Python – Extract Kth element of every Nth tuple in List
Last Updated :
10 Apr, 2023
Given list of tuples, extract Kth column element of every Nth tuple.
Input :test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)], K = 2, N = 3
Output : [3, 8, 10]
Explanation : From 0th, 3rd, and 6th tuple, 2nd elements are 3, 8, 10.
Input :test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)], K = 0, N = 3
Output : [4, 4, 1]
Explanation : From 0th, 3rd, and 6th tuple, 0th elements are 4, 4, 1.
Method #1 : Using loop
In this, we iterate by skipping N values using 3rd parameter of range(), and append every Kth value in list using loop.
Step-by-step approach:
- Create an empty list named res.
- Start a loop using the range() function to iterate over the indices of test_list, with a step of N.
- Extract the Kth element from the tuple at the current index, and append it to res.
- Print the extracted elements.
Below is the implementation of the above approach:
Python3
test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8),
(6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
print("The original list is : " + str(test_list))
K = 1
N = 3
res = []
for idx in range(0, len(test_list), N):
res.append(test_list[idx][K])
print("The extracted elements : " + str(res))
|
Output
The original list is : [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
The extracted elements : [5, 7, 9]
Time Complexity: O(n), where n is the elements of tuple
Auxiliary Space: O(n), where n is the size of tuple
Method #2 : Using list comprehension
In this, we perform task of extracted elements using one-liner using list comprehension, using same method as above.
Python3
test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8),
(6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
print("The original list is : " + str(test_list))
K = 1
N = 3
res = [test_list[idx][K] for idx in range(0, len(test_list), N)]
print("The extracted elements : " + str(res))
|
Output
The original list is : [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
The extracted elements : [5, 7, 9]
Time Complexity: O(n), where n is the length of the input list.
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the list “test_list”.
Method #3: Using enumerate
This code extracts the Kth element from every Nth tuple in the list test_list. Here’s the step-by-step algorithm for this approach:
- Initialize the list test_list of tuples
- Initialize the values of K and N
- Use a list comprehension to iterate through the tuples in test_list.
- Check if the index of the current tuple is divisible by N using the modulo operator %. If the index is not divisible by N, skip this iteration.
- If the index is divisible by N, extract the Kth element of the tuple and add it to the result list.
- Return the result list.
Python3
test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8),
(6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
print("The original list is : " + str(test_list))
K = 1
N = 3
res = [t[K] for i, t in enumerate(test_list) if i % N == 0]
print("The extracted elements : " + str(res))
|
Output
The original list is : [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
The extracted elements : [5, 7, 9]
Time complexity: O(n), where n is the number of tuples in the list test_list.
Auxiliary space: O(n), since we are creating a new list to store the extracted elements.
Method 4: Using the map() function with a lambda expression.
Step-by-step approach:
- Initialize the list of tuples:
- Initialize the values of K and N
- Use the map() function with a lambda expression to extract the Kth element of every Nth tuple in the
- Print the extracted elements
Below is the implementation of the above approach:
Python3
test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8),
(6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
print("The original list is: " + str(test_list))
K = 1
N = 3
res = list(map(lambda t: t[K], test_list[::N]))
print("The extracted elements: " + str(res))
|
Output
The original list is: [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
The extracted elements: [5, 7, 9]
Time complexity: O(n), where n is the length of the input list.
Auxiliary space: O(n) as we are creating a new list to store the extracted elements.
Method 5: Using numpy array indexing
Step-by-step approach:
- Convert the list of tuples to a numpy array using the numpy.array() function.
- Use array indexing to extract the Kth element of every Nth row in the array.
- Convert the resulting numpy array to a Python list using the .tolist() method.
Python3
import numpy as np
test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8),
(6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
print("The original list is: " + str(test_list))
K = 1
N = 3
arr = np.array(test_list)
res = arr[::N, K].tolist()
print("The extracted elements: " + str(res))
|
OUTPUT:
The original list is: [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
The extracted elements: [5, 7, 9]
The time complexity of this method is O(N), where N is the length of the original list.
The auxiliary space is O(N) as well, because we create a new numpy array to store the data.
Method 6: Using a generator expression with the yield keyword.
Step-by-step approach:
- Define a generator function that takes in the list, K and N as parameters.
- Use a loop to iterate through the indices of the list from 0 to the length of the list, incrementing by N.
- Use the yield keyword to return the Kth element of the tuple at the current index.
- Call the generator function with the test list, K, and N.
- Convert the generator object to a list using the list() function and assign it to the variable res.
- Print the extracted elements using the print() function and string concatenation.
Below is the implementation of the above approach:
Python3
test_list = [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8),
(6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
print("The original list is: " + str(test_list))
K = 1
N = 3
def extract_tuple_element(lst, K, N):
for i in range(0, len(lst), N):
yield lst[i][K]
res = list(extract_tuple_element(test_list, K, N))
print("The extracted elements: " + str(res))
|
Output
The original list is: [(4, 5, 3), (3, 4, 7), (4, 3, 2), (4, 7, 8), (6, 4, 7), (2, 5, 7), (1, 9, 10), (3, 5, 7)]
The extracted elements: [5, 7, 9]
Time complexity: O(N), where N is the length of the list divided by the step size N.
Auxiliary space: O(1), as only constant amount of extra space is used.
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