Notes on Lists in Python for Class 12 Computer Science

Notes on Lists in Python  for  Class  12 Computer Science

1. Definition

A list is a built-in data structure in Python used to store multiple items in a single variable. It is one of the most versatile and commonly used sequence types.

2. Key Characteristics

• Ordered: Items hold a specific position (index).

• Mutable: You can add, remove, or change items after the list is created.

• Heterogeneous: Can store elements of different data types (integers, strings, booleans, even other lists) together.

• Allow Duplicates: The same value can appear multiple times.

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3. Syntax & Creation

Lists are created using square brackets [], separating elements with commas.

Python

# Empty List
my_list = []

# Integers
numbers = [10, 20, 30, 40]

# Mixed Data Types
mixed = [1, "Hello", 3.14, True]

# Nested List (List inside a list)
nested = [1, [2, 3], 4]

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4. Indexing and Slicing

Python uses zero-based indexing. You can access elements from the start (positive index) or the end (negative index).

• Accessing Elements:

  • numbers[0] → First item.

  • numbers[-1] → Last item.

• Slicing (list[start:stop:step]):

  • numbers[1:3] → Items from index 1 up to (but not including) 3.

  • numbers[::-1] → Reverses the list.

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5. Common List Methods

Method	Description	Example
append(x)	Adds an item x to the end.	lst.append(5)
insert(i, x)	Inserts item x at index i.	lst.insert(1, "Hi")
extend(seq)	Adds all elements of a sequence to the end.	lst.extend([6, 7])
remove(x)	Removes the first occurrence of item x.	lst.remove(5)
pop(i)	Removes and returns item at index i (default last).	lst.pop()
index(x)	Returns the index of the first item with value x.	lst.index("Hi")
count(x)	Returns how many times x appears.	lst.count(5)
sort()	Sorts the list in ascending order (modifies original).	lst.sort()
reverse()	Reverses the order of elements (modifies original).	lst.reverse()

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6. Modifying Lists

Because lists are mutable, you can change them directly using their index.

Python

fruits = ["apple", "banana", "cherry"]

# Change an item
fruits[1] = "mango"  
# Result: ["apple", "mango", "cherry"]

# Delete an item using 'del' keyword
del fruits[0]
# Result: ["mango", "cherry"]

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7. List Comprehension

A concise way to create lists based on existing lists. It is often faster and more readable than a for loop.

Syntax: [expression for item in iterable if condition]

Python

# Standard Loop way
squares = []
for x in range(5):
    squares.append(x**2)

# List Comprehension way (One-liner)
squares = [x**2 for x in range(5)] 
# Result: [0, 1, 4, 9, 16]

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8. Essential Built-in Functions

• len(list): Returns the number of items.

• min(list): Returns the smallest item.

• max(list): Returns the largest item.

• sum(list): Returns the sum of all items (if numbers).

• sorted(list): Returns a new sorted list (does not modify the original).

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