Python | Alternate range slicing in list

List slicing is quite common utility in Python, one can easily slice certain elements from a list, but sometimes, we need to perform that task in non-contiguous manner and slice alternate ranges. Let's discuss how this particular problem can be solved. 

Method #1 : Using list comprehension List comprehension can be used to perform this particular task with ease as it can be used to run a loop and only filter the elements that leave a remainder more than half of target slice size multiplied by 2. By this way we can extract the sliced numbers in range alternatively. 

# Python3 code to demonstrate
# alternate range slicing 
# using list comprehension

# initializing list 
test_list = [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]

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

# Select range size 
N = 3

# using list comprehension
# alternate range slicing
res = [test_list[i] for i in range(len(test_list)) 
                              if i % (N * 2) >= N]

# print result
print("The alternate range sliced list : " + str(res))

The original list : [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]
The alternate range sliced list : [8, 9, 10, 20, 7, 30]

Time complexity: O(n), where n is the length of the list.
Auxiliary space: O(n), as a new list of size n is created to store the result of list comprehension.

  Method #2 : Using enumerate() + list comprehension The list comprehension can also be combined with the enumerate function to perform this task. The advantage of using enumerate is that we can track of index along with the value and it's more efficient and has lesser run-time than the above function. 

# Python3 code to demonstrate
# alternate range slicing 
# using list comprehension + enumerate()

# initializing list 
test_list = [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]

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

# Select range size 
N = 3

# using list comprehension + enumerate()
# alternate range slicing
res = [val for i, val in enumerate(test_list)
                         if i % (N * 2) >= N]

# print result
print("The alternate range sliced list : " + str(res))

The original list : [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]
The alternate range sliced list : [8, 9, 10, 20, 7, 30]

Time complexity: O(n), where n is the length of the list, as the list comprehension and the enumerate() function both have linear time complexity.
Auxiliary space: O(n), as a new list of size n is created to hold the result of the list comprehension.

Method #3 : Using itertools +for loop

In this code, a list test_list is initialized with a set of values, and a variable N is defined as the range size for the alternate slicing.

Then, a loop is used to iterate over the elements of the test_list using the range() function. For each iteration, the index i is checked to see if it leaves a remainder of more than half of N when divided by N multiplied by 2. If the condition is true, the islice() function is used to slice the list and return only the element at the current index i. The result of the islice() function is then appended to the res list using the extend() method.

Finally, the res list is printed to the console.

from itertools import islice

# initializing list 
test_list = [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]

# Select range size 
N = 3

# using islice()
# alternate range slicing
res = []
for i in range(len(test_list)):
    if i % (N * 2) >= N:
        res.extend(islice(test_list, i, i+1))

# print result
print("The alternate range sliced list:", res)
#This code is contributed by Edula Vinay Kumar Reddy

The alternate range sliced list: [8, 9, 10, 20, 7, 30]

The time complexity of this code is O(n), where n is the length of the test_list. This is because the loop iterates over the elements of the list once. The space complexity of this code is also O(n), because the res list is created and grows by one element for each iteration of the loop.

Method 4: Using filter() +  enumerate() +  method:

This approach uses the enumerate() function to get the index and value pairs of the test_list. The filter() function is then used to filter out the elements that do not satisfy the alternate range slicing condition. Finally, a list comprehension is used to extract the values from the filtered list. 

Step-by-step approach:

• Initialize a list named test_list with some integer values.
• Print the original list using the print() function.
• Set a variable N equal to the desired range size.
• Use the enumerate() function to get the index and value pairs of the test_list.
• Use the filter() function to filter out the elements that do not satisfy the alternate range slicing condition.
• In this case, we want to keep the elements whose index modulo (N * 2) is greater than or equal to N.
• Convert the filtered values into a list using the list() function.
• Extract the values from the filtered list using a list comprehension.
• Print the resulting list using the print() function.

Below is the implementation of the above approach:

# Python3 code to demonstrate
# alternate range slicing 
# using filter()

# initializing list 
test_list = [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]

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

# Select range size 
N = 3

# using filter()
# alternate range slicing
res = list(filter(lambda x: x[0] % (N * 2) >= N, enumerate(test_list)))

# extracting values from filtered list
res = [x[1] for x in res]

# print result
print("The alternate range sliced list : " + str(res))

The original list : [2, 4, 6, 8, 9, 10, 12, 16, 18, 20, 7, 30]
The alternate range sliced list : [8, 9, 10, 20, 7, 30]

Time complexity: O(n),
 Auxiliary space: O(n), because it creates a new list to store the filtered values.