`join()`

Introduction

The join() method is the most efficient and Pythonic way to combine a list of strings into a single string.

Because strings are immutable in Python, repeatedly using the + operator to concatenate strings inside a loop leads to poor performance. join() solves this by computing the total memory needed and allocating it all at once.

How `join()` Works

Unlike many other programming languages where join is called on the list itself, in Python, join is a string method called on the delimiter.

words = ["coding", "interviews", "are", "fun"]

# Join with a space
sentence = " ".join(words)
print(sentence) # "coding interviews are fun"

# Join with a comma
csv = ",".join(words)
print(csv) # "coding,interviews,are,fun"

Joining Characters

If you have a list of characters, use an empty string "" as the delimiter.

chars = ['a', 'b', 'c']
result = "".join(chars)
print(result) # "abc"

When to use `join()`

You should use join() whenever you need to build a string from multiple pieces in a loop.

Example: Removing Vowels

def remove_vowels(s):
    vowels = set("aeiouAEIOU")
    result = []

    for char in s:
        if char not in vowels:
            result.append(char)

    # Much faster than result_string += char
    return "".join(result)

Common Interview Mistakes

Mistake: `TypeError` with Non-Strings

The join() method only works on iterables containing strings. If the list contains integers or other data types, it will raise a TypeError.

Incorrect:

nums = [1, 2, 3]
print("-".join(nums)) # TypeError

Correct: You must convert the elements to strings first, usually with a generator expression or list comprehension.

nums = [1, 2, 3]
print("-".join(str(n) for n in nums)) # "1-2-3"

Mistake: Repeated String Concatenation (`+`)

Never do this in an interview:

result = ""
for word in words:
    result += word + " "

This takes \(O(N^2)\) time because a new string is created and copied during every iteration. Using " ".join(words) takes \(O(N)\) time.

Time and Space Complexity

Time Complexity: \(O(N)\) where \(N\) is the total length of all strings combined.
Space Complexity: \(O(N)\) to store the newly created string.

Summary

Use "delimiter".join(iterable).
Use "".join() to convert a list of characters back into a string.
Never use + to build a string iteratively in a loop. Use join() instead to guarantee \(O(N)\) time complexity.
Remember to convert numbers to strings before calling join().

join()