Python Lists (Part 1)

A list is one of the most versatile and widely used data structures in Python. A list is an ordered collection of items that can store elements of different data types such as integers, floating-point numbers, strings, Boolean values, and even other lists. Unlike strings, lists are mutable, which means their elements can be modified after the list is created.

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Learning Objectives

• Understand the concept of lists.
• Create and initialize lists.
• Access elements using positive and negative indexing.
• Perform basic list operations.
• Traverse a list using loops.

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What is a List?

A list is an ordered collection of elements enclosed within square brackets []. Each element in a list is separated by a comma.

Examples

numbers=[10,20,30,40]

names=["Aman","Riya","Rahul"]

mixed=[101,"Python",95.5,True]

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Characteristics of Lists

• Lists are ordered collections.
• Lists are mutable (modifiable).
• Lists can store duplicate values.
• Lists can contain different data types.
• Each element has an index.
• Lists can contain other lists (Nested Lists).

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Creating Lists

List of Integers

marks=[85,90,78,92]

List of Strings

cities=["Jaipur","Delhi","Mumbai"]

Mixed List

student=[101,"Rahul",91.5,True]

Empty List

data=[]

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List Indexing

Every element in a list has an index. Python supports both positive and negative indexing.

subjects=["English","Physics","Chemistry","Mathematics","Computer"]

Element	English	Physics	Chemistry	Mathematics	Computer
Positive Index	0	1	2	3	4
Negative Index	-5	-4	-3	-2	-1

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Accessing List Elements

subjects=["English","Physics","Chemistry","Mathematics","Computer"]

print(subjects[0])

print(subjects[2])

print(subjects[-1])

Output

English
Chemistry
Computer

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Updating List Elements

Since lists are mutable, elements can be modified.

marks=[85,90,78]

marks[1]=95

print(marks)

Output

[85,95,78]

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List Operations

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1. Concatenation (+)

The + operator joins two lists.

list1=[1,2,3]

list2=[4,5]

print(list1+list2)

Output

[1,2,3,4,5]

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2. Repetition (*)

The * operator repeats a list.

numbers=[1,2]

print(numbers*3)

Output

[1,2,1,2,1,2]

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3. Membership Operators

The in and not in operators check whether an element exists in a list.

numbers=[10,20,30]

print(20 in numbers)

print(50 in numbers)

print(50 not in numbers)

Output

True
False
True

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4. List Slicing

Slicing extracts a portion of a list.

Syntax

list[start:stop]

Example 1

numbers=[10,20,30,40,50]

print(numbers[1:4])

Output

[20,30,40]

Example 2

print(numbers[:3])

Output

[10,20,30]

Example 3

print(numbers[2:])

Output

[30,40,50]

Example 4

print(numbers[::-1])

Output

[50,40,30,20,10]

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Traversing a List Using for Loop

colors=["Red","Blue","Green"]

for color in colors:
    print(color)

Output

Red
Blue
Green

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Traversing a List Using while Loop

colors=["Red","Blue","Green"]

i=0

while i<len(colors):
    print(colors[i])
    i=i+1

Output

Red
Blue
Green

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Real-Life Applications of Lists

• Storing marks of students.
• Maintaining attendance records.
• Managing product inventories.
• Creating shopping carts.
• Storing employee details.
• Managing examination results.
• Processing survey responses.

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Common Programming Mistakes

1. Accessing an Invalid Index

numbers=[10,20,30]

print(numbers[5])

Result: IndexError

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2. Forgetting Square Brackets

numbers=10,20,30

Explanation: This creates a tuple-like sequence instead of a list.

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3. Assuming Slicing Includes the Stop Index

numbers=[10,20,30,40,50]

print(numbers[1:4])

Output

[20,30,40]

The element at index 4 is not included.

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CBSE Important Programs

1. Create a list of five numbers and display it.
2. Print all elements using a for loop.
3. Print all elements using a while loop.
4. Display the first and last element of a list.
5. Reverse a list using slicing.
6. Concatenate two lists.
7. Repeat a list using the * operator.
8. Check whether an element exists in a list.
9. Extract a sub-list using slicing.
10. Update an element in a list.

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Viva Questions

1. What is a list?
2. Why are lists called mutable?
3. Can a list contain different data types?
4. What is list indexing?
5. Differentiate between positive and negative indexing.
6. How can you traverse a list?
7. What is list slicing?
8. Which operators are used for concatenation and repetition?
9. What are membership operators?
10. Give two real-life applications of lists.

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Exam Tips

• Remember that lists are mutable.
• Indexing starts from 0.
• Negative indexing starts from -1.
• The stop index in slicing is excluded.
• Practice traversing lists using both for and while loops.

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