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))

Output :

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]

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))

Output :

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]