List Manipulation in Python | Computer Science for Class 11 - Humanities/Arts PDF Download

Introduction to List

In Python, Multiple values (example, Number, Character, Date etc.) can be stored in a single variable by using lists., a list is an ordered sequence of elements that can be changed or modified. A list’s items are any elements or values that are contained within it. Lists are defined by having values inside square brackets [] just as strings are defined by characters inside quotations.

Example 1 – 
>>> list1 = [2,4,6,8,10,12]
>>> print(list1)
[2, 4, 6, 8, 10, 12]

Example 2 – 
>>> list2 = [‘a’,’e’,’i’,’o’,’u’]
>>> print(list2)
[‘a’, ‘e’, ‘i’, ‘o’, ‘u’]

Example 3 – 
>>> list3 = [100,23.5,’Hello’]
>>> print(list3)
[100, 23.5, ‘Hello’]

Accessing Elements in a List (Display Element from the List)

The elements of a list are accessed in the same way as characters are accessed in a string.

Example –
>>> list1 = [2,4,6,8,10,12]
>>> list1[0]
2
>>> list1[3]
8
>>> list1[15]
IndexError: list index out of range

Lists are Mutable
In Python, lists are mutable. It means that the contents of the list can be changed after it has been created.

Example – 
>>> list1 = [‘Red’,’Green’,’Blue’,’Orange’]
>>> list1[3] = ‘Black’
>>> list1
[‘Red’, ‘Green’, ‘Blue’, ‘Black’]

List Operations

• The data type list allows manipulation of its contents through various operations as shown below.

Concatenation

• Python allows us to join two or more lists using concatenation operator depicted by the symbol +.

Example 1 –
>>> list1 = [1,3,5,7,9]
>>> list2 = [2,4,6,8,10]
>>> list1 + list2
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]

Example 2 –
>>> list3 = [‘Red’,’Green’,’Blue’]
>>> list4 = [‘Cyan’, ‘Magenta’, ‘Yellow’ ,’Black’]
>>> list3 + list4
[‘Red’,’Green’,’Blue’,’Cyan’,’Magenta’, ‘Yellow’,’Black’]

Repetition
Python allows us to replicate a list using repetition operator depicted by symbol *.
>>> list1 = [‘Hello’]
>>> list1 * 4
[‘Hello’, ‘Hello’, ‘Hello’, ‘Hello’]

Membership
Like strings, the membership operators in checks if the element is present in the list and returns True, else returns False.
>>> list1 = [‘Red’,’Green’,’Blue’]
>>> ‘Green’ in list1
True
>>> ‘Cyan’ in list1
False

Slicing
Like strings, the slicing operation can also be applied to lists.

Example 1 –
>>> list1 =[‘Red’,’Green’,’Blue’,’Cyan’, ‘Magenta’,’Yellow’,’Black’]
>>> list1[2:6]
[‘Blue’, ‘Cyan’, ‘Magenta’, ‘Yellow’]

Example 2 –
>>> list1[2:20] #second index is out of range
[‘Blue’, ‘Cyan’, ‘Magenta’, ‘Yellow’, ‘Black’]

Traversing a List

We can access each element of the list or traverse a list using a for loop or a while loop.

List Traversal Using for Loop –
Example –
>>> list1 = [‘Red’,’Green’,’Blue’,’Yellow’, ‘Black’]
>>> for item in list1:
print(item)

Output:

• Red
• Green
• Blue
• Yellow
• Black

List Traversal Using while Loop –

Example –
>>> list1 = [‘Red’,’Green’,’Blue’,’Yellow’, ‘Black’]
>>> i = 0
>>> while i < len(list1):
print(list1[i])
i += 1

Output:

• Red
• Green
• Blue
• Yellow
• Black

List Methods and Built-in Functions

• The data type list has several built-in methods that are useful in programming.

len()

• Returns the length of the list passed as the argument
  >>> list1 = [10,20,30,40,50]
  >>> len(list1)
  5

list()

• Creates an empty list if no argument is passed Creates a list if a sequence is passed as an argument

Example –
>>> list1 = list()
>>> list1
[ ]
>>> str1 = ‘aeiou’
>>> list1 = list(str1)
>>> list1
[‘a’, ‘e’, ‘i’, ‘o’, ‘u’]

append()

• Appends a single element passed as an argument at the end of the list. The single element can also be a list.

Example –
>>> list1 = [10,20,30,40]
>>> list1.append(50)
>>> list1
[10, 20, 30, 40, 50]
>>> list1 = [10,20,30,40]
>>> list1.append([50,60])
>>> list1
[10, 20, 30, 40, [50, 60]]

extend()

• Appends each element of the list passed as argument to the end of the given list.

Example –
>>> list1 = [10,20,30]
>>> list2 = [40,50]
>>> list1.extend(list2)
>>> list1
[10, 20, 30, 40, 50]

insert()

• Inserts an element at a particular index in the list

Example –
>>> list1 = [10,20,30,40,50]
>>> list1.insert(2,25)
>>> list1
[10, 20, 25, 30, 40, 50]
>>> list1.insert(0,5)
>>> list1
[5, 10, 20, 25, 30, 40, 50]

count()

• Returns the number of times a given element appears in the list

Example –
>>> list1 = [10,20,30,10,40,10]
>>> list1.count(10)
3
>>> list1.count(90)
0

index()

• Returns index of the first occurrence of the element in the list. If the element is not present, ValueError is generated.

Example –
>>> list1 = [10,20,30,20,40,10]
>>> list1.index(20)
1
>>> list1.index(90)
ValueError: 90 is not in list

remove()

• Removes the given element from the list. If the element is present multiple times, only the first occurrence is removed. If the element is not present, then ValueError is generated.

Example –
>>> list1 = [10,20,30,40,50,30]
>>> list1.remove(30)
>>> list1
[10, 20, 40, 50, 30]
>>> list1.remove(90)
ValueError:list.remove(x):x not in list

pop()

• Returns the element whose index is passed as parameter to this function and also removes it from the list. If no parameter is given, then it returns and removes the last element of the list.

Example –
>>> list1 = [10,20,30,40,50,60]
>>> list1.pop(3)
40
>>> list1
[10, 20, 30, 50, 60]
>>> list1 = [10,20,30,40,50,60]
>>> list1.pop()
60
>>> list1
[10, 20, 30, 40, 50]

reverse()

• Reverses the order of elements in the given list

Example –
>>> list1 = [34,66,12,89,28,99]
>>> list1.reverse()
>>> list1
[ 99, 28, 89, 12, 66, 34]
>>> list1 = [ ‘Tiger’ ,’Zebra’ , ‘Lion’ , ‘Cat’ ,’Elephant’ ,’Dog’]
>>> list1.reverse()
>>> list1
[‘Dog’, ‘Elephant’, ‘Cat’, ‘Lion’, ‘Zebra’, ‘Tiger’]

sort()

• Sorts the elements of the given list in-place

Example –
>>>list1 = [‘Tiger’,’Zebra’,’Lion’, ‘Cat’, ‘Elephant’ ,’Dog’]
>>> list1.sort()
>>> list1
[‘Cat’, ‘Dog’, ‘Elephant’, ‘Lion’, ‘Tiger’, ‘Zebra’]
>>> list1 = [34,66,12,89,28,99]
>>> list1.sort(reverse = True)
>>> list1
[99,89,66,34,28,12]

sorted()

• It takes a list as parameter and creates a new list consisting of the same elements arranged in sorted order.

Example –
>>> list1 = [23,45,11,67,85,56]
>>> list2 = sorted(list1)
>>> list1
[23, 45, 11, 67, 85, 56]
>>> list2
[11, 23, 45, 56, 67, 85]

min()

• Returns minimum or smallest element of the list

Example –
>>> list1 = [34,12,63,39,92,44]
>>> min(list1)
12

max()

• Returns maximum or largest element of the list

Example –
>>> list1 = [34,12,63,39,92,44]
>>> max(list1)
92

sum()

• Returns sum of the elements of the list

Example –
>>> list1 = [34,12,63,39,92,44]
>>> sum(list1)
284

Nested Lists

• When a list appears as an element of another list, it is called a nested list.

Example –
>>> list1 = [1,2,’a’,’c’,[6,7,8],4,9]
>>> list1[4]
[6, 7, 8]

Copying Lists

• The simplest way to make a copy of the list is to assign it to another list.

Example –
>>> list1 = [1,2,3]
>>> list2 = list1
>>> list1
[1, 2, 3]
>>> list2
[1, 2, 3]

List as Argument to a Function
Whenever a list is passed as an argument to a function, we have to consider two scenarios:
(A) A modification to the list in the function will be mirrored back in the calling function, which allows for changes to the original list’s elements.
Q. Program to increment the elements of a list. The list is passed as an argument to a function.
#Program
#Function to increment the elements of the list passed as argument
def increment(list2):
for i in range(0,len(list2)):
list2[i] += 5
print(‘Reference of list Inside Function’,id(list2))
list1 = [10,20,30,40,50]
print(“Reference of list in Main”,id(list1))
print(“The list before the function call”)
print(list1)
increment(list1)
print(“The list after the function call”)
print(list1)

Output:
Reference of list in Main 70615968
The list before the function call
[10, 20, 30, 40, 50]
Reference of list Inside Function 70615968
The list after the function call
[15, 25, 35, 45, 55]

(B) If the list is given a new value inside the function, a new list object is generated and it becomes the local copy of the function. Any updates made inside the local copy of the function are not updated in the calling function.

Q. Program to increment the elements of the list passed as parameter.
#Program
#Function to increment the elements of the list passed as argument
def increment(list2):
print(“\nID of list inside function before assignment:”, id(list2))
list2 = [15,25,35,45,55]
print(“ID of list changes inside function after assignment:”, id(list2))
print(“The list inside the function after assignment is:”)
print(list2)
list1 = [10,20,30,40,50]
print(“ID of list before function call:”,id(list1))
print(“The list before function call:”)
print(list1)
increment(list1) #list1 passed as parameter to function
print(‘\nID of list after function call:’,id(list1))
print(“The list after the function call:”)
print(list1)

Output:
ID of list before function call: 65565640
The list before function call:
[10, 20, 30, 40, 50]
ID of list inside function before assignment:65565640
ID of list changes inside function after assignment:65565600
The list inside the function after assignment is:
[15, 25, 35, 45, 55]
ID of list after function call: 65565640
The list after the function call:
[10, 20, 30, 40, 50]