Python – Filter Sorted Rows

Given a Matrix, extract rows that are sorted, either by ascending or descending.

Input : test_list = [[3, 6, 8, 10], [1, 8, 2, 4, 3], [8, 5, 3, 2], [1, 4, 5, 3]] 
Output : [[3, 6, 8, 10], [8, 5, 3, 2]] 
Explanation : Both lists are ordered, 1st ascending, second descending.

Input : test_list = [[3, 6, 8, 10], [1, 8, 2, 4, 3], [8, 5, 7, 2], [1, 4, 5, 3]] 
Output : [[3, 6, 8, 10]] 
Explanation : List ordered in ascending order. 

Method #1 : Using list comprehension + sorted() + reverse

In this, we check for each row, perform sort by sorted(), and reverse sorted by passing reverse as key. 

Python3

# Python3 code to demonstrate working of
# Filter Sorted Rows
# Using list comprehension + sorted() + reverse

# initializing list
test_list = [[3, 6, 8, 10], [1, 8, 2, 4, 3], [8, 5, 3, 2], [1, 4, 5, 3]]

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

# filtering using sorted() and reverse as key
res = [sub for sub in test_list if sub == list(
    sorted(sub)) or sub == list(sorted(sub, reverse=True))]

# printing result
print("Extracted rows : " + str(res))

Output:

The original list is : [[3, 6, 8, 10], [1, 8, 2, 4, 3], [8, 5, 3, 2], [1, 4, 5, 3]] Extracted rows : [[3, 6, 8, 10], [8, 5, 3, 2]]

Time Complexity: O(n*nlogn)
Auxiliary Space: O(n)

Method #2 : Using filter() + lambda + sorted() + reverse

In this, we perform task of filtering using lambda and sorted() and reverse can be used to check for equality with ordered list.

Python3

# Python3 code to demonstrate working of
# Filter Sorted Rows
# Using filter() + lambda + sorted() + reverse

# initializing list
test_list = [[3, 6, 8, 10], [1, 8, 2, 4, 3], [8, 5, 3, 2], [1, 4, 5, 3]]

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

# filtering using sorted() and reverse as key
res = list(filter(lambda sub: sub == list(sorted(sub)) or sub ==
                  list(sorted(sub, reverse=True)), test_list))

# printing result
print("Extracted rows : " + str(res))

Output:

The original list is : [[3, 6, 8, 10], [1, 8, 2, 4, 3], [8, 5, 3, 2], [1, 4, 5, 3]] Extracted rows : [[3, 6, 8, 10], [8, 5, 3, 2]]

Time Complexity: O(n*logn), where n is the length of the input list. This is because we’re using the built-in sorted() function which has a time complexity of O(nlogn) in the worst case.
Auxiliary Space: O(n), where n is the length of the input list as we’re using additional space other than the input list itself.