Python | Merging duplicates to list of list

Last Updated : 27 Mar, 2019

Sometimes, we need to perform the conventional task of grouping some like elements to a separate list and thus forming a list of list. This can also helps in counting and also get the sorted order of elements. Let’s discuss certain ways in which this can be done.

Method #1 : Using collections.Counter()
This particular function can prove to be quite useful to perform this particular task as it counts the frequency of elements in list and then we can pair them using the list comprehension.

# Python3 code to demonstrate
# grouping like elements as list 
# using collections.Counter()
import collections
  
# initializing list 
test_list = [1, 3, 4, 2, 1, 3, 4, 2, 3, 4, 1]
  
# printing original list 
print("The original list : " + str(test_list))
  
# using collections.Counter()
# grouping like elements as list 
temp = collections.Counter(test_list)
res = [[i] * j for i, j in temp.items()]
  
# print result
print("The elements after grouping are : " + str(res))

Output :

The original list : [1, 3, 4, 2, 1, 3, 4, 2, 3, 4, 1]
The elements after grouping are : [[1, 1, 1], [2, 2], [3, 3, 3], [4, 4, 4]]

Method #2 : Using itertools.groupby()
This problem can easily solved by traditional groupby functionality that is offered by Python via groupby function, which groups the like elements as suggested by name.

# Python3 code to demonstrate
# grouping like elements as list 
# using itertools.groupby()
import itertools
  
# initializing list 
test_list = [1, 3, 4, 2, 1, 3, 4, 2, 3, 4, 1]
  
# printing original list 
print("The original list : " + str(test_list))
  
# using itertools.groupby()
# grouping like elements as list 
res = [list(i) for j, i in itertools.groupby(sorted(test_list))]
  
# print result
print("The elements after grouping are : " + str(res))

Output :

The original list : [1, 3, 4, 2, 1, 3, 4, 2, 3, 4, 1]
The elements after grouping are : [[1, 1, 1], [2, 2], [3, 3, 3], [4, 4, 4]]

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