Strings and String Operations in Python

🔶 What is a String?

A string is a sequence of characters enclosed in single quotes ('), double quotes ("), or even triple quotes (''' or """).

✅ Examples:

text1 = 'Hello'
text2 = "Python"
text3 = '''Data Science'''

A string can contain:

• Letters: 'abc'
• Numbers: '123' (as characters)
• Symbols: '@#$'
• Spaces: 'Data Science'

In Python, a string is immutable, meaning once it's created, it cannot be changed.

🔶 How to Create a String

🔸 Using quotes:

name = "Alice"
greeting = 'Hello!'

🔸 Using str() constructor:

number = 123
string_number = str(number)   # '123'

🔶 String Indexing and Slicing

✅ Indexing:

Each character in a string has a position called an index. Indexing starts from 0.

word = "Python"
print(word[0])   # 'P'
print(word[3])   # 'h'

You can also use negative indexing:

print(word[-1])  # 'n' (last character)
print(word[-2])  # 'o'

✅ Slicing:

Slicing means getting a part of the string.

word = "DataScience"
print(word[0:4])   # 'Data'
print(word[4:])    # 'Science'
print(word[:4])    # 'Data'
print(word[::2])   # 'Dt cec' (skips every second character)

🔶 Basic String Operations

Operation	Syntax / Example	Description
Concatenation	"Data" + "Science"	Joins two strings → 'DataScience'
Repetition	"Python" * 3	Repeats string → 'PythonPythonPython'
Length	len("Python")	Returns number of characters → 6
Membership	'D' in "Data"	Checks if character exists → True
Looping	for char in "AI": print(char)	Iterates over characters

🔶 String Methods (Built-in Functions)

Python provides many helpful functions (methods) to work with strings.

🟩 Common String Methods

Method	Description	Example
.lower()	Converts to lowercase	'PYTHON'.lower() → 'python'
.upper()	Converts to uppercase	'data'.upper() → 'DATA'
.title()	Capitalizes each word	'machine learning'.title()
.strip()	Removes spaces from both ends	' data '.strip() → 'data'
.replace(old, new)	Replaces a part of the string	'AI'.replace('A','I')
.split(separator)	Splits string into list	'a,b,c'.split(',') → ['a','b','c']
.join(list)	Joins list into string	','.join(['a','b','c']) → 'a,b,c'
.find(substring)	Finds first index of substring	'python'.find('t') → 2
.count(substring)	Counts how many times a substring appears	'banana'.count('a') → 3
.startswith(sub)	Checks if string starts with sub	'hello'.startswith('he') → True
.endswith(sub)	Checks if string ends with sub	'file.txt'.endswith('.txt') → True

🔶 Escape Characters

Escape characters are used to include special characters in strings.

Escape Code	Description	Example Output
\n	New line	Line 1 Line 2
\t	Tab (space)	Adds tab space
\'	Single quote	It's Python
\"	Double quote	He said "Hi"
\\	Backslash	\

print("Hello\nWorld")      # prints on two lines
print("She said, \"Yes\"") # uses double quotes inside string

🔶 String Formatting

Used to insert variables into strings.

✅ 1. Using f-strings (Modern & Preferred)

name = "Alice"
age = 25
print(f"My name is {name} and I am {age} years old.")

✅ 2. Using format()

print("My name is {} and I am {} years old.".format(name, age))

🟦 Real-life Applications in Data Science

Task	How Strings Help
Reading file content	File data is read as strings
Preprocessing text (NLP)	Clean and format input strings
Parsing CSV/JSON/XML data	Split and analyze text-based files
Converting input values	Convert raw strings to usable formats
Displaying messages or dashboards	Present data using formatted strings

🧠 Key Points to Remember

• Strings are sequences of characters and are immutable.
• You can use indexing and slicing to access parts of a string.
• There are many built-in string methods to manipulate text.
• Strings can be formatted using f-strings or .format().
• Strings play a key role in file handling, web scraping, NLP, and more.

📝 Practice Questions

1. Take a sentence from user input and count how many words it has.
2. Write a program to reverse a string using slicing.
3. Ask the user to enter a name, and check if it starts with a capital letter.
4. Replace all spaces in a sentence with underscores (_).
5. Check if a string is a palindrome (same forward and backward).