What is list in python

What is list in python

Lists are just like dynamically sized arrays, declared in other languages (vector in C++ and ArrayList in Java). A list in Python is an ordered collection of items that can hold multiple values or elements of different types (e.g., integers, strings, or even other lists).

The elements in a list are indexed according to a definite sequence and the indexing of a list is done with 0 being the first index. Each element in the list has its definite place in the list, which allows duplicating of elements in the list, with each element having its own distinct place and credibility.

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Basic Properties of a List:

Ordered: The items in a list have a specific order, and that order is maintained.
Mutable: Lists can be modified, meaning you can add, remove, or change items in the list after it has been created.
Allows Duplicates: Lists can have duplicate elements (i.e., the same value can appear multiple times).
Heterogeneous: Lists can hold elements of different types (e.g., integers, strings, objects).

Syntax for Creating a List:

We create a list by placing elements inside square brackets ([]), separated by commas.

List_Name=[element1,element2,.....]
my_list = [1, 2, 3, 4, 5]

Basic Operations on Lists:

Creating a List:

We can create a list containing different types of elements.

my_list = [1, "apple", 3.14, True]
print(my_list)

Output:

[1, 'apple', 3.15, True]

Accessing List Items:

Lists are indexed, starting from 0. You can access an element by using its index.

my_list = [1, "apple", 3.14, True]
print(my_list[1])  # Accessing the element at index 1 (second element)

Output:

apple

Negative Indexing:

Negative indices allow us to access list elements starting from the end. -1 refers to the last item, -2 refers to the second last item, and so on.

print(my_list[-1])  # Accessing the last element

Output:

True

Slicing a List:

We can extract a part of a list (a sublist) using slicing. The syntax is my_list[start:end], where start is the index where the slice starts, and end is the index where it ends (but excluding the element at the end index).

my_list = [1, 2, 3, 4, 5]
print(my_list[1:4])  # Slice from index 1 to index 3 (not including index 4)

Output:

[2, 3, 4]

Changing List Elements:

Since lists are mutable, we can change their values by assigning a new value to an index.

my_list = [1, 2, 3, 4, 5]
my_list[0] = 10  # Changing the first element
print(my_list)

Output:

[10, 2, 3, 4, 5]

Adding Elements to a List:

There are different ways to add elements to a list:

Using append(): Adds an element to the end of the list.
Using insert(): Inserts an element at a specific position in the list.
Using extend(): Adds multiple elements to the end of the list.

my_list = [1, 2, 3]
# Using append()
my_list.append(4)
# Using insert() (insert 99 at index 1)
my_list.insert(1, 99)
# Using extend() (adding multiple elements)
my_list.extend([5, 6])
print(my_list)

Output:

[1, 99, 2, 3, 4, 5, 6]

Example: Enter five elements in list then print those elements along with sum value.

n=[];
for i in range(5):
    num=int(input("enter elements"));
    n.append(num);
print(n);
print(sum(n));

Output:

enter elements5

enter elements6

enter elements3

enter elements9

enter elements3

[5, 6, 3, 9, 3]

Syn: list.insert(index, element)

l = [8, 'Harry', 9.3]
print(l)
ele=input("Enter Element")
ind=int(input("Enter Index no."))
l.insert(ind, ele)
print(l)

Removing Elements from a List:

We can remove elements from a list in several ways:

Using remove(): Removes the first occurrence of a specified element.
Using pop(): Removes and returns an element at a specified index (if no index is provided, it removes the last element).
Using del: Removes an element at a specified index.
Using clear(): Removes all elements from the list.

Example:

my_list = [1, 2, 3, 4, 5]

# Using remove() (removes first occurrence of 3)

my_list.remove(3)

Example: Enter five list elements then remove all elements.

n=[];
for i in range(5):
    num=int(input("Enter elements"));
    n.append(num);
print(n);
ch=input("Do you want remove all elements(y/n):");
if ch=='y':
    n.clear();
print(n);

Output:

Enter elements5

Enter elements6

Enter elements9

Enter elements3

Enter elements7

[5, 6, 9, 3, 7]

Do you want remove all elements(y/n):y

[]

# Using pop() (removes element at index 2, returns the value)

popped_element = my_list.pop(2)

# Using del (removes element at index 0)

del my_list[0]

# Using clear() (removes all elements)

my_list.clear()

print(my_list) # After clearing the list

Output:

css

[]

List Length:

You can find the number of elements in a list using the len() function.

my_list = [1, 2, 3, 4, 5]

print(len(my_list))

Output:

List Membership:

We can check if an element exists in a list using the in keyword.

my_list = [1, 2, 3, 4, 5]

print(3 in my_list) # Returns True if 3 is in the list

print(6 in my_list) # Returns False if 6 is not in the list

Output:

True

False

List Comprehension:

A concise way to create lists using a single line of code. List comprehension is often more efficient and readable.

squares = [x**2 for x in range(1, 6)]

print(squares)

Output:

[1, 4, 9, 16, 25]

List Methods:

Here are some commonly used methods associated with Python lists:

append(x): Adds item x to the end of the list.
insert(i, x): Inserts item x at position i.
remove(x): Removes the first occurrence of item x.
pop([i]): Removes and returns item at position i. If no index is provided, removes and returns the last item.
clear(): Removes all items from the list.
index(x): Returns the index of the first occurrence of item x.
count(x): Returns the number of times item x appears in the list.
sort(): Sorts the list in ascending order.
reverse(): Reverses the order of the list.

List = []

print("Blank List: ")

print(List)

# Creating a List of numbers

List = [10, 20, 14]

print("\nList of numbers: ")

print(List)

# Creating a List of strings and accessing

# using index

List = ["Geeks", "For", "Geeks"]

print("\nList Items: ")

print(List[0])

print(List[2])

Summary:

A list is an ordered, mutable collection of items in Python.
Lists can contain items of different types.
You can access, modify, and manipulate lists using various methods like append(), insert(), remove(), pop(), and others.
Lists support indexing, slicing, and can be iterated over with loops.