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Python | Index specific cyclic iteration in list
  • Last Updated : 21 Feb, 2019

The problem of cyclic iteration is quite common, but sometimes, we come through the issue in which we require to process the list in a way in which it is cyclic iterated that too starting from a specific index. Let’s discuss certain ways in which this problem can be solved.

Method #1 : Using % operator + loop
The % operator can be used to cycle the out of bound index value to begin from the beginning of list to form a cycle and hence help in the cyclic iteration.




# Python3 code to demonstrate 
# cyclic iteration in list 
# using % operator and loop
  
# initializing tuple list 
test_list = [5, 4, 2, 3, 7]
  
# printing original list
print ("The original list is : " + str(test_list))
  
# starting index 
K = 3
  
# using % operator and loop
# cyclic iteration in list 
res = []
for i in range(len(test_list)):
    res.append(test_list[K % len(test_list)])
    K = K + 1
  
# printing result 
print ("The cycled list is : " +  str(res))
Output:
The original list is : [5, 4, 2, 3, 7]
The cycled list is : [3, 7, 5, 4, 2]

 
Method #2 : Using itertools.cycle() + itertools.islice() + itertools.dropwhile()
The itertools library has built in functions that can help achieve to the solution of this particular problem. The cycle function performs the cycling part, dropwhile function brings the cycle to begin of list and islice function specifies the cycle size.




# Python3 code to demonstrate 
# cyclic iteration in list using itertools
from itertools import cycle, islice, dropwhile
  
# initializing tuple list 
test_list = [5, 4, 2, 3, 7]
  
# printing original list
print ("The original list is : " + str(test_list))
  
# starting index 
K = 3
  
# using itertools methods for
# cyclic iteration in list 
cycling = cycle(test_list)  
skipping = dropwhile(lambda x: x != K, cycling) 
slicing = islice(skipping, None, len(test_list))
slicing = list(slicing)
  
# printing result 
print ("The cycled list is : " +  str(slicing))
Output:
The original list is : [5, 4, 2, 3, 7]
The cycled list is : [3, 7, 5, 4, 2]

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