Chapter 14: String Indexing and Slicing

🚀 Open Notebook

📺 Video Tutorial

String indexing in Python is easy ✂️ (6:17)

📚 What You’ll Learn

Access and extract parts of strings using indexing and slicing - essential for text processing!

🎯 Learning Objectives

• Understand zero-based indexing

• Use positive and negative indices

• Slice strings with [start:end:step]

• Reverse strings and extract substrings

📖 Concept Explanation

Indexing

Each character in a string has a position (index) starting from 0.

String: "PYTHON"
Index:   0 1 2 3 4 5
Negative:-6-5-4-3-2-1

Slicing Syntax

string[start:end:step]

• start: Beginning index (inclusive)

• end: Ending index (exclusive)

• step: Interval between characters

💡 Examples

Single Character Access

word = "Python"
print(word[0])   # P (first character)
print(word[3])   # h (fourth character)
print(word[-1])  # n (last character)
print(word[-2])  # o (second to last)

Slicing Ranges

text = "Programming"
print(text[0:4])   # "Prog" (index 0 to 3)
print(text[3:7])   # "gram" (index 3 to 6)
print(text[:4])    # "Prog" (start to index 3)
print(text[4:])    # "ramming" (index 4 to end)
print(text[:])     # "Programming" (entire string)

Step Parameter

numbers = "0123456789"
print(numbers[::2])   # "02468" (every 2nd char)
print(numbers[1::2])  # "13579" (every 2nd, starting at 1)
print(numbers[::3])   # "0369" (every 3rd char)

Reversing Strings

word = "Python"
print(word[::-1])  # "nohtyP" (reversed)

# Practical use: palindrome checker
word = "racecar"
if word == word[::-1]:
    print("It's a palindrome!")

✍️ Practice Exercises

1. Extract first 3 characters from a string

2. Get the last 4 characters

3. Extract every other character

4. Reverse a user’s name

5. Check if a string is a palindrome

6. Extract domain from email (after @)

7. Get file extension from filename

🎮 Real-World Examples

Credit Card Formatter

card = "1234567812345678"
formatted = f"{card[0:4]}-{card[4:8]}-{card[8:12]}-{card[12:16]}"
print(formatted)  # 1234-5678-1234-5678

# Hide middle digits
hidden = f"{card[0:4]}-****-****-{card[-4:]}"
print(hidden)  # 1234-****-****-5678

Extract Information

date = "2024-01-15"
year = date[0:4]    # "2024"
month = date[5:7]   # "01"
day = date[8:10]    # "15"
print(f"Year: {year}, Month: {month}, Day: {day}")

URL Parser

url = "https://www.example.com/page"
protocol = url[0:5]        # "https"
domain = url[12:23]        # "example.com"
path = url[23:]            # "/page"

📝 Common Patterns

First/Last N Characters

text = "Hello World"
first_5 = text[:5]    # "Hello"
last_5 = text[-5:]    # "World"

Remove First/Last N Characters

text = "Hello World"
without_first = text[1:]   # "ello World"
without_last = text[:-1]   # "Hello Worl"

Every Nth Character

text = "ABCDEFGHIJ"
every_2nd = text[::2]  # "ACEGI"
every_3rd = text[::3]  # "ADGJ"

🚀 Challenge Projects

1. Social Security Number Formatter: XXX-XX-XXXX

2. Phone Number Parser: Extract area code, prefix, line number

3. Acronym Maker: Take first letter of each word

4. Text Truncator: Limit text to N characters + “…”

5. Caesar Cipher: Shift characters by N positions

🔍 Slicing Tricks

Copy a String

original = "Python"
copy = original[:]  # Creates a copy

Reverse Words

sentence = "Hello World"
words = sentence.split()
reversed_words = [word[::-1] for word in words]
result = " ".join(reversed_words)
print(result)  # "olleH dlroW"

Extract Extension

filename = "document.pdf"
extension = filename[filename.rfind('.')+1:]
print(extension)  # "pdf"

🎓 Key Takeaways from Video

1. Strings are text data enclosed in quotes

2. Follow along with the video for hands-on practice

3. Experiment with the code examples to deepen understanding

💡 These points cover the main concepts from the video tutorial to help reinforce your learning.

🔗 Next Chapter

Continue to Chapter 15: Format Specifiers to learn advanced string formatting!