Chapter 6: Strings in Python

A string is a sequence of characters enclosed in single quotes ('...'), double quotes ("..."), or triple quotes ('''...''' for multi-line strings). In Python, strings are immutable, meaning their contents cannot be changed after they are created.

6.1 String Operations

Python provides several powerful operators to manipulate strings.

Concatenation (+): Joins two strings together.
```
"Hello" + " World" # Output: 'Hello World'
```
Repetition (*): Repeats a string a specified number of times.
```
"Py" * 3 # Output: 'PyPyPy'
```
Membership (in, not in): Checks if a substring exists within a string.
```
"a" in "Apple" # Output: False (case-sensitive)
```

Slicing ([start:stop:step]): Extracts a portion of the string.

text = "COMPUTER"
print(text[1:4])  # Output: 'OMP'
print(text[::-1]) # Output: 'RETUPMOC' (Reverses string)

6.2 Traversing a String

You can access each character of a string one by one using a for or while loop.

word = "PYTHON"
for char in word:
    print(char)

6.3 Built-in String Methods

Strings come with numerous built-in methods. Note that string methods do not modify the original string; they return a new string.

len(str): Returns the length (number of characters) of the string.
capitalize(): Capitalizes the first letter of the string.
title(): Capitalizes the first letter of every word.
lower() / upper(): Converts the string to all lowercase or all uppercase.
count(sub): Returns the number of times substring sub appears.
find(sub) / index(sub): Returns the lowest index where sub is found. find() returns -1 if not found, while index() raises an error.
startswith(prefix) / endswith(suffix): Returns True if string starts/ends with the specified substring.
isalnum(): True if all characters are alphanumeric (letters or numbers).
isalpha(): True if all characters are alphabets.
isdigit(): True if all characters are digits.
islower() / isupper() / isspace(): Checks if all characters are lowercase, uppercase, or whitespace respectively.
lstrip() / rstrip() / strip(): Removes leading, trailing, or both leading and trailing whitespace characters.
replace(old, new): Replaces occurrences of the old substring with new.
split(sep): Splits the string into a list of words using sep as the delimiter.
partition(sep): Splits the string into a tuple of 3 elements: (before sep, sep, after sep).
join(iterable): Joins elements of an iterable (like a list) into a single string, using the string as a separator.
```
"-".join(["A", "B", "C"]) # Output: 'A-B-C'
```

Competency Based Questions

Q1. Predict the Output What is the output of the following code block?

s = " Kendriya Vidyalaya "
s = s.strip()
print(s.replace("a", "@").split())

Q2. Find the Error Riya wants to change the first letter of her name stored in a variable name = "riya" to a capital letter by directly modifying the character at index 0.

name = "riya"
name[0] = "R"
print(name)

Why does this code throw an error? How can she achieve her goal using string methods?

Q3. Case-Based Scenario A password verification system requires a password to satisfy the following rules:

It must contain only letters and numbers (no special characters).
It must end with “123”. Which two Python string methods should the programmer use to verify these conditions?

Q4. Application-Oriented Write a Python program to input a string and determine whether it is a palindrome or not without using any loops. (A palindrome reads the same forwards and backwards, e.g., “MADAM”).

Answers to Competency Based Questions

A1. ['Kendriy@', 'Vidy@l@y@'] Explanation:

strip() removes leading/trailing spaces -> "Kendriya Vidyalaya"
replace("a", "@") -> "Kendriy@ Vidy@l@y@"
split() splits by space into a list -> ['Kendriy@', 'Vidy@l@y@'].

A2. Error: Strings in Python are immutable. You cannot assign a value to a specific index of an existing string (name[0] = "R" causes a TypeError). Correction: She should use the capitalize() or title() method.

name = "riya"
name = name.capitalize()
print(name)

A3. The programmer should use:

isalnum() (Returns True if characters are only letters and numbers).
endswith("123") (Returns True if the string ends with “123”).

A4.

text = input("Enter a string: ")
# Reversing the string using slicing
reversed_text = text[::-1]

if text == reversed_text:
    print("It is a palindrome")
else:
    print("It is not a palindrome")