 3, 1], [8, 9], [2],
             [10, 12, 7], [4, 11]]

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

# Using list comprehension and dictionary
lengths = [len(sub) for sub in test_list]
res = {length: lengths.count(length) for length in lengths}

# printing result
print("Row length frequencies : " + str(res))

Output

The original list is : [[6, 3, 1], [8, 9], [2], [10, 12, 7], [4, 11]]
Row length frequencies : {3: 2, 2: 2, 1: 1}

Time complexity: O(n), where n is the length of test_list.
Auxiliary space: O(k), where k is the number of distinct row lengths in test_list.