Python program to find all duplicate characters in a string

This article teaches you how to write a Python program to find all duplicate characters in a string using different approaches.

Characters that repeat themselves within a string are referred to as duplicate characters. When we refer to finding duplicate characters in a string, we mean identifying every character that appears more than once in the string.

Input-Output Scenario

Following is the input-output scenario to find all the duplicate characters in a string ?

Input: TutorialsPoint
Output: t, o, i

As we can see, the duplicate characters in the given string "TutorialsPoint" are "t" with 3 repetitions, "o" with 2 repetitions and "i" with 2 repetitions.

Algorithm

The following algorithm will search a string for duplicate characters ?

• Create a string and store it in a variable
• Use nested loops to check each character against all other characters
• Count occurrences of each character
• If a character's count is more than 1, it's a duplicate
• Print the duplicate characters

Using Nested Loops

The basic approach uses nested loops to compare each character with all subsequent characters in the string ?

text = "tutorialspoint"
duplicates = []

print("Duplicate characters:")
for i in range(len(text)):
    count = 1
    # Skip if character already processed
    if text[i] in duplicates:
        continue
    
    for j in range(i + 1, len(text)):
        if text[i] == text[j]:
            count += 1
    
    if count > 1:
        duplicates.append(text[i])
        print(f"{text[i]} appears {count} times")

Duplicate characters:
t appears 3 times
o appears 2 times
i appears 2 times

Using count() Method

The count() method returns the frequency of a character in the string. This approach is more concise ?

text = "tutorialspoint"
duplicate_chars = []

for character in text:
    # Check if character appears more than once
    if text.count(character) > 1:
        # Add to list if not already present
        if character not in duplicate_chars:
            duplicate_chars.append(character)

print("Duplicate characters:", *duplicate_chars)

Duplicate characters: t o i

Using Counter() Method

The Counter class from the collections module creates a dictionary with characters as keys and their frequencies as values ?

from collections import Counter

def find_duplicates(text):
    # Create dictionary with character frequencies
    char_count = Counter(text)
    duplicates = []
    
    # Find characters with count > 1
    for char, count in char_count.items():
        if count > 1:
            duplicates.append(char)
    
    return duplicates

# Test the function
text = 'TutorialsPoint'
result = find_duplicates(text)
print("Duplicate characters:", result)

Duplicate characters: ['t', 'o', 'i']

Using Dictionary Approach

Manually creating a dictionary to track character frequencies provides more control over the counting process ?

def find_duplicates_dict(text):
    char_freq = {}
    duplicates = []
    
    # Count frequency of each character
    for char in text:
        char_freq[char] = char_freq.get(char, 0) + 1
    
    # Find duplicates
    for char, freq in char_freq.items():
        if freq > 1:
            duplicates.append(char)
    
    return duplicates

text = "tutorialspoint"
result = find_duplicates_dict(text)
print("Duplicate characters:", result)
print("Character frequencies:")
for char in result:
    print(f"'{char}': {text.count(char)} times")

Duplicate characters: ['t', 'o', 'i']
Character frequencies:
't': 3 times
'o': 2 times
'i': 2 times

Comparison

Conclusion

Use Counter() from collections for the most efficient solution. For simple cases, the count() method provides readable code. The dictionary approach offers flexibility for custom requirements.