Python | Alternate Cycling in list

Sometimes, while working with a Python list, we can have a problem in which we need to perform the access/printing of list in different ways. In some variations, there might be a need for printing the list in an alternate cyclic way, i.e printing elements from front and read alternatively. This is a popular problem in-school programming. Let’s discuss a certain way in which this task can be performed.

Method : Using reversed() + islice() + iter() + cycle() + next() + list comprehension
The combination of above functions can be employed to perform this task. In this, reversed() and iter() are used to create iterators of reversed and normal sequence list respectively, cycle() performs the task of alternate access. The islice() performs the task of extracting the elements and construct to new list. The next() performs the task of accessing elements.

Code :



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# Python3 code to demonstrate working of
# Alternate Cycling in list
# using reversed() + islice() + iter() + cycle() + next() + list comprehension
from itertools import islice, cycle
  
# initialize list
test_list = [5, 6, 8, 9, 10, 21, 3]
  
# printing original list
print("The original list is : " + str(test_list))
  
# Alternate Cycling in list
# using reversed() + islice() + iter() + cycle() + next() + list comprehension
res = [next(i) for i in islice(cycle((iter(test_list), 
                                     reversed(test_list))), len(test_list))]
  
# printing result
print("Alternate Cyclic iteration is : " + str(res))

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Output :

The original list is : [5, 6, 8, 9, 10, 21, 3]
Alternate Cyclic iteration is : [5, 3, 6, 21, 8, 10, 9]


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