Python - Merging duplicates to list of list

We are given a list we need to merge the duplicate to lists of list. For example a list a=[1,2,2,3,4,4,4,5] we need to merge all the duplicates in lists so that the output should be [[2,2],[4,4,4]]. This can be done by using multiple functions like defaultdict from collections and Counter and various approaches with combinations of loop

Using collections.defaultdict

We can use defaultdict from collections can be used to group duplicates together efficiently.

from collections import defaultdict

a = [1, 2, 2, 3, 4, 4, 4, 5]

# Initialize defaultdict with list as default factory
b = defaultdict(list)

# Group duplicates together in lists
for item in a:
    b[item].append(item)

# Get the lists of duplicates
c = [group for group in b.values() if len(group) > 1]

print("Merged List:", c)

Merged List: [[2, 2], [4, 4, 4]]

Explanation:

• Code uses a defaultdict with a list as the default factory to group duplicate elements from the list a. Each unique element from a is added to the corresponding list in the dictionary.
• After grouping, the code filters out groups with only one element, leaving only those lists with duplicates, which are then stored in c and printed as the merged list.

Using a Manual Approach

A more manual approach where we can iterate through the list and merge duplicates.

a = [1, 2, 2, 3, 4, 4, 4, 5]

# Initialize an empty list to store merged lists
m = []

# Iterate over the list and merge duplicates
while a:
    current = a[0]
    duplicates = [current]
    
    # Find all occurrences of the current item
    a = [item for item in a[1:] if item != current]  # Remove the current item
    m.append(duplicates + [item for item in a if item == current])

print("Merged List:", m)

Merged List: [[1], [2], [3], [4], [5]]

Explanation:

• Code iterates through the list a and extracts duplicates of the current element, adding them to a duplicates list while filtering out occurrences of the current element from a.
• Each processed group of duplicates is appended to the m list, and this process repeats until all elements in a have been processed.

Using collections.Counter

We can use Counter to count occurrences and then create lists of duplicates.

from collections import Counter

a = [1, 2, 2, 3, 4, 4, 4, 5]

# Count occurrences of each element
c = Counter(a)

# Create lists of duplicates
d = [[key] * c[key] for key in c if c[key] > 1]

print("Merged List:", d)

Merged List: [[2, 2], [4, 4, 4]]

Explanation:

• Counter from the collections module is used to count the occurrences of each element in the list a.
• Code then creates a list of lists (d), where each list contains duplicates of elements that occur more than once in the original list