Python Strings

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1. What is a String?

• A string is a sequence of characters enclosed in quotes.
• Strings are immutable, meaning their contents cannot be changed after creation.
• Python supports single quotes ('), double quotes ("), and triple quotes (''' or """) for defining strings.

name = "Raj"
message = 'Hello, World!'
paragraph = """This is a multi-line
string in Python."""

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2. String Operations

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1. Concatenation

• Combining two or more strings using the + operator.

first_name = "Ram"
last_name = "Kumar"
full_name = first_name + " " + last_name
print(full_name)  # Output: Ram Kumar

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2. Repetition

• Repeating a string using the * operator.

greeting = "Hello! "
print(greeting * 3)  # Output: Hello! Hello! Hello!

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3. Indexing

• Accessing individual characters in a string using their index.
• Python uses zero-based indexing (the first character has index 0).

name = "Anand"
print(name[0])  # Output: A
print(name[-1])  # Output: d (negative indexing starts from the end)

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4. Slicing

• Extracting a substring using a range of indices.
• Syntax: string[start:stop:step]

text = "Python Programming"
print(text[0:6])  # Output: Python (from index 0 to 5)
print(text[7:])   # Output: Programming (from index 7 to the end)
print(text[:6])   # Output: Python (from the start to index 5)
print(text[::2])  # Output: Pto rgamn (every second character)

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5. Length

• Finding the length of a string using the len() function.

name = "Bala"
print(len(name))  # Output: 4

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3. String Methods

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1. upper() and lower()

• Convert a string to uppercase or lowercase.

text = "Hello, World!"
print(text.upper())  # Output: HELLO, WORLD!
print(text.lower())  # Output: hello, world!

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2. strip(), lstrip(), and rstrip()

• Remove leading and trailing whitespace (or specified characters).

text = "   Python   "
print(text.strip())  # Output: Python

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3. replace()

• Replace a substring with another substring.

text = "Hello, World!"
new_text = text.replace("World", "Karthik")
print(new_text)  # Output: Hello, Karthik!

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4. split()

• Split a string into a list of substrings based on a delimiter.

text = "Python,Java,C++"
languages = text.split(",")
print(languages)  # Output: ['Python', 'Java', 'C++']

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5. join()

• Join a list of strings into a single string using a delimiter.

words = ["Python", "is", "fun"]
sentence = " ".join(words)
print(sentence)  # Output: Python is fun

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6. find() and index()

• Find the index of a substring in a string.
• find() returns -1 if the substring is not found, while index() raises an error.

text = "Hello, World!"
print(text.find("World"))  # Output: 7
print(text.index("World"))  # Output: 7

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7. startswith() and endswith()

• Check if a string starts or ends with a specific substring.

text = "Hello, World!"
print(text.startswith("Hello"))  # Output: True
print(text.endswith("!"))        # Output: True

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8. isalpha(), isdigit(), isalnum()

• Check if all characters in a string are alphabetic, numeric, or alphanumeric.

print("Python".isalpha())  # Output: True
print("123".isdigit())     # Output: True
print("Python3".isalnum()) # Output: True

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4. String Formatting

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1. f-strings (Python 3.6+)

• Embed expressions inside string literals using {}.

name = "David"
age = 30
print(f"{name} is {age} years old.")  # Output: David is 30 years old.

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2. format() Method

• Insert values into a string using placeholders {}.

name = "Kannan"
age = 25
print("{0} is {1} years old.".format(name, age))  # Output: Kannan is 25 years old.

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3. % Formatting (Older Style)

• Use % as a placeholder for values.

name = "Siva"
age = 28
print("%s is %d years old." % (name, age))  # Output: Siva is 28 years old.

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5. Escape Sequences

• Special characters used to represent non-printable or special characters in strings.
• Common escape sequences:
  • \n: Newline
  • \t: Tab
  • \\: Backslash
  • \": Double quote
  • \': Single quote

print("Hello\nWorld!")  # Output: Hello
                        #         World!

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6. Additional Examples

• Concatenation:
  first_name = "Suresh"
  last_name = "Kumar"
  full_name = first_name + " " + last_name
  print(full_name)  # Output: Suresh Kumar
  
• Slicing:
  name = "Sathish"
  print(name[0:4])  # Output: Sath
  
• String Methods:
  text = "   Sujatha   "
  print(text.strip())  # Output: Sujatha
  
• String Formatting:
  name = "Kavitha"
  age = 22
  print(f"{name} is {age} years old.")  # Output: Kavitha is 22 years old.
  

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7. Best Practices

• Use f-strings for string formatting (Python 3.6+).
• Use triple quotes for multi-line strings.
• Avoid using + for concatenating many strings (use join() instead for efficiency).