String Creation and Indexing

String Creation

Python offers several ways to create strings, each with its own use cases and nuances.

1. Using Quotes

Strings in Python can be created using single quotes (''), double quotes (""), or triple quotes (''' ''' or """ """).

single_quoted = 'Hello, World!'
double_quoted = "Python is awesome!"
triple_single_quoted = '''This is a
multi-line string.'''
triple_double_quoted = """Another
multi-line string."""

• Single and double quotes are interchangeable for single-line strings.
• Triple quotes are used for multi-line strings or docstrings.

Choosing Between Quote Types

• Use single quotes if the string contains double quotes:

  message = 'He said, "Hello!"'
• Use double quotes if the string contains single quotes:

  message = "It's a beautiful day!"
• Use triple quotes for strings that span multiple lines or contain both single and double quotes:

  long_message = """He said, "It's a beautiful day!"
  I couldn't agree more."""

2. String Concatenation

You can create strings by concatenating other strings:

first_name = "John"
last_name = "Doe"
full_name = first_name + " " + last_name  # "John Doe"

3. String Replication

Strings can be replicated using the * operator:

repeated_string = "Echo " * 3  # "Echo Echo Echo "

4. String Conversion

You can create strings from other data types using the str() function:

number = 42
number_string = str(number)  # "42"

pi = 3.14159
pi_string = str(pi)  # "3.14159"

5. f-Strings (Formatted String Literals)

Available in Python 3.6+, f-strings provide a concise way to embed expressions inside string literals:

name = "Alice"
age = 30
introduction = f"My name is {name} and I'm {age} years old."

6. String Methods

Certain string methods return new strings:

original = "hello"
uppercase = original.upper()  # "HELLO"

String Indexing

Python strings are sequences of characters, and each character can be accessed by its index.

Basic Indexing

• Indexing starts at 0 for the first character.
• Negative indices count from the end of the string, with -1 being the last character.

text = "Python"
first_char = text[0]   # 'P'
third_char = text[2]   # 't'
last_char = text[-1]   # 'n'
second_last = text[-2] # 'o'

Indexing Behavior

1. Accessing Valid Indices:

   text = "Hello"
   print(text[1])  # 'e'

2. Index Out of Range: Accessing an index that doesn't exist raises an IndexError:

   text = "Hello"
   # print(text[10])  # Raises IndexError: string index out of range

3. Empty String Indexing: Attempting to index an empty string will always raise an IndexError:

   empty = ""
   # print(empty[0])  # Raises IndexError: string index out of range

Advanced Indexing Techniques

1. Slicing: Slicing allows you to extract a substring using the syntax [start:end:step]:

   text = "Python Programming"
   print(text[0:6])    # "Python"
   print(text[7:])     # "Programming"
   print(text[:5])     # "Pytho"
   print(text[::2])    # "Pto rgamn"
   print(text[::-1])   # "gnimmargorP nohtyP" (reverses the string)

2. Stride Slicing: The third parameter in slicing specifies the stride or step:

   text = "Python"
   print(text[::2])  # "Pto"
   print(text[1::2]) # "yhn"

3. Slice Assignment: Unlike individual characters, you can assign to slices of mutable sequences (but not strings, as they're immutable):

   # This works for lists, but not for strings
   my_list = list("Python")
   my_list[1:3] = list("aaaaa")
   print(''.join(my_list))  # "Paaaaahon"

String Immutability and Indexing

Remember that strings in Python are immutable. This means you cannot change a string by assigning to an index:

text = "Python"
# text[0] = "J"  # This raises a TypeError: 'str' object does not support item assignment

To modify a string, you need to create a new string:

text = "Python"
new_text = "J" + text[1:]  # "Jython"

Practical Applications

1. Extracting Substrings:

   email = "[email protected]"
   username = email[:email.index("@")]  # "user"
   domain = email[email.index("@")+1:]  # "example.com"

2. String Validation:

   def is_palindrome(s):
       return s == s[::-1]
   
   print(is_palindrome("radar"))  # True
   print(is_palindrome("python")) # False

3. Formatting and Alignment:

   title = "Python Programming"
   print(f"{title:^30}")  # Centered in 30 characters
   print(f"{title:>30}")  # Right-aligned in 30 characters

Understanding string creation and indexing is crucial for effective text processing in Python. These concepts form the foundation for more advanced string manipulation techniques and are essential for working with textual data in various applications.