How to Replace the Kth word in a String using Python?

Replacing the kth word in a string is a common string manipulation task in Python. This involves identifying the word at a specific position (k) and substituting it with a new word. Python offers several approaches using built-in methods like split(), list comprehension, and regular expressions.

Using Split Method and Indexing

The simplest approach uses split() to convert the string into a list of words, then directly accesses the kth word using indexing.

Syntax

string.split(delimiter)

The split() method divides a string based on a delimiter (space by default) and returns a list of words.

Example

def replace_kth_word(string, k, new_word):
    words = string.split()
    if 1 <= k <= len(words):
        words[k - 1] = new_word
    return ' '.join(words)

sentence = "Hello I will replace dog with cat"
k = 5
new_word = "lion"
updated_sentence = replace_kth_word(sentence, k, new_word)

print(f"Original string: {sentence}")
print(f"Updated string: {updated_sentence}")

Original string: Hello I will replace dog with cat
Updated string: Hello I will replace lion with cat

Using List Comprehension

List comprehension provides a more concise way to replace the kth word by creating a new list with the replacement in a single line.

Example

def replace_kth_word_comprehension(string, k, new_word):
    words = string.split()
    updated_words = [new_word if i == k - 1 else word for i, word in enumerate(words)]
    return ' '.join(updated_words)

sentence = "Apple is my favourite fruit"
k = 1
new_word = "Orange"
updated_sentence = replace_kth_word_comprehension(sentence, k, new_word)

print(f"Original string: {sentence}")
print(f"Updated string: {updated_sentence}")

Original string: Apple is my favourite fruit
Updated string: Orange is my favourite fruit

Using Regular Expressions

The re module provides pattern-based word replacement using regular expressions, which is useful for complex text processing scenarios.

Example

import re

def replace_kth_word_regex(string, k, new_word):
    pattern = r'\b(\w+)\b'
    words = re.findall(pattern, string)
    if 1 <= k <= len(words):
        words[k - 1] = new_word
    return re.sub(pattern, lambda m: words.pop(0), string)

sentence = "Python is a powerful language"
k = 3
new_word = "versatile"
updated_sentence = replace_kth_word_regex(sentence, k, new_word)

print(f"Original string: {sentence}")
print(f"Updated string: {updated_sentence}")

Original string: Python is a powerful language
Updated string: Python is versatile powerful language

Using Slice and Join

This method combines list slicing with the join() method to reconstruct the string with the replacement word.

Example

def replace_kth_word_slice(string, k, new_word):
    words = string.split()
    if 1 <= k <= len(words):
        updated_sentence = " ".join(words[:k-1] + [new_word] + words[k:])
        return updated_sentence
    return string

sentence = "He has a house in London"
k = 4
new_word = "flat"
updated_sentence = replace_kth_word_slice(sentence, k, new_word)

print(f"Original string: {sentence}")
print(f"Updated string: {updated_sentence}")

Original string: He has a house in London
Updated string: He has a flat in London

Comparison

Conclusion

The split and indexing method is the most straightforward approach for replacing the kth word in a string. Use list comprehension for concise code or regular expressions when you need advanced pattern matching capabilities.