notes on Strings in Python

Here’s the enriched class note with more detailed explanations and examples based on the content from your PDF:

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Strings: Introduction

• A string is a sequence of characters enclosed within single ('...'), double ("..."), or triple quotes ('''...''' or """...""").
• Strings are immutable, meaning once a string is created, it cannot be changed.
• Examples:

  string1 = "Hello, World!"
  string2 = 'Python Programming'
  string3 = '''This is a
  multiline string'''

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String Operations

1. Concatenation: Joining two or more strings using the + operator.

• Example from PDF:
  python A = "Tom" B = "Jerry" C = A + " & " + B print(C) # Output: "Tom & Jerry"

1. Repetition: Repeating a string multiple times using the * operator.

• Example from PDF:
  python Line = "go" print(Line * 3, "Govinda") # Output: "gogogo Govinda"

1. Membership: Using in and not in operators to check if a substring exists.

• Example from PDF:
  python 'a' in 'python' # Output: False 'a' in 'java' # Output: True 'per' in 'operators' # Output: True

1. Slicing: Extracting parts of a string using slice notation.

• Example from PDF:
  python str1 = "wonderful" print(str1[0:6]) # Output: "wonder" print(str1[0::2]) # Output: "wnefl" print(str1[::-1]) # Output: "lufrednow" (reverse string)

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Traversing a String Using Loops

• Traversing means accessing each character of the string one by one.
• Example from PDF:

  name = "lovely"
  for ch in name:
      print(ch, '-', end='')  # Output: l-o-v-e-l-y-

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Built-in String Functions/Methods

1. len(): Returns the number of characters in the string, including spaces.

   len("Python")  # Output: 6

2. capitalize(): Converts the first character of the string to uppercase.

• Example from PDF:
  python "python".capitalize() # Output: "Python"

2. title(): Converts the first character of each word to uppercase.

   "hello world".title()  # Output: "Hello World"

4. lower(): Converts all characters to lowercase.

   "HELLO".lower()  # Output: "hello"

5. upper(): Converts all characters to uppercase.

   "hello".upper()  # Output: "HELLO"

6. count(substring): Counts occurrences of a substring within the string.

• Example from PDF:
  python "banana".count("a") # Output: 3

6. find(substring): Returns the index of the first occurrence of the substring, or -1 if not found.

• Example from PDF:
  python "hello".find("e") # Output: 1

6. index(substring): Similar to find(), but raises an error if the substring is not found.

• Example from PDF:
  python "hello".index("e") # Output: 1

6. endswith(suffix): Checks if the string ends with the given suffix.

   "hello".endswith("lo")  # Output: True

10. startswith(prefix): Checks if the string starts with the given prefix. "hello".startswith("he") # Output: True
11. isalnum(): Checks if the string contains only alphanumeric characters. "abc123".isalnum() # Output: True
12. isalpha(): Checks if the string contains only alphabetic characters. "abc".isalpha() # Output: True
13. isdigit(): Checks if the string contains only digits. "123".isdigit() # Output: True
14. islower(): Checks if all alphabetic characters are lowercase. "hello".islower() # Output: True
15. isupper(): Checks if all alphabetic characters are uppercase. "HELLO".isupper() # Output: True
16. isspace(): Checks if the string contains only whitespace. " ".isspace() # Output: True
17. lstrip(): Removes leading whitespace from the string. " hello".lstrip() # Output: "hello"
18. rstrip(): Removes trailing whitespace from the string. "hello ".rstrip() # Output: "hello"
19. strip(): Removes leading and trailing whitespace. " hello ".strip() # Output: "hello"
20. replace(old, new): Replaces occurrences of old with new.
    • Example from PDF:
    "hello world".replace("world", "Python") # Output: "hello Python"
21. join(iterable): Joins elements of an iterable (e.g., list) into a string, separated by a specified delimiter. "-".join(["a", "b", "c"]) # Output: "a-b-c"
22. partition(sep): Splits the string into three parts: before the separator, the separator itself, and after the separator.
    • Example from PDF:
    "apple#banana#cherry".partition("#") # Output: ('apple', '#', 'banana#cherry')
23. split(sep): Splits the string at the specified delimiter and returns a list of parts.
    python "apple,banana,cherry".split(",") # Output: ['apple', 'banana', 'cherry']

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Additional Examples from PDF:

• Reversing a String:

  string1 = "karan"
  length = len(string1)
  for ch in range(-1, -length-1, -1):
      print(string1[ch])
  # Output: n a r a k

• Program to Count Vowels:

  string = input("Enter a string: ")
  vowels = 'aeiouAEIOU'
  count = 0
  for ch in string:
      if ch in vowels:
          count += 1
  print("Number of vowels:", count)

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