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Python | Move given element to List Start
  • Last Updated : 27 Feb, 2020

The conventional problem involving the element shifts has been discussed many times earlier, but sometimes we have strict constraints performing them and knowledge of any possible variation helps. This article talks about one such problem of shifting K’s at start of list, catch here is it checks for just K’s excluding the conventional ‘None’ (False) values. Let’s discuss certain ways in which this can be performed.

Method #1 : Using list comprehension + isinstance()
In this method, we perform the operation of shifting in 2 steps. In 1st step we get all the values that we need to get at front and at end we just push the K’s to front. The isinstance method is used to filter out the Boolean False entity.




# Python3 code to demonstrate 
# Move all K element to List Start
# using list comprehension + isinstance() 
  
# initializing list 
test_list = [1, 4, None, "Manjeet", False, 4, False, 4, "Nikhil"
  
# printing original list 
print("The original list : " + str(test_list)) 
  
# initializing K 
K = 4
  
# using list comprehension + isinstance() 
# Move all K element to List Start
temp = [ele for ele in test_list if ele != K and ele or ele is None or isinstance(ele, bool) ] 
res = [K] * (len(test_list) - len(temp)) + temp
  
# print result 
print("The list after shifting K's to start : " + str(res)) 
Output :
The original list : [1, 4, None, 'Manjeet', False, 4, False, 4, 'Nikhil']
The list after shifting K's to start : [4, 4, 4, 1, None, 'Manjeet', False, False, 'Nikhil']

 

Method #2 : Using list comprehension + isinstance() + list slicing
This method is similar to the above method, the only modification is that to reduce number of steps, list slicing is used to attach the K’s to perform whole task in just 1 step.






# Python3 code to demonstrate 
# Move all K element to List Start
# using list comprehension + isinstance() + list slicing 
  
# initializing list 
test_list = [1, 4, None, "Manjeet", False, 4, False, 4, "Nikhil"
  
# printing original list 
print("The original list : " + str(test_list)) 
  
# initializing K 
K = 4
  
# using list comprehension + isinstance() + list slicing 
# Move all K element to List Start
res = ([K] * len(test_list) + [ele for ele in test_list if ele != K and ele or not isinstance(ele, int
    or isinstance(ele, bool)])[len([ele for ele in test_list if ele != K and ele or not isinstance(ele, int
    or isinstance(ele, bool)]):] 
  
# print result 
print("The list after shifting K's to end : " + str(res)) 
Output :
The original list : [1, 4, None, 'Manjeet', False, 4, False, 4, 'Nikhil']
The list after shifting K's to start : [4, 4, 4, 1, None, 'Manjeet', False, False, 'Nikhil']

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