Chapter 6: String Manipulation

📝 What are Strings?

A string is a sequence of characters - basically any text you want to work with. Think of it as a necklace made of letter beads, where each bead is a character!

You've already used strings in previous chapters, but now we'll dive deep into all the amazing things you can do with text!

Creating Strings - Different Ways

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⚡ Quick Tip: When to Use Which Quotes?

# Single quotes when your string contains double quotes
sentence = 'She said, "Hello!"'

# Double quotes when your string contains single quotes
sentence = "It's a beautiful day!"

# Triple quotes for multiple lines or when you have both ' and "
paragraph = """He said, "It's wonderful!"
I agreed completely."""

# Generally: Pick one style and be consistent!

Escape Characters - Special Characters in Strings

Using Escape Sequences

Use backslash (\) to include special characters:

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Common Escape Sequences

Escape Sequence	Meaning	Example
`\n`	Newline	`"Line 1\nLine 2"`
`\t`	Tab	`"Name:\tJohn"`
`\'`	Single quote	`'It\'s great'`
`\"`	Double quote	`"She said \"Hi\""`
`\\`	Backslash	`"C:\\folder"`

🔢 String Indexing & Slicing

Strings are sequences! Just like lists, each character has a position (index). You can access individual characters or extract portions of the string.

Accessing Individual Characters

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String Slicing - Extracting Substrings

Slicing Syntax: string[start:stop:step]

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🔍 Understanding String Slicing

For string: "PYTHON"
Indices:     0 1 2 3 4 5

text[1:4]   → "YTH"    (indices 1, 2, 3)
text[:3]    → "PYT"    (from start to 3)
text[3:]    → "HON"    (from 3 to end)
text[::2]   → "PTO"    (every 2nd char: 0, 2, 4)
text[::-1]  → "NOHTYP" (reversed!)

Remember:
- start is INCLUSIVE
- stop is EXCLUSIVE
- step is how many to skip

🛠️ Powerful String Methods

Python strings come with many built-in methods that make text manipulation easy!

Case Conversion Methods

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Searching and Checking Methods

text = "Python is awesome and Python is fun"

# find() - returns index of first occurrence (-1 if not found)
print("find('Python'):", text.find("Python"))
print("find('Java'):", text.find("Java"))

# index() - like find() but raises error if not found
print("\nindex('Python'):", text.index("Python"))
# print(text.index('Java'))  # Would raise ValueError!

# count() - count occurrences
print("\ncount('Python'):", text.count("Python"))
print("count('is'):", text.count("is"))

# Checking what string starts/ends with
sentence = "hello world"
print("\nChecking start/end:")
print("starts with 'hello':", sentence.startswith("hello"))
print("starts with 'world':", sentence.startswith("world"))
print("ends with 'world':", sentence.endswith("world"))

# Check if string contains only certain characters
print("\nChecking character types:")
print("'123'.isdigit():", "123".isdigit())
print("'abc'.isalpha():", "abc".isalpha())
print("'abc123'.isalnum():", "abc123".isalnum())
print("'   '.isspace():", "   ".isspace())

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Trimming and Cleaning Strings

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💡 Most Commonly Used String Methods

# Case conversion
.upper()      →  "hello" → "HELLO"
.lower()      →  "HELLO" → "hello"
.title()      →  "hello world" → "Hello World"
.capitalize() →  "hello" → "Hello"

# Searching
.find(sub)       →  Returns index or -1
.count(sub)      →  Count occurrences
.startswith(sub) →  True/False
.endswith(sub)   →  True/False

# Cleaning
.strip()    →  Remove whitespace from both ends
.replace()  →  Replace substring with another

# Splitting/Joining (covered next section!)
.split()    →  Split string into list
.join()     →  Join list into string

🎨 String Formatting - Building Strings

There are several ways to build strings with variables in Python. Let's learn them all!

Method 1: String Concatenation (The Old Way)

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Method 2: % Formatting (Old Style)

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Method 3: str.format() (Better Way)

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🚀 F-Strings - The Modern Way!

F-strings (formatted string literals) are the newest and best way to format strings in Python! They're fast, readable, and powerful. Available in Python 3.6+.

Basic F-String Usage

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Advanced F-String Formatting

# Number formatting with f-strings
pi = 3.14159265359

print("Default:", f"{pi}")
print("2 decimals:", f"{pi:.2f}")
print("4 decimals:", f"{pi:.4f}")
print("As percentage:", f"{0.85:.1%}")

# Alignment and padding
name = "Krishna"
print(f"\nAlignment:")
print(f"Left:   '{name:<15}'")
print(f"Right:  '{name:>15}'")
print(f"Center: '{name:^15}'")

# Padding with specific character
print(f"\nPadding:")
print(f"'{name:*<15}'")  # Pad with *
print(f"'{name:_>15}'")  # Pad with _
print(f"'{name:=^15}'")  # Pad with =

# Number formatting
number = 1234567
print(f"\nNumber formatting:")
print(f"With commas: {number:,}")
print(f"Binary: {255:b}")
print(f"Hex: {255:x}")
print(f"Octal: {255:o}")

# Table with f-strings
print("\n" + "=" * 50)
print(f"{'Product':<20} {'Price':>10} {'Quantity':>10}")
print("=" * 50)
print(f"{'Apple':<20} ${19.99:>9.2f} {5:>10}")
print(f"{'Banana':<20} ${12.50:>9.2f} {10:>10}")
print(f"{'Cherry':<20} ${25.00:>9.2f} {3:>10}")
print("=" * 50)

# Datetime formatting
from datetime import datetime
now = datetime.now()
print(f"\nCurrent time: {now:%Y-%m-%d %H:%M:%S}")

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🔍 F-String Format Specifiers

Syntax: f"{value:format_spec}"

Common format specifiers:
{value:.2f}    → Float with 2 decimal places
{value:10}     → Min width of 10 characters
{value:<10}    → Left aligned, width 10
{value:>10}    → Right aligned, width 10
{value:^10}    → Center aligned, width 10
{value:,}      → Add thousands separator
{value:.2%}    → Display as percentage

Examples:
f"{3.14159:.2f}"      → "3.14"
f"{42:05}"            → "00042"
f"{'hi':>5}"          → "   hi"
f"{1000000:,}"        → "1,000,000"
f"{0.85:.1%}"         → "85.0%"

💡 Why F-Strings are the Best!

Readable: Variables directly in string - clear and intuitive
Fast: Faster than other formatting methods
Powerful: Can include any Python expression
Concise: Less typing than concatenation or .format()
Modern: The recommended way to format strings in Python

Use f-strings whenever possible! They're the Pythonic way to format strings.

🔒 String Immutability - Strings Can't Change!

Strings are immutable - once created, they cannot be changed. Any operation that "modifies" a string actually creates a new string!

What Immutability Means

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🔍 Why Are Strings Immutable?

Python makes strings immutable for several good reasons:

Safety: Can't accidentally modify strings - prevents bugs
Performance: Python can optimize memory for identical strings
Hashable: Immutable strings can be dictionary keys and set members
Thread-safe: Multiple parts of program can use same string safely

Example of string interning (optimization):
a = "hello"
b = "hello"
print(a is b)  # True! Same object in memory!

Python reuses the same "hello" object to save memory!

Working with "Changing" Strings

If you need to build or modify strings efficiently:

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✂️ Splitting and Joining Strings

Two of the most useful string operations for working with text data!

split() - Breaking Strings Into Lists

# Split on whitespace (default)
sentence = "Python is awesome and fun"
words = sentence.split()
print("Sentence:", sentence)
print("Words:", words)
print("Number of words:", len(words))

# Split on specific character
csv_data = "apple,banana,cherry,mango"
fruits = csv_data.split(",")
print("\nCSV data:", csv_data)
print("Fruits list:", fruits)

# Split on multiple characters
date = "2024-12-15"
parts = date.split("-")
year, month, day = parts  # Unpack!
print(f"\nDate: {date}")
print(f"Year: {year}, Month: {month}, Day: {day}")

# Limit splits with maxsplit
text = "one:two:three:four:five"
parts = text.split(":", 2)  # Split only twice
print("\nOriginal:", text)
print("Split max 2:", parts)

# splitlines() - split on line breaks
multi_line = """Line 1
Line 2
Line 3"""
lines = multi_line.splitlines()
print("\nLines:", lines)

# Practical: Parse user input
user_input = "Krishna Kumar, 25, Mumbai"
name, age, city = user_input.split(", ")
print(f"\nParsed: Name={name}, Age={age}, City={city}")

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join() - Combining Lists Into Strings

# Basic join
words = ["Python", "is", "awesome"]
sentence = " ".join(words)
print("Words:", words)
print("Sentence:", sentence)

# Join with different separators
fruits = ["apple", "banana", "cherry"]
print("\nFruits:", fruits)
print("With comma:", ", ".join(fruits))
print("With dash:", " - ".join(fruits))
print("With newline:", "\n".join(fruits))

# Join numbers (must convert to strings first!)
numbers = [1, 2, 3, 4, 5]
str_numbers = [str(n) for n in numbers]
result = "-".join(str_numbers)
print("\nNumbers joined:", result)

# Practical: Build CSV
data = ["Krishna", "25", "Mumbai"]
csv_row = ",".join(data)
print("\nCSV row:", csv_row)

# Build file path
path_parts = ["home", "user", "documents", "file.txt"]
path = "/".join(path_parts)
print("\nFile path:", path)

# Create formatted list
items = ["apples", "bananas", "cherries"]
formatted = "I bought: " + ", ".join(items) + "."
print("\n" + formatted)

# Empty string join - concatenate without separator
letters = ['P', 'y', 't', 'h', 'o', 'n']
word = "".join(letters)
print("\nLetters:", letters)
print("Word:", word)

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💡 split() and join() - Perfect Partners!

# They're opposites!
text = "apple,banana,cherry"

# Split: string → list
fruits = text.split(",")      # ["apple", "banana", "cherry"]

# Join: list → string
text_again = ",".join(fruits) # "apple,banana,cherry"

# Common pattern: modify words
sentence = "hello world python"
words = sentence.split()           # Split into words
words = [w.capitalize() for w in words]  # Capitalize each
result = " ".join(words)           # Join back
print(result)  # "Hello World Python"

Real-World Examples

# Example 1: Parse CSV data
csv_line = "Krishna,25,Mumbai,krishna@email.com"
name, age, city, email = csv_line.split(",")
print("=== Parsed User ===")
print(f"Name: {name}")
print(f"Age: {age}")
print(f"City: {city}")
print(f"Email: {email}")

# Example 2: Clean and reformat text
messy_text = "  PYTHON   is   GREAT  "
words = messy_text.split()  # Splits on any whitespace, removes extra
clean = " ".join(words)
proper = clean.capitalize()
print("\nMessy:", repr(messy_text))
print("Clean:", proper)

# Example 3: Extract domain from email
email = "user@example.com"
username, domain = email.split("@")
print(f"\nEmail: {email}")
print(f"Username: {username}")
print(f"Domain: {domain}")

# Example 4: Parse log entry
log = "2024-12-15 10:30:45 ERROR User login failed"
parts = log.split(" ", 3)  # Split into 4 parts max
date, time, level, message = parts
print("\n=== Log Entry ===")
print(f"Date: {date}")
print(f"Time: {time}")
print(f"Level: {level}")
print(f"Message: {message}")

# Example 5: Build URL
base = "https://api.example.com"
endpoint = "users"
user_id = "12345"
url = "/".join([base, endpoint, user_id])
print(f"\nAPI URL: {url}")

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📺 Video Resources

Watch these videos to reinforce your understanding of string manipulation:

🤖 AI Learning Prompts

Use these prompts with AI assistants to deepen your understanding:

Understanding String Basics

I'm learning about Python strings. Please help me understand:

1. What are strings and how are they stored in memory?
2. Explain string indexing and slicing with 10 different examples
3. Why are strings immutable and what does that mean in practice?
4. Show me the difference between single, double, and triple quotes
5. Give me 5 exercises to practice string indexing and slicing

Use simple language and include solutions!

Mastering String Methods

Help me become an expert with string methods:

1. List and explain the 15 most commonly used string methods
2. Give me real-world examples for each method
3. What's the difference between find() and index()?
4. Show me how to chain string methods together
5. Create 10 practice problems using different string methods
6. Explain string methods that check character types (isdigit, isalpha, etc.)

Include step-by-step examples!

String Formatting Mastery

I want to master all string formatting methods in Python:

1. Compare concatenation, %, .format(), and f-strings
2. Which method should I use and when?
3. Show me 20 examples of f-string formatting
4. How do I format numbers, dates, and align text?
5. Give me practice exercises for each formatting method
6. Show me how to build complex formatted output (like tables)

Make it beginner-friendly with plenty of examples!

Practical String Applications

Help me apply string knowledge to real problems:

1. How do I parse CSV data using string methods?
2. Show me how to clean and validate user input
3. How can I extract information from strings (emails, URLs, etc.)?
4. Give me 5 mini-projects that focus on string manipulation
5. How do I work with multi-line strings and text files?

Include complete working examples!

💡 Tips for Using These Prompts:

Practice regularly: Ask for new examples daily
Build projects: Request small programs that use multiple string concepts
Debug with AI: Share your string code and ask for improvements
Compare approaches: Ask "what's the best way to..." for different scenarios
Request challenges: Ask for progressively harder string problems

✏️ Practice Exercises

How to Approach These Exercises:

Read carefully: Make sure you understand the requirements
Plan first: Think about which string methods you'll need
Test thoroughly: Try different inputs
Experiment: Try multiple approaches to solve each problem

Part 1: Guided Exercises

Guided Exercise 1: Text Analyzer

What You'll Learn: How to analyze text using string methods.

Time: 15 minutes

Step 1: Create a Simple Text Analyzer

# Text to analyze
text = "Python is awesome! Python is fun and Python is powerful."

# Count total characters
total_chars = len(text)
print(f"Total characters: {total_chars}")

# Count words
words = text.split()
word_count = len(words)
print(f"Total words: {word_count}")

# Count specific word
python_count = text.lower().count("python")
print(f"'Python' appears: {python_count} times")

# Find all unique words
unique_words = set(word.lower().strip("!.,") for word in words)
print(f"Unique words: {len(unique_words)}")
print(f"Unique word list: {sorted(unique_words)}")

# Check if contains certain words
if "awesome" in text.lower():
    print("\nText contains 'awesome'!")

# Count vowels
vowels = "aeiouAEIOU"
vowel_count = sum(1 for char in text if char in vowels)
print(f"\nVowel count: {vowel_count}")

# Display statistics
print("\n=== Text Statistics ===")
print(f"Characters: {total_chars}")
print(f"Words: {word_count}")
print(f"Unique words: {len(unique_words)}")
print(f"Vowels: {vowel_count}")

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Guided Exercise 2: User Input Formatter

What You'll Learn: How to clean and format user input.

Time: 20 minutes

Clean and Format Various Types of Input

# Example 1: Clean name input
name_input = "  kRiShNa KuMaR  "
cleaned_name = name_input.strip().title()
print(f"Input: '{name_input}'")
print(f"Cleaned: '{cleaned_name}'")

# Example 2: Format phone number
phone = "1234567890"
formatted_phone = f"({phone[:3]}) {phone[3:6]}-{phone[6:]}"
print(f"\nPhone input: {phone}")
print(f"Formatted: {formatted_phone}")

# Example 3: Validate and clean email
email_input = "  USER@EXAMPLE.COM  "
cleaned_email = email_input.strip().lower()
print(f"\nEmail input: '{email_input}'")
print(f"Cleaned: '{cleaned_email}'")

# Check email format
if "@" in cleaned_email and "." in cleaned_email.split("@")[1]:
    print("✓ Valid email format")
else:
    print("✗ Invalid email format")

# Example 4: Create username from name
full_name = "Krishna Kumar Sharma"
username = full_name.lower().replace(" ", "_")
print(f"\nFull name: {full_name}")
print(f"Username: {username}")

# Example 5: Format currency
amount = 1234567.89
formatted_amount = f"${amount:,.2f}"
print(f"\nAmount: {amount}")
print(f"Formatted: {formatted_amount}")

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Part 2: Independent Practice

Challenge 1: Password Validator

Difficulty: Medium | Time: 20 minutes

Task: Create a password validator that checks:

At least 8 characters long
Contains at least one uppercase letter
Contains at least one lowercase letter
Contains at least one digit
Print whether password is valid

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# Password Validator Solution
password = "Test1234"

# Check all conditions
length_ok = len(password) >= 8
has_upper = any(c.isupper() for c in password)
has_lower = any(c.islower() for c in password)
has_digit = any(c.isdigit() for c in password)

# Display results
print("=== Password Validation ===")
print(f"Password: {password}")
print(f"Length >= 8: {'✓' if length_ok else '✗'}")
print(f"Has uppercase: {'✓' if has_upper else '✗'}")
print(f"Has lowercase: {'✓' if has_lower else '✗'}")
print(f"Has digit: {'✓' if has_digit else '✗'}")

# Overall validation
is_valid = length_ok and has_upper and has_lower and has_digit
print(f"\nPassword is {'VALID ✓' if is_valid else 'INVALID ✗'}")

Challenge 2: Text Formatter

Difficulty: Medium | Time: 25 minutes

Task: Create a text formatter that:

Takes a multi-line paragraph
Removes extra whitespace
Capitalizes each sentence
Counts sentences
Displays formatted output

Output will appear here...

# Text Formatter Solution
text = """   this is  a    test.  python is   great.
it can   do many things.   """

print("Original:")
print(repr(text))

# Remove extra whitespace
words = text.split()
cleaned = " ".join(words)

# Split into sentences
sentences = cleaned.split(". ")
formatted_sentences = [s.capitalize() for s in sentences]
formatted_text = ". ".join(formatted_sentences)

# Add period at end if missing
if not formatted_text.endswith("."):
    formatted_text += "."

# Display results
print("\n=== Formatted Text ===")
print(formatted_text)
print(f"\nSentences: {len(formatted_sentences)}")
print(f"Words: {len(words)}")
print(f"Characters: {len(formatted_text)}")

🎉 Congratulations!

You've completed Chapter 6! You now know:

How to create and manipulate strings
String indexing and slicing
Essential string methods
All string formatting methods (especially f-strings!)
String immutability and why it matters
How to split and join strings

Next Step: Move on to Chapter 7 to learn about file handling!

🎯 Knowledge Check

Test your understanding of string manipulation concepts!

🎓 Ready for More?

Great job completing Chapter 6! You've mastered string manipulation - one of the most important skills in Python programming. In the next chapter, we'll learn about file handling and how to read and write data to files!

String Manipulation

📝 What are Strings?

Creating Strings - Different Ways

⚡ Quick Tip: When to Use Which Quotes?

Escape Characters - Special Characters in Strings

Using Escape Sequences

Common Escape Sequences

🔢 String Indexing & Slicing

Accessing Individual Characters

String Slicing - Extracting Substrings

Slicing Syntax: string[start:stop:step]

🔍 Understanding String Slicing

🛠️ Powerful String Methods

Case Conversion Methods

Searching and Checking Methods

Trimming and Cleaning Strings

💡 Most Commonly Used String Methods

🎨 String Formatting - Building Strings

Method 1: String Concatenation (The Old Way)

Method 2: % Formatting (Old Style)

Method 3: str.format() (Better Way)

🚀 F-Strings - The Modern Way!

Basic F-String Usage

Advanced F-String Formatting

🔍 F-String Format Specifiers

💡 Why F-Strings are the Best!

🔒 String Immutability - Strings Can't Change!

What Immutability Means

🔍 Why Are Strings Immutable?

Working with "Changing" Strings

✂️ Splitting and Joining Strings

split() - Breaking Strings Into Lists

join() - Combining Lists Into Strings

💡 split() and join() - Perfect Partners!

Real-World Examples

📺 Video Resources

Python String Methods

Python F-Strings Tutorial

String Manipulation in Python

🤖 AI Learning Prompts

Understanding String Basics

Mastering String Methods

String Formatting Mastery

Practical String Applications

💡 Tips for Using These Prompts:

✏️ Practice Exercises

How to Approach These Exercises:

Part 1: Guided Exercises

Guided Exercise 1: Text Analyzer

Step 1: Create a Simple Text Analyzer

Guided Exercise 2: User Input Formatter

Clean and Format Various Types of Input

Part 2: Independent Practice

Challenge 1: Password Validator

Challenge 2: Text Formatter

🎉 Congratulations!

🎯 Knowledge Check

Quiz Results

🎓 Ready for More?