Python – Sort from Kth index in List

Given a list of elements, perform sort from Kth index of List.

Input  :  test_list = [7, 3, 7, 6, 4, 9], K = 2

Output : [7, 3, 4, 6, 7, 9]

Explanation :  List is unsorted till 3 (1st index), From 2nd Index, its sorted. 

Input  :  test_list = [5, 4, 3, 2, 1], K= 3

Output : [5, 4, 3, 1, 2]

Explanation :  Only last two elements, 1, 2 are sorted. 

Method #1 : Using loop + list slicing

This is one of the ways in which this task can be performed. In this, we extract each element before K in list and then perform list slice using slice symbol and perform sort over the extracted sliced list.

Python3

# Python3 code to demonstrate working of
# Perform sort from Kth index
# Using loop + list slicing

# initializing list
test_list = [8, 4, 7, 3, 2, 14, 6]

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

# initializing K
K = 3

# Using loop + list slicing
res = []

# Using loop to extract elements till K
for idx, ele in enumerate(test_list):
    if idx < K:
        res.append(ele)

# join sorted and unsorted segments
res = res + sorted(test_list[K:])

# printing result
print("Partially sorted list : " + str(res))

Output

The original list : [8, 4, 7, 3, 2, 14, 6]
Partially sorted list : [8, 4, 7, 2, 3, 6, 14]

Time Complexity: O(n*logn), as sorted function is used.
Auxiliary Space: O(n), where n is the size of list

Method #2 : Using double List slicing

This is yet another way in which we perform this task. In this, we perform the task of list slicing for slicing the unsort part as well and perform concatenation, delivering a one liner alternative to solve this problem.

Python3

# Python3 code to demonstrate working of
# Perform sort from Kth index
# Using  double List slicing

# initializing list
test_list = [8, 4, 7, 3, 2, 14, 6]

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

# initializing K
K = 3

# Using loop + list slicing
res = []

# Using loop to extract elements till K
# Concatenating unsort and sorted part as one liner
res = test_list[:K] + sorted(test_list[K:])

# printing result
print("Partially sorted list : " + str(res))

Output

The original list : [8, 4, 7, 3, 2, 14, 6]
Partially sorted list : [8, 4, 7, 2, 3, 6, 14]

Time Complexity: O(n*logn), as sorted function is used.
Auxiliary Space: O(n) where n is the number of elements in the list “test_list”.