Python – Replace duplicate Occurrence in String

Last Updated : 08 May, 2023

Sometimes, while working with Python strings, we can have problem in which we need to perform the replace of a word. This is quite common task and has been discussed many times. But sometimes, the requirement is to replace occurrence of only duplicate, i.e from second occurrence. This has applications in many domains. Let’s discuss certain ways in which this task can be performed.

Method #1 : Using split() + enumerate() + loop

The combination of above functions can be used to perform this task. In this, we separate the words using split. In this, we memoize the first occurrence in set and check if the value is saved before and then is replaced is already occurred.

Python3

   # Python3 code to demonstrate working of 
# Replace duplicate Occurrence in String 
# Using split() + enumerate() + loop 
 
# initializing string 
test_str = 'Gfg is best . Gfg also has Classes now. \ 
                Classes help understand better . ' 
 
# printing original string 
print("The original string is : " + str(test_str)) 
 
# initializing replace mapping 
repl_dict = {'Gfg' : 'It', 'Classes' : 'They' } 
 
# Replace duplicate Occurrence in String 
# Using split() + enumerate() + loop 
test_list = test_str.split(' ') 
res = set() 
for idx, ele in enumerate(test_list): 
    if ele in repl_dict: 
        if ele in res: 
            test_list[idx] = repl_dict[ele] 
        else: 
            res.add(ele) 
res = ' '.join(test_list) 
 
# printing result 
print("The string after replacing : " + str(res)) 
 
  

Output :

The original string is : Gfg is best . Gfg also has Classes now. Classes help understand better . The string after replacing : Gfg is best . It also has Classes now. They help understand better .

Time Complexity: O(n)

Space Complexity: O(n)

Method #2 : Using keys() + index() + list comprehension

This is yet another way in which this task can be performed. In this, we don’t require memoization. This is one liner approach to solve this problem.

Python3

   # Python3 code to demonstrate working of 
# Replace duplicate Occurrence in String 
# Using keys() + index() + list comprehension 
 
# initializing string 
test_str = 'Gfg is best . Gfg also has Classes now. Classes help understand better . '
 
# printing original string 
print("The original string is : " + str(test_str)) 
 
# initializing replace mapping 
repl_dict = {'Gfg' : 'It', 'Classes' : 'They' } 
 
# Replace duplicate Occurrence in String 
# Using keys() + index() + list comprehension 
test_list = test_str.split(' ') 
res = ' '.join([repl_dict.get(val) if val in repl_dict.keys() and test_list.index(val) != idx 
                                else val for idx, val in enumerate(test_list)]) 
 
# printing result 
print("The string after replacing : " + str(res)) 
 
  

Output :

The original string is : Gfg is best . Gfg also has Classes now. Classes help understand better . The string after replacing : Gfg is best . It also has Classes now. They help understand better .

Time Complexity: O(n)

Space Complexity: O(n)

Method #3 : Using list comprehension + set + keys This is yet another way in which this task can be performed.

The code initializes a string variable named test_str with the value ‘Gfg is best . Gfg also has Classes now. Classes help understand better . ‘.
The code prints the original string using the print() function and string concatenation.
The code initializes a dictionary variable named repl_dict with key-value pairs, where each key is a string that needs to be replaced, and its corresponding value is the string to replace it with.
The code splits the test_str string into a list of words using the split() method, and initializes an empty set named seen to keep track of which words have been replaced.
The code uses a list comprehension to replace duplicate occurrences of words in the list of words generated in the previous step. The list comprehension iterates over each word in the list and checks if it is present in the repl_dict dictionary and also not present in the seen set. If the word is present in the dictionary and not in the seen set, it is replaced with its corresponding value from the dictionary, and the word is added to the seen set. Otherwise, the original word is used.
The code joins the list of words generated in the previous step using the join() method with a space separator to form the final string, and assigns the result to the res variable.
The code prints the final string using the print() function and string concatenation.

Python3

   # initializing string
test_str = 'Gfg is best . Gfg also has Classes now. Classes help understand better . '
 
# printing original string
print("The original string is : " + str(test_str))
 
# initializing replace mapping
repl_dict = {'Gfg': 'It', 'Classes': 'They'}
 
# Replace duplicate Occurrence in String
# Using set() + loop + list comprehension
words = test_str.split()
seen = set()
res = [repl_dict[word] if word in repl_dict and word not in seen and not seen.add(word) else word for word in words]
 
# join the list of words to form the final string
res = ' '.join(res)
 
# printing result
print("The string after replacing : " + str(res))
 
  

Output

The original string is : Gfg is best . Gfg also has Classes now. Classes help understand better .  The string after replacing : It is best . Gfg also has They now. Classes help understand better .

Time complexity: O(n), where n is the length of the input string. This is because we iterate over each word in the string exactly once.

Space complexity: O(n), where k is the number of keys in the replacement dictionary. This is because we use a set to store the words that have already been replaced.

Method #4: Using regular expressions

This method uses the re module to search for the repeated occurrences of the words in the string and replaces them with their corresponding values from the repl_dict.

step-by-step approach:

Import the re module.
Initialize the test_str and repl_dict variables.
Define a regular expression pattern to match repeated occurrences of the words in the repl_dict.
Use the re.sub() method to replace the matched patterns with their corresponding values from the repl_dict.
Print the original and replaced strings.

Python3

   import re
 
# initializing string 
test_str = 'Gfg is best . Gfg also has Classes now. \
                Classes help understand better . '
 
# initializing replace mapping 
repl_dict = {'Gfg': 'It', 'Classes': 'They'}
 
# regular expression pattern to match repeated occurrences
pattern = r'\b(' + '|'.join(repl_dict.keys()) + r')\b(?=.*\b\1\b)'
 
# Replace duplicate Occurrence in String 
res = re.sub(pattern, lambda match: repl_dict[match.group()], test_str)
 
# printing result 
print("The original string is : " + str(test_str))
print("The string after replacing : " + str(res))
 
  

Output

The original string is : Gfg is best . Gfg also has Classes now.                 Classes help understand better .  The string after replacing : It is best . Gfg also has They now.                 Classes help understand better .

Time complexity: O(n), where n is the length of the input string.
Auxiliary space: O(1)