Strings 11 Computer Science Chapter 8 CBSE Notes 2025-26

Understanding strings in Python is essential as they are used to handle textual data in most programs. In Python, a string is a sequence of one or more Unicode characters, and it can include letters, digits, spaces, or symbols. You can create strings by enclosing characters within single, double, or triple quotes. Triple quotes allow strings to span multiple lines, which can be useful for paragraphs or docstrings.

Creating and Accessing Strings Python strings are easy to create, whether by using 'Hello', "Hello", or even triple quotes like '''Hello'''. Each character in a string is accessible using indexing, starting from 0 on the left. For example, in the string "Hello World!", H is at index 0, W is at index 6, and the exclamation point is at index 11. Python also supports negative indexing, where -1 represents the last character, -2 the second to last, and so forth. If you try to access an index outside the string's length, Python will raise an error.

String Immutability One important property of strings in Python is that they are immutable. Once a string is created, its contents cannot be changed. If you try to assign a new character to a specific position in a string (like str1[1] = 'a'), Python will display an error. This means that any changes to string content require creating a new string.

String Operations Python lets you combine, repeat, and check membership within strings using various operators:

• Concatenation: Use the + operator to join two strings. For example, 'Hello' + 'World!' becomes 'HelloWorld!'.
• Repetition: Use the * operator to repeat a string. 'Hello' * 3 results in 'HelloHelloHello'.
• Membership: Use the in operator to check if a substring exists in a string, and not in for the opposite.

Slicing Strings Slicing allows you to extract parts of a string by specifying a start and end index using the syntax str1[start:end]. The character at the start index is included, but the end is excluded. For instance, str1[1:5] gives characters from index 1 to 4. Omitting the start index begins slicing from the start; omitting the end index goes to the string's end. You can also use a 'step' value to skip characters, such as str1[0:10:2], which returns every second character from index 0 to 9. Slicing with negative indices or reversing strings (str1[::-1]) is also possible.

String Traversal You can process each character in a string using loops. The for loop lets you go through each character naturally (e.g., for ch in str1:), while the while loop can access characters using their indices, running from 0 to len(string)-1. Both approaches efficiently allow you to perform tasks like searching or modifying characters.

Built-in String Methods Python provides many useful string methods to work with string values:

• len(str): Returns the length of the string.
• title(): Converts the string to title case (first letter capital, rest small).
• lower()/upper(): Converts string to all lowercase or uppercase letters.
• count(substring, start, end): Counts how many times a substring occurs.
• find(substring, start, end): Finds first index of a substring, or -1 if not found.
• startswith()/endswith(): Checks if the string starts or ends with a specific substring.
• replace(old, new): Replaces all occurrences of a substring with another.
• split(separator): Splits a string into a list, using the separator.
• strip()/lstrip()/rstrip(): Removes whitespace from both ends, the left, or the right side.
• isalnum(), isalpha(), isdigit(), islower(), isupper(), issapce(), istitle(): Check the type or property of the string (e.g., only alphabets, only digits, all lowercase, etc.).

These methods make working with strings easier and help with tasks like formatting, searching, and checking data types within strings.

Handling Strings with Functions Working with strings often requires writing functions for specific tasks. Some common user-defined functions are:

• Counting characters: A function might count occurrences of a particular letter in a string.
• Replacing vowels: Replace all vowels with a special symbol (like *) for effects or security.
• Reversing strings: This can be achieved by slicing or looping through the string backwards, creating either a new string or printing in reverse directly.
• Palindrome checking: By comparing characters from the start and end moving inwards, you can check if a string reads the same forwards and backward.

These practice programs help in building a strong foundation in string manipulation which is vital for most programming tasks.

Examples and Exercises Exercises include predicting outputs from various string operations such as slicing, repetition, or method calls. For example, using mySubject = "Computer Science", questions may ask the outcome of mySubject[::2] (which skips every second character). Other exercises make you use functions like lower(), replace(), or split() on address strings to see practical results. These questions help reinforce understanding and application of concepts.

Programming Practice To strengthen your skills, practice problems can include tasks like:

• Counting alphabets, digits, symbols, and words in user-input text
• Converting a sentence into title case by capitalising the first letter of every word
• Deleting all occurrences of a specified character from a string
• Calculating the sum of all digits present in a string
• Replacing each blank in a sentence with a hyphen

Summary of Key Points

• Strings are sequences of characters, created using quotes.
• Indexing and negative indexing help in accessing characters directly.
• Strings cannot be changed in place—they are immutable.
• Operators like +, *, in, not in allow concatenation, repetition, and substring checks.
• Slicing is used to extract substrings by specifying ranges.
• Traversal of strings can be done using for or while loops.
• Python’s built-in string methods make formatting, searching, and changing string cases easy.
• Practicing with small programs strengthens understanding of essential string operations.

CBSE Class 11 Computer Science Chapter 8 Notes – Strings: Quick Revision Key Points

These CBSE Class 11 Computer Science Notes Chapter 8 make string concepts easy to understand by organizing all the facts, methods, and examples from your NCERT textbook. The notes cover key string operations, slicing, traversing, and relevant Python functions in a clear and concise manner for efficient last-minute revision.

Use these handy revision notes to quickly recap important coding patterns and functions needed for your exams or Python programming tasks. Covering both the theory and applied side, these Class 11 Computer Science String notes will help you gain confidence and secure better marks in Chapter 8.