Python program to print duplicates from a list of integers?

A duplicate element in a list is an item that appears more than once. Python provides several efficient methods to identify and extract duplicate elements from a list of integers.

Let's say we have the following input list ?

[5, 10, 15, 10, 20, 25, 30, 20, 40]

The output should display duplicate elements ?

[10, 20]

Using For Loop with Dictionary

This approach uses a dictionary to count occurrences and a nested condition to track duplicates ?

# Create a List
numbers = [5, 10, 15, 18, 20, 25, 30, 30, 40, 50, 50, 50]
duplicates = []
counts = {}

# Display the List
print("List =", numbers)

for x in numbers:
    if x not in counts:
        counts[x] = 1
    else:
        if counts[x] == 1:
            duplicates.append(x)
        counts[x] += 1

print("Duplicate Elements =", duplicates)

List = [5, 10, 15, 18, 20, 25, 30, 30, 40, 50, 50, 50]
Duplicate Elements = [30, 50]

Using Counter from Collections

The Counter class automatically counts occurrences and makes duplicate detection straightforward ?

from collections import Counter

# Create a List
numbers = [5, 10, 15, 10, 20, 25, 30, 20, 40]

# Display the List
print("List =", numbers)

# Get the Count of each element in the list
count_dict = Counter(numbers)

# Display the Duplicate elements
duplicates = [item for item in count_dict if count_dict[item] > 1]
print("Duplicate Elements =", duplicates)

List = [5, 10, 15, 10, 20, 25, 30, 20, 40]
Duplicate Elements = [10, 20]

Using List Comprehension with Set

This method combines list comprehension with the count() method and uses a set to remove duplicate entries ?

# Create a List
numbers = [5, 10, 15, 10, 20, 25, 30, 20, 40]

# Display the List
print("List =", numbers)

# Find duplicates using list comprehension and set
duplicates = list(set([x for x in numbers if numbers.count(x) > 1]))
print("Duplicate Elements =", duplicates)

List = [5, 10, 15, 10, 20, 25, 30, 20, 40]
Duplicate Elements = [10, 20]

Comparison

Conclusion

Use Counter for the most efficient and readable approach. The dictionary method offers more control, while list comprehension provides concise one-liners for smaller datasets.