`replace()`

Introduction

The replace() method replaces occurrences of a substring with another substring. Since strings are immutable, replace() returns a completely new string.

It is useful for data cleaning, formatting, and simple character substitutions.

How `replace()` Works

The method takes two required arguments: the old substring and the new substring.

text = "I love Java"
new_text = text.replace("Java", "Python")

print(new_text) # "I love Python"

The `count` Parameter

You can provide a third argument to limit the number of replacements.

text = "apple apple apple"

# Replace only the first two occurrences
print(text.replace("apple", "orange", 2))
# "orange orange apple"

When to use `replace()`

Use replace() when you need to swap entire substrings or remove specific characters (by replacing them with "").

phone = "(555) 123-4567"

# Removing formatting
clean = phone.replace("(", "").replace(")", "").replace(" ", "").replace("-", "")
print(clean) # "5551234567"

(Note: If you are replacing many different characters, regular expressions or list comprehensions might be cleaner).

Common Interview Mistakes

Mistake: Forgetting that strings are immutable

Because strings are immutable, replace() does not modify the string in-place. If you don't reassign the result, nothing happens.

Incorrect:

s = "hello"
s.replace("h", "j")
print(s) # "hello"

Correct:

s = "hello"
s = s.replace("h", "j")
print(s) # "jello"

Mistake: Overlapping replacements

replace() does not perform overlapping replacements. It processes the string from left to right.

text = "aaaa"
# The first "aa" is replaced, then the second "aa" is replaced.
print(text.replace("aa", "b")) # "bb"

Time and Space Complexity

Time Complexity: \(O(N)\) where \(N\) is the length of the string.
Space Complexity: \(O(N)\) to create the new string.

Summary

s.replace(old, new, [count]) returns a new string with replacements made.
Remember to reassign the result because strings are immutable.
Use it to easily remove unwanted substrings by replacing them with "".

replace()