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Python | Combining duplicates into a list of list

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Method # 1: Using collections.Counter()
This particular function can be very useful for this particular task as it counts the frequency of items in list, and then we can link them using a list comprehension.

# Python3 demo code
# group similar items as a list
# use collection.Counter ()

import collections

 
# initializing list

test_list = [ 1 , 3 , 4 , 2 , 1 , 3 , 4 , 2 , 3 , 4 , 1 ]

 
# print original list

print ( "The original list:" + str (test_list))

 
# using collection.Counter () # grouping similar items as a 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 be easily solved with the traditional groupby functionality offered by Python through the groupby function, which groups similar elements as suggested by name.

# Python3 demo code
# grouping similar elements in a list
# using itertools.groupby ()

import itertools

 
# initializing list

test_list = [ 1 , 3 , 4 , 2 , 1 , 3 , 4 , 2 , 3 , 4 , 1 ]

 
# print original list

print ( "The original list:" + str (test_list))

 
# using ite rtools.groupby ()
# grouping similar elements as a 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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