Python program to convert a list into a list of lists using a step value
Last Updated :
21 Apr, 2023
Given a List, the task here is to write a python program that can split the list into list of lists using by a step value here denoted via K.
Input : test_list = [5, 6, 3, 2, 7, 1, 9, 10, 8], K = 3
Output : [[5, 2, 9], [6, 7, 10], [3, 1, 8]]
Explanation : 5, 2 and 9 are 0th, 3rd and 6th element respectively, and hence ( difference 3 ) grouped together.
Input : test_list = [5, 6, 3, 2, 7, 1], K = 3
Output : [[5, 2], [6, 7], [3, 1]]
Explanation : 5 and 2 are 0th and 3rd element respectively, and hence ( difference 3 ) grouped together.
Method 1: Using loop and slicing
In this, loop is employed to skip numbers as required, and each subsequent skipped sliced list is extracted using slicing and is appended to result.
Example:
Python3
test_list = [5, 6, 3, 2, 7, 1, 9, 10, 8]
print("The original list is : " + str(test_list))
K = 3
res = []
for idx in range(0, K):
res.append(test_list[idx::K])
print("Stepped splitted List : " + str(res))
|
Output
The original list is : [5, 6, 3, 2, 7, 1, 9, 10, 8]
Stepped splitted List : [[5, 2, 9], [6, 7, 10], [3, 1, 8]]
Time Complexity: O(n)
Auxiliary Space: O(n)
Method 2: Using list comprehension and slicing
Similar to above method, only difference being usage of list comprehension for task of iteration instead of loop for shorthand alternative.
Step-by-step approach:
- Initialize a variable called K with the value 3.
- Use a list comprehension to create a new list called res that contains sublists of test_list. The sublist at index i in res is created by selecting every Kth element starting from index i in test_list.
- Print the resulting list.
Example:
Python3
test_list = [5, 6, 3, 2, 7, 1, 9, 10, 8]
print("The original list is : " + str(test_list))
K = 3
res = [test_list[idx::K] for idx in range(0, K)]
print("Stepped splitted List : " + str(res))
|
Output:
The original list is : [5, 6, 3, 2, 7, 1, 9, 10, 8]
Stepped splitted List : [[5, 2, 9], [6, 7, 10], [3, 1, 8]]
Time Complexity: O(n)
Auxiliary Space: O(n)
Method 4: Using numpy array slicing
- Import the numpy library.
- Initialize the input list.
- Convert the input list to a numpy array.
- Initialize the skips variable.
- Use numpy array slicing to split the array into K chunks.
- Convert the resulting arrays back to Python lists.
- Store the sliced lists in the “res” variable.
- Print the resulting “res” variable.
Python3
import numpy as np
test_list = [5, 6, 3, 2, 7, 1, 9, 10, 8]
arr = np.array(test_list)
K = 3
res = [arr[i::K] for i in range(K)]
res = [list(x) for x in res]
print("Stepped splitted List : " + str(res))
|
Output:
Stepped splitted List : [[5, 2, 9], [6, 7, 10], [3, 1, 8]]
Time complexity: O(n), where n is the length of the input list.
Auxiliary space: O(n), where n is the length of the input list. This is because we need to store the sliced arrays in memory as separate lists.
Method 5: Using a list comprehension and the enumerate() function:
Python3
test_list = [5, 6, 3, 2, 7, 1, 9, 10, 8]
print("The original list is : " + str(test_list))
K = 3
res = [[test_list[i] for i in range(len(test_list)) if idx == i % K] for idx, val in enumerate(test_list)]
print("Stepped splitted List : " + str(res[:K]))
|
Output
The original list is : [5, 6, 3, 2, 7, 1, 9, 10, 8]
Stepped splitted List : [[5, 2, 9], [6, 7, 10], [3, 1, 8]]
Time complexity: O(N), where N is the length of the given list.
Auxiliary space: O(N/K), which is the space required to store the sliced sublists.
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