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Python | Subgroups of i’th index size in list

  • Last Updated : 18 Mar, 2019

Sometimes we need to group elements and grouping techniques and requirements vary accordingly. One such way to group the elements is by the i’th size in list which stores the dictionary of index keys with values as list elements of subsequent size i.

Input : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
Output: {1: [4], 2: [7, 8], 3: [10, 12, 15], 4: [13, 17, 14, 5]}

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Let’s discuss certain ways in which this can be done.



Method #1 : Using islice() + dictionary comprehension
The slice method can be used to group the chunks of list required to be made as values of the dictionaries which are then assigned to their destined index key using the dictionary comprehension.




# Python3 code to demonstrate
# Subgrouping of i'th index size in list
# using islice() + dictionary comprehension
from itertools import islice
  
# initializing list
test_list = [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
  
# printing original list 
print("The original list : " + str(test_list))
  
# using islice() + dictionary comprehension
# Subgrouping of i'th index size in list
temp = iter(test_list)
res = {key: val for key, val in ((i, list(islice(temp, i)))
                for i in range(1, len(test_list))) if val}
  
# printing result
print("The grouped dictionary is : " + str(res))
Output :

The original list : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
The grouped dictionary is : {1: [4], 2: [7, 8], 3: [10, 12, 15], 4: [13, 17, 14, 5]}

 

Method #2 : Using itemgetter() + takewhile() + islice()
In order to increase the computation speed, we introduce new functions to perform this particular task, takewhile and itemgetter functions which performs the task of grouping the sliced values.




# Python3 code to demonstrate
# Subgrouping of i'th index size in list
# using itemgetter() + takewhile() + islice()
from itertools import islice, takewhile
from operator import itemgetter
  
# initializing list
test_list = [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
  
# printing original list 
print("The original list : " + str(test_list))
  
# using itemgetter() + takewhile() + islice()
# Subgrouping of i'th index size in list
temp = iter(test_list)
res =  {key: val for key, val in 
        takewhile(itemgetter(1), ((i, list(islice(temp, i)))
        for i in range(1, len(test_list))))}
  
# printing result
print("The grouped dictionary is : " + str(res))
Output :

The original list : [4, 7, 8, 10, 12, 15, 13, 17, 14, 5]
The grouped dictionary is : {1: [4], 2: [7, 8], 3: [10, 12, 15], 4: [13, 17, 14, 5]}




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