Python | Get Kth element till N

Sometimes, we may come across a utility in which we require to get the first N sublist elements that too only a particular index. This can have an application in queuing to get only the Kth N person’s name. Let’s discuss certain ways in which this can be done. 

Method #1 : Using list comprehension and list slicing The above two powerful Python utilities can be useful here as well to get the result as list comprehension can extract the element slicing can restrict the size we need to extract. 

Python3

# Python3 code to demonstrate
# Get Kth element till N
# using list comprehension + list slicing

# initializing list
test_list = [['Geeks', 1, 15], ['for', 3, 5], ['Geeks', 3, 7]]

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

# initializing N
N = 2

# initializing K
K = 1

# using list comprehension + list slicing
# Get Kth element till N
res = [i[K] for i in test_list[ : N]]

# print result
print("The Kth element of sublist till N : " + str(res))

Output : 

The original list : [['Geeks', 1, 15], ['for', 3, 5], ['Geeks', 3, 7]]
The Kth element of sublist till N : [1, 3]

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

Method #2 : Using map() + itemgetter() + islice() The combination of above 3 functions can be used to perform this particular task. The itemgetter function gets the element to extract, islice slices till N and map function combines the result. 

Python3

# Python3 code to demonstrate
# Get Kth element till N
# using map() + itemgetter() + islice()
from operator import itemgetter
from itertools import islice

# initializing list
test_list = [['Geeks', 1, 15], ['for', 3, 5], ['Geeks', 3, 7]]

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

# initializing N
N = 2

# initializing K
K = 1

# using map() + itemgetter() + islice()
# Get Kth element till N
res = list(map(itemgetter(K), islice(test_list, 0, N)))

# print result
print("The Kth element of sublist till N : " + str(res))

Output : 

The original list : [['Geeks', 1, 15], ['for', 3, 5], ['Geeks', 3, 7]]
The Kth element of sublist till N : [1, 3]

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

Method 3: Using a simple for loop

Use simple for loop to iterate over the list and extract the Kth element of each sublist till N. 

Python3

# initializing list
test_list = [['Geeks', 1, 15], ['for', 3, 5], ['Geeks', 3, 7]]

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

# initializing N
N = 2

# initializing K
K = 1

# using for loop
# Get Kth element till N
res = []
for i in range(N):
    res.append(test_list[i][K])

# print result
print("The Kth element of sublist till N : " + str(res))

Output

The original list : [['Geeks', 1, 15], ['for', 3, 5], ['Geeks', 3, 7]]
The Kth element of sublist till N : [1, 3]

Time complexity: O(N)
Auxiliary space: O(N), as we need to store the Kth element of each sublist till N in a separate list.