Unit 4 python -list methods

LIST METHODS
Prepared by
V.Kumararaja, AP/IT
S.Thanga Prasad, AP/CSE
Er.Perumal Manimekalai College of Engineering, Hosur
1

LIST CREATION
A list is created by placing all the items
(elements) inside a square bracket [ ],
separated by commas.
It can have any number of items and they
may be of different types (integer, float,
string etc.).
Example:
list = [1, 2, 3] # list with same data-types
list = [1, "Hello", 3.4] # list with mixed data-types 2

ACCESS ITEMS FROM A LIST
 It can be access in several ways
 Use the index operator [] to access an item
in a list. Index starts from 0. So, a list having
5 elements will have index from 0 to 4.
 Example:
list = ['p','r','o','b','e']
print(list[2]) #Positive Indexing
print(list[-2]) #Negative Indexing
Output
O
b
3

SLICE LISTS
 Accessing a range of items in a list by using
the slicing operator [ ] using (colon :).
 Slicing can be best visualized by considering
the index to be between the elements.
 Example:
list = ['p','r','o','b','e']
print(list[0:4]) #Positive
print(list[-2:-1]) # Negative
Output
['p', 'r', 'o', 'b']
['b']
4

LIST METHODS
 append() - Add an element to the end of the list.
 count() - Returns the count of number of items passed as
an argument.
 extend() - Add all elements of a list to the another list.
 index() - Returns the index of the first matched item.
 insert() - Insert an item at the defined index.
 pop() - Removes and returns an element at the given
index.
 copy() - Returns a shallow copy of the list
 remove() - Removes an item from the list.
 reverse() - Reverse the order of items in the list.
 sort() - Sort items in a list in ascending order.
5

APPEND() METHODS
The append() method adds a single item
to the existing list.
The append() method only modifies the
original list. It doesn't return any value.
The append() method takes a single
item and adds it to the end of the list.
The item can be numbers, strings,
another list, dictionary etc.
Syntax
list.append(item)
6

Example 1: Adding Element to a List
list = ['hi', 'hello']
print('old list: ', list)
list.append('welcome!')
print('Updated list: ', list)
Example 2: Adding List to a List
list = ['hi', 'hello']
print('old list: ', list)
list1=['welcome‘]
list.append(list1)
print('Updated list: ', list)
APPEND() METHODS - PROGRAMS
Output
old list: ['hi', 'hello']
Updated list: ['hi', 'hello‘,'welcome!']
Output
old list: ['hi', 'hello']
Updated list: ['hi', 'hello', ['welcome']]
7

COUNT() METHODS
 Count() method counts how many times an
element has occurred in a list and returns it.
 The count() method takes a single argument:
 element - element whose count is to be
found.
 The count() method returns the number of
occurrences of an element in a list.
 Syntax
list.count(element)
8

Example: Count the occurrence of an element in the list
list = ['h','i','h','o','w','y','o','u','!','!']
print('The count of i is:',list.count('i'))
print('The count of o is:',list.count('o'))
Example 2: Count the occurrence of tuple and list inside the list
list = ['a', ('h', 'i'), ('a', 'b'), [8, 4]]
count = list.count(('a', 'b'))
print("The count of ('a', 'b') is:", count)
count = list.count([3, 4])
print("The count of [3, 4] is:", count)
COUNT() METHODS - PROGRAMS
Output
The count of i is: 1
The count of o is: 2
Output
The count of ('a', 'b') is: 1
The count of [3, 4] is: 0
9

 The extend() extends the list by adding all items of a list
to the end.
 Extend() method takes a single argument (a list) and
adds it to the end.
 This method also add elements of other native
data_types ,like tuple and set to the list,
 Syntax
EXTEND() METHODS
list1.extend(list2)
Note : The elements
of list2 are added to
the end of list1.
list.extend(list(tuple_type)) Note : The elements
of tuple are added to
the end of list1.
10

EXTEND() METHODS - PROGRAMS
Example 1: Using extend() Method
language = ['C', 'C++', 'Python']
language1 = ['JAVA', 'COBOL']
language.extend(language1)
print('Language List: ', language)
Output
Language List: ['C', 'C++', 'Python', 'JAVA', 'COBOL']
11

Example 2: Add Elements of Tuple and Set to List
language = ['C', 'C++', 'Python']
language_tuple = ['JAVA', 'COBOL']
language_set = {'.Net', 'C##'}
language.extend(language_tuple)
print('New Language List: ', language)
language.extend(language_set)
print('Newest Language List: ', language)
EXTEND() METHODS - PROGRAMS
Output
New Language List: ['C', 'C++', 'Python', 'JAVA', 'COBOL'] Newest
Language List: ['C', 'C++', 'Python', 'JAVA', 'COBOL',
'.Net', 'C##']
12

INDEX( ) METHODS
 Index() method search and find given element in
the list and returns its position.
 However, if the same element is present more than
once, index() method returns its smallest/first
position.
 Index in Python starts from 0 not 1.
 The index() method returns the index of the
element in the list.
 The index method takes a single argument:
 element - element that is to be searched.
 Syntax
list.index(element)
13

INDEX( ) METHODS - PROGRAMS
Example 1: Find position of element present in the
list and not present in the list
list = ['h','i','h','o','w','y','o','u','!','!']
print('The count of i is:',list.index('i'))
print('The count of o is:',list.index('o'))
print('The count of o is:',list.index('z'))
Output
The count of i is: 1
The count of o is: 3
ValueError: 'z' is not in list
14

Example 2: Find index of tuple and list inside a list
random = ['a', ('a', 'd'),('a','b'), [3, 4]]
index = random.index(('a', 'b'))
print("The index of ('a', 'b'):", index)
index = random.index([3, 4])
print("The index of [3, 4]:", index)
INDEX( ) METHODS - PROGRAMS
Output
The index of ('a', 'b'): 2
The index of [3, 4]: 3
15

 The insert() method inserts the element to the list at
the given index.
 The insert() function takes two parameters:
 index - position where element needs to be
inserted
 element - this is the element to be inserted in
the list
 The insert() method only inserts the element to the list.
It doesn't return any value.
 Syntax
INSERT( ) METHODS
list.insert(index, element) 16

Example 1: Inserting Element to List
vowels=['a','i','o','u']
print("old list =",vowels)
vowels.insert(1,'e')
print("new list =",vowels)
INSERT( ) METHODS - PROGRAMS
Output
old list = ['a', 'i', 'o', 'u']
new list = ['a', 'e', 'i', 'o', 'u']
17

Example 2: Inserting a Tuple (as an Element) to the List
list = [{1, 2}, [5, 6, 7]]
print('old List: ', list)
number_tuple = (3, 4)
list.insert(1, number_tuple)
print('Updated List: ', list)
INSERT( ) METHODS - PROGRAMS
Output
old List: [{1, 2}, [5, 6, 7]]
Updated List: [{1, 2}, (3, 4), [5, 6, 7]]
18

 The pop() method takes a single argument (index) and
removes the element present at that index from the
list.
 If the index passed to the pop() method is not in the
range, it throws IndexError: pop index out of
range exception.
 The parameter passed to the pop() method is optional.
If no parameter is passed, the default index -1 is
passed as an argument.
 Also, the pop() method removes and returns the
element at the given index and updates the list.
 Syntax
POP( ) METHODS
list.pop(index) 19

Example 1: Pop Element at the Given Index from the List
list = ['Python', 'Java', 'C++', 'French', 'C‘]
return_value = language.pop(3)
print('Return Value: ', return_value)
print('Updated List: ', language)
POP( ) METHODS - PROGRAMS
Output
old list = ['Python', 'Java', 'C++', 'French', 'C']
Return Value: French
Updated List: ['Python', 'Java', 'C++', 'C']
20

Example 2: pop() when index is not passed
language = ['Python', 'Java', 'C++‘, 'C']
print('Return Value: ', language.pop())
print('Return Value: ', language.pop(-1))
POP( ) METHODS - PROGRAMS
Output
Return Value: C
Updated List: ['Python', 'Java', 'C++']
Return Value: C++ Updated List:
['Python', 'Java']
21

 The copy() method returns a shallow copy of the list.
 A list can be copied with = operator.
old_list = [1, 2, 3]
new_list = old_list
 The problem with copying the list in this way is that if
you modify the new_list, the old_list is also modified.
 Syntax
COPY( ) / ALIASING METHODS
new_list = old_list 22

Example 1: Copying a List
old_list = [1, 2, 3]
new_list = old_list
new_list.append('a')
print('New List:', new_list )
print('Old List:', old_list )
COPY( ) / ALIASING METHODS - PROGRAMS
Output
New List: [1, 2, 3, 'a']
Old List: [1, 2, 3, 'a']
23

 We need the original list unchanged when the new list
is modified, you can use copy() method.
 This is called shallow copy.
 The copy() method doesn't take any parameters.
 The copy() function returns a list. It doesn't modify the
original list.
 Syntax
SHALLOW COPY( ) / CLONING METHODS
new_list = list.copy()
24

Example 2: Shallow Copy of a List Using Slicing
list = [1,2,4]
new_list = list[:]
new_list.append(5)
print('Old List: ', list)
print('New List: ', new_list)
SHALLOW COPY( ) / CLONING METHODS - PROGRAMS
Output
Old List: [1, 2, 4]
New List: [1, 2, 4, 5]
25

 The remove() method searches for the given element
in the list and removes the first matching element.
 The remove() method takes a single element as an
argument and removes it from the list.
 If the element(argument) passed to the remove()
method doesn't exist, valueError exception is thrown.
 Syntax
list.remove(element)
REMOVE( ) METHODS
26

Example 1 :Remove Element From The List
list = [5,78,12,26,50]
print('Original list: ', list)
list.remove(12)
print('Updated list elements: ', list)
REMOVE( ) METHODS - PROGRAMS
Output
Original list: [5, 78, 12, 26,
50]Updated list: [5, 78, 26, 50]
27

 The reverse() method reverses the elements of a
given list.
 The reverse() function doesn't return any value. It
only reverses the elements and updates the list.
 Syntax
REVERSE( ) METHODS
list.reverse()
28

REVERSE( ) METHODS - PROGRAM
Example 1 :Reverse a List Using Slicing Operator
list = [1,5,8,6,11,55]
print('Original List:', list)
reversed_list = list[::-1]
print(‘Reversed List:', reversed_list)
Example 2: Reverse a List
list = [1,5,8,6,11,55]
list.reverse()
print('Reversed List:', list)
Output
Original List: [1, 5, 8, 6, 11, 55]
Reversed List: [55, 11, 6, 8, 5, 1]
Output
Original List: [1, 5, 8, 6, 11, 55]
Reversed List: [55, 11, 6, 8, 5, 1]
29

 The sort() method sorts the elements of a given list.
 The sort() method sorts the elements of a given list
in a specific order - Ascending or Descending.
 Syntax
SORT( ) METHODS
list.sort()
30

REVERSE( ) METHODS - PROGRAM
Example 1 :Sort a given List in ascending order
list = [1,15,88,6,51,55]
list.sort()
print('sorted List:', list)
Example 2 : Sort the list in Descending order
list = [1,15,88,6,51,55]
list.sort(reverse=True)
print('sorted List:', list)
Output
Original List: [1, 15, 88, 6, 51, 55]
sorted List: [1, 6, 15, 51, 55, 88]
Output
Original List: [1, 15, 88, 6, 51, 55]
sorted List: [88, 55, 51, 15, 6, 1] 31

PYTHON DICTIONARY
 Python dictionary is an unordered collection of items.
While other compound data types have only value as
an element, a dictionary has a key: value pair.
Creating a dictionary
 Creating a dictionary is as simple as placing items
inside curly braces {} separated by comma.
 An item has a key and the corresponding value
expressed as a pair, key: value.
 While values can be of any data type and can repeat,
keys must be of immutable type (string, number or tuple
with immutable elements) and must be unique.
32

ACCESS ELEMENTS FROM A DICTIONARY
 While indexing is used with other container types to access
values, dictionary uses keys. Key can be used either inside
square brackets or with the get() method.
d = {'name':'Ram', 'age': 26}
print("name is :",d['name'])
d['age']=33
d['Initial']='S‘
print("Updated dict :",d)
del d['Initial']
print("Updated dict :",d)
Output
name is : Ram
Updated dict : {'name': 'Ram', 'age': 33, 'Initial': 'S'}
Updated dict : {'name': 'Ram', 'age': 33}
33

Unit 4 python -list methods

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Unit 4 python -list methods