����Python Programming �

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UNIT – II

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Chapter-8

Python Programming - Lists, Tuples and Dictionaries

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Data structures-Sequence

• Data structure:
• A Data structure is a group of elements that are put together under one name.
• It defines a particular way of storing and organizing data in a computer so that it can be used efficiently
• Sequence:
• It is most basic data structure in Python
• In sequence, each element has a specific index
• Index starts with 0 and it is automatically incremented for the next element
• Example: string(It is a sequence of characters), List, Tuple, Dictionary
• Python has built-in functions to manipulate the elements
• Operations: slicing, indexing, adding, multiplying and checking for membership

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Lists

• It is sequence data structure. Like a string, List is a sequence of values. In a string, the values are characters;
• In a list, they can be any type. The values in list are called elements or sometimes items.
• There are several ways to create a new list; the simplest is to enclose the elements in square brackets ([ and ]):

[10, 20, 30, 40]

['Tamilnadu', 'Karnataka', 'Kerala', ‘Andhra’]

• The first example is a list of four integers.
• The second is a list of three strings.

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Mar 17, 2022

Lists

• It is sequence data structure
• Elements are written as a list of comma separated values between square brackets ([ ])
• Elements may be different data types
• List is mutable – means elements can be changed

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• Example

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Lists

• Lists are just like dynamically sized arrays, declared in other languages (vector in C++ and ArrayList in Java)
• A single list may contain DataTypes like Integers, Strings, as well as Objects.
• Lists are mutable, and hence, they can be altered even after their creation. List us mutable – Unlike strings, lists are mutable because you can change the order of items in a list or reassign an item in a list.
• List in Python are ordered and have a definite count.
• 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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Creation of List�

# Creating a List

List = [ ]

print("Blank List: ")

print(List)

Output: Blank List: [ ]

# Creating a List of numbers

List = [10, 20, 14]

print("\nList of numbers: ")

print(List)

  Output : List of numbers: [10, 20, 14]

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List

# Creating a List of strings and accessing using index

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

print("\nList Items: ")

print(List[0])

print(List[2])

Output : List Items Geeks Geeks

# Creating a Multi-Dimensional List

# (By Nesting a list inside a List)

List = [['Geeks', 'For'], ['Geeks']]

print("\nMulti-Dimensional List: ")

print(List)

Multi-Dimensional List:

Output : [ ['Geeks', 'For'], ['Geeks'] ]

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# Creating a List with the use of Numbers

# (Having duplicate values)

List = [1, 2, 4, 4, 3, 3, 3, 6, 5]

print("\nList with the use of Numbers: ")

print(List)

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Output : List with the use of Numbers:

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

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# Creating a List with mixed type of values

# (Having numbers and strings)

List = [1, 2, 'Geeks', 4, 'For', 6, 'Geeks']

print("\nList with the use of Mixed Values: ")

print(List)

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Output :

List with the use of Mixed Values:

[1, 2, 'Geeks', 4, 'For', 6, 'Geeks']

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Knowing the size of List�

# Creating a List

List1 = [ ]

print(len(List1))

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Output : 0

# Creating a List of numbers

List2 = [10, 20, 14]

print(len(List2))

Output : 3

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llist_1 = ['a','b','c','d','e','f']

print(list_1[1:3])

print(list_1[:4])

print(list_1[3:])

Output:

['b', 'c']

['a', 'b', 'c', 'd']

['d', 'e', 'f']

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Access values in List

• Lists can be sliced and concatenated
• Syntax of Slice operation:

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Updating Value in a List

• Append new values in the list with append()

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Updating Value in a List

• Remove existing value from the list with del

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Updating Value in a List

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Nested List

• List within another list

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Cloning List

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== and is in list

list1=[1,2,3]

list2=[1,2,3]

#shallow copy

list3=list1

print("list1==list2", list1==list2)

print("list3==list2", list3==list2)

print("list1 is list2", list1 is list2)

print("list3  is list2", list3 is list2)

print("list3  is list1", list3 is list1)

list3.append(4)

print(list1, list3)

# copy

list4=list1.copy()

print("list4  is list1", list4 is list1)

print("list4 == list1", list4 == list1)

list4.append(5)

print(list1, list4)

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List methods

• Method Description append()Method Description append() Adds an element at the end of the list clear() Removes all the elements from the list

copy() Returns a copy of the list

count() Returns the number of elements with the specified

value

extend() Add the elements of a list (or any iterable), to the end of

the current list

index() Returns the index of the first element with the specified

value

insert() Adds an element at the specified position

pop() Removes the element at the specified position

remove() Removes the first item with the specified value

reverse() Reverses the order of the list

sort() Sorts the list

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Basic List Operations

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List operations

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List Methods

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List Methods

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Example

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Example

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Using List as Stack

• It is a Last-In-First-Out data structure

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• Use of Stack: Function call

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Demonstration of Stack Operation

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Using List as Queue

• It is the First-in-First-Out data structure (FIFO)
• Real time Scenario

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• Applications of Queue in Computer Systems:

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Demonstration of Queue Operation

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Example

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Looping in List

• List elements can be accessed by using,

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• Example

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Looping in List-enumerate() function

• Used to get index as well as item from the list

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List- using the range() function

• used to print index of the items

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List – using iterator (iter() function)

• An iterator is an object that contains a countable number of values.
• Used to loop over the elements of the list.
• Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().

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����Python Programming �

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​

​

​

​

​

UNIT – II

​

Chapter-8

Python Programming - Lists, Tuples and Dictionaries

​

​

Data structures-Sequence

• Data structure:
• A Data structure is a group of elements that are put together under one name.
• It defines a particular way of storing and organizing data in a computer so that it can be used efficiently
• Sequence:
• It is most basic data structure in Python
• In sequence, each element has a specific index
• Index starts with 0 and it is automatically incremented for the next element
• Example: string(It is a sequence of characters), List, Tuple, Dictionary
• Python has built-in functions to manipulate the elements
• Operations: slicing, indexing, adding, multiplying and checking for membership

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Lists

• It is sequence data structure. Like a string, List is a sequence of values. In a string, the values are characters;
• In a list, they can be any type. The values in list are called elements or sometimes items.
• There are several ways to create a new list; the simplest is to enclose the elements in square brackets ([ and ]):

[10, 20, 30, 40]

['Tamilnadu', 'Karnataka', 'Kerala', ‘Andhra’]

• The first example is a list of four integers.
• The second is a list of three strings.

​

Mar 17, 2022

Lists

• It is sequence data structure
• Elements are written as a list of comma separated values between square brackets ([ ])
• Elements may be different data types
• List is mutable – means elements can be changed

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​

• Example

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*

Lists

• Lists are just like dynamically sized arrays, declared in other languages (vector in C++ and ArrayList in Java)
• A single list may contain DataTypes like Integers, Strings, as well as Objects.
• Lists are mutable, and hence, they can be altered even after their creation. List us mutable – Unlike strings, lists are mutable because you can change the order of items in a list or reassign an item in a list.
• List in Python are ordered and have a definite count.
• 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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Creation of List�

# Creating a List

List = [ ]

print("Blank List: ")

print(List)

Output: Blank List: [ ]

# Creating a List of numbers

List = [10, 20, 14]

print("\nList of numbers: ")

print(List)

  Output : List of numbers: [10, 20, 14]

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List

# Creating a List of strings and accessing using index

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

print("\nList Items: ")

print(List[0])

print(List[2])

Output : List Items Geeks Geeks

# Creating a Multi-Dimensional List

# (By Nesting a list inside a List)

List = [['Geeks', 'For'], ['Geeks']]

print("\nMulti-Dimensional List: ")

print(List)

Multi-Dimensional List:

Output : [ ['Geeks', 'For'], ['Geeks'] ]

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# Creating a List with the use of Numbers

# (Having duplicate values)

List = [1, 2, 4, 4, 3, 3, 3, 6, 5]

print("\nList with the use of Numbers: ")

print(List)

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Output : List with the use of Numbers:

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

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# Creating a List with mixed type of values

# (Having numbers and strings)

List = [1, 2, 'Geeks', 4, 'For', 6, 'Geeks']

print("\nList with the use of Mixed Values: ")

print(List)

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Output :

List with the use of Mixed Values:

[1, 2, 'Geeks', 4, 'For', 6, 'Geeks']

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Knowing the size of List�

# Creating a List

List1 = [ ]

print(len(List1))

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Output : 0

# Creating a List of numbers

List2 = [10, 20, 14]

print(len(List2))

Output : 3

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llist_1 = ['a','b','c','d','e','f']

print(list_1[1:3])

print(list_1[:4])

print(list_1[3:])

Output:

['b', 'c']

['a', 'b', 'c', 'd']

['d', 'e', 'f']

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Access values in List

• Lists can be sliced and concatenated
• Syntax of Slice operation:

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Updating Value in a List

• Append new values in the list with append()

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Updating Value in a List

• Remove existing value from the list with del

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Updating Value in a List

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Nested List

• List within another list

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Cloning List

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== and is in list

list1=[1,2,3]

list2=[1,2,3]

#shallow copy

list3=list1

print("list1==list2", list1==list2)

print("list3==list2", list3==list2)

print("list1 is list2", list1 is list2)

print("list3  is list2", list3 is list2)

print("list3  is list1", list3 is list1)

list3.append(4)

print(list1, list3)

# copy

list4=list1.copy()

print("list4  is list1", list4 is list1)

print("list4 == list1", list4 == list1)

list4.append(5)

print(list1, list4)

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List methods

• Method Description append()Method Description append() Adds an element at the end of the list clear() Removes all the elements from the list

copy() Returns a copy of the list

count() Returns the number of elements with the specified

value

extend() Add the elements of a list (or any iterable), to the end of

the current list

index() Returns the index of the first element with the specified

value

insert() Adds an element at the specified position

pop() Removes the element at the specified position

remove() Removes the first item with the specified value

reverse() Reverses the order of the list

sort() Sorts the list

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Basic List Operations

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List operations

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List Methods

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List Methods

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Example

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Example

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Using List as Stack

• It is a Last-In-First-Out data structure

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• Use of Stack: Function call

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Demonstration of Stack Operation

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Using List as Queue

• It is the First-in-First-Out data structure (FIFO)
• Real time Scenario

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• Applications of Queue in Computer Systems:

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Demonstration of Queue Operation

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Example

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Looping in List

• List elements can be accessed by using,

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• Example

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Looping in List-enumerate() function

• Used to get index as well as item from the list

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List- using the range() function

• used to print index of the items

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List – using iterator (iter() function

• An iterator is an object that contains a countable number of values.
• Used to loop over the elements of the list.
• Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().

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TUPLES

• Sequence of immutable objects separated by comma.
• Defined within parenthesis ()
• Creating a tuple:

tup=(val1, val2, …..valn)

• Val1, val2 …. valn may be same or different datatypes
• Any set of multiple comma-separated values without [ ] or { } or ( ) are treated as tuples by default.

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TUPLES-Example

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TUPLES-Example

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Utility of TUPLES

• Tuples are useful for representing records/structures as in other programming languages.
• Some built-in functions return tuples.

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Accessing Values in a Tuple

• Using index each element can be accessed.
• Index starts with 0.
• Operations – slice, concatenate

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Extracting Values from a Tuple

• Updation and deletion is not possible in Tuple since it is immutable
• Extracting the values from one tuple to another tuple is possible

Sorted sorted((9,4,7,1)) (1,4,7,9)

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Zip zip((1,2),[3,4]) [(1, 3), (2, 4)]

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Tuple methods

• Only two methods
• count() and index()

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Find the index of element in Tuples

• To find the index of an element in the tuple, index() method is used.
• Syntax: tup.index(obj)
• If the object is not present, error is generated.

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Counting the Elements in Tuples

• To find the number of specific elements
• Syntax: tup.count(obj)

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Tuple Assignment

* Number of values in the right hand side should be same as number of variables in the left hand side

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Tuples for Returning Multiple values

• If we need to return more than one values from function we can use tuples.

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Nested Tuples

• We can define a tuple in another tuple called nested tuple.
• We can also specify list within tuple.

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Dictionary

• Dictionary is a data structure in which we store values as a pair of key and value.
• Each key is separated by a colon (:) and consecutive items are separated by commas.
• All the items are enclosed in {}
• The syntax is :

dictionary_name={ key1:values, key2:value2…., keyn:valuen}

• We can also write as
• dictionary_name={ key1:values,

key2:value2

….,

keyn:valuen}

• Keys in a dictionary must be unique and be of any immutable objects (like int, string, tuple )
• Dictionaries are not sequences, but they are mappings. Mappings are collections of objects that store objects by key instead of by position (index)

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Creating a dictionary

• Creating an empty dictionary

Dictionary_name={}

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Example 1:

mydict = {}

print("Empty Dictionary: ")

print(mydict)

#This code creates an empty dictionary.

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• To create a dictionary with items

Example 2:

mydict = {1: ‘Mango’, 2: ‘Guava‘, 3:’Strawberry’}

#creates a dictionary with integer keys

print(mydict)

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Creating a dictionary

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Example 3:

mydict = {‘xxx’: 123, ‘yyy’: 456, ‘zzz’ : 789}

print(mydict)

# creates a dictionary with string keys

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Example 4:

# creates dictionary with mixed keys

mydict = {‘name’: ‘xxx’, 1: ‘yyy’}

print(mydict)

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Example 5:

# creates dictionary with mixed keys and values, that is with string and list

mydict = {‘name’: ‘xxx’, 1:[1,2,3,4]}

print(mydict)

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Using dict()

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# Creating a Dictionary using dict()

mydict = dict({1: 'apple', 2: 'grapes', 3:'mangoes'})

print("\nDictionary using dict(): ")

print(mydict)

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# Creating a Dictionary with each item as a Pair

mydict = dict([(1, 'apple'), (2, 'grapes'),(3, 'mangoes')])

print("\nDictionary with each item as a pair: ")

print(mydict)

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Dictionary comprehension

• Another way of creating a dictionary

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• Example

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Dictionary comprehension

• Another way of creating a dictionary

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• Example

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Dictionary

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Dictionary

• Data structure
• Store values as a pair of key and value
• Each key is separated from its value by a colon(:)
• Consecutive items are separated by comma
• Entire item in dictionary is enclosed in curly brackets ({})
• Syntax

dictionary_name={

key_1:value_1,

key_2:value_2,

key_3:value_3

}

• keys in the dictionary must be of unique and be of any immutable data type (strings, numbers, or tuples)
• values need not be unique and can be of any data type
• keys are case-sensitive

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Dictionary

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dict ( ) function – used to create dictionary

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>>> print(dict([('aaa',1),('bbb',6.6),('ccc','rrr')]))

{'aaa': 1, 'bbb': 6.6, 'ccc': 'rrr'}

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>>> print(dict((('aaa',1),('bbb',6.6),('ccc','rrr'))))

{'aaa': 1, 'bbb': 6.6, 'ccc': 'rrr'}

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>>> x=(dict([('aaa',1),('bbb',6.6),('ccc','rrr')]))

>>> x

{'aaa': 1, 'bbb': 6.6, 'ccc': 'rrr'}

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Dictionary

• Dictionary comprehension–create dictionary based on existing dictionary
• Syntax

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• expression generates elements of dictionary from items in the sequence that satisfy the condition

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Accessing Values

• square brackets are used along with the key to obtain its value

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• a KeyError is generated, if we try to access an item with a key, which is not specified in the dictionary

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Adding an item in a dictionary

• syntax

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Modifying an item in a dictionary

• just overwrite the existing value

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Deleting items in a dictionary

• del – one or more items
• clear()- delete or remove all the items
• To remove an entire dictionary form the memory

del dictionary_name

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Deleting items in a dictionary

• pop() method – removes an item from the dictionary and returns its value
• if specified key is not present, default value is returned
• if specified key is not present, and if the default value is not specified, then it returns the KeyError

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• value can be of any type, mutable or immutable

>>> dict={1:['aaa']}

>>> print(dict)

{1: ['aaa']}

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• key can be only immutable, it cannot be mutable,
• so, key cannot be a List, since list is mutable

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>>> dict={['aaa']:1}

Traceback (most recent call last):

File "<pyshell#10>", line 1, in <module>

dict={['aaa']:1}

TypeError: unhashable type: 'list'

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• if key contains any mutable object directly or indirectly in a tuple, then TypeError is generated

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Program to check a single key in a dictionary

• in keyword is used

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x={'aaa': 1, 'bbb': 6.6, 'ccc': 'rrr'}

if 'eee' in x:

print('eee exists')

else:

print('eee not exists in dictionary')

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Output:

eee not exists in dictionary

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Sorting items in a dictionary

• keys() method-returns a list of all the keys in the dictionary in an arbitrary order

>>> x={'fff': 1, 'bbb': 6.6, 'zzz': 'rrr'}

>>> print(x.keys())

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dict_keys(['fff', 'bbb', 'zzz'])

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• sorted() function used to sort the keys

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>>> print(sorted(x.keys()))

['bbb', 'fff', 'zzz']

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Looping over a dictionary

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Nested dictionary

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Built-in Dictionary Functions and Methods

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Built-in Dictionary Functions and Methods

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Built-in Dictionary Functions and Methods

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Built-in Dictionary Functions and Methods

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Built-in Dictionary Functions and Methods

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Difference between a list and a dictionary

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String formatting with dictionary

%s, %d, %f etc., can be used to represent string, integer, floating point number, or any other data

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x={'fff': 1, 'bbb': 2, 'zzz': 3}

for key, val in x.items():

print("%s key : value %d" %(key,val))

Output:

fff key : value 1

bbb key : value 2

zzz key : value 3

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�when to use which data structure?�

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• Use lists to store a collection of data that does not need random access
• Use lists if the data has to be modified frequently
• Use tuples when data should not be altered

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�List vs Tuple vs Dictionary�

• Can append a new item in a list, since list is mutable
• Cannot append new item in tuple, since tuple is immutable
• Tuple has fixed size, cannot add or delete items from it

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�List vs Tuple vs Dictionary�

• tuples are easier on memory and processor, and achieves performance optimization
• tuples are best used as heterogeneous collections
• lists are best used as homogeneous collections
• Dictionary are best data structure for frequent lookup operations

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mcm={x:x*100 for x in range(1,11)}

temp=mcm.values()

print(temp)

cmm={x:x/100 for x in temp}

print("meter:centimeter",mcm)

print("centimeter:meter",cmm)

Output:

dict_values([100, 200, 300, 400, 500, 600, 700, 800, 900, 1000])

meter:centimeter {1: 100, 2: 200, 3: 300, 4: 400, 5: 500, 6: 600, 7: 700, 8: 800, 9: 900, 10: 1000}

centimeter:meter {100: 1.0, 200: 2.0, 300: 3.0, 400: 4.0, 500: 5.0, 600: 6.0, 700: 7.0, 800: 8.0, 900: 9.0, 1000: 10.0}

>>>

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dict={0:0,1:1}

def fib(n):

if n not in dict:

val=fib(n-1)+ fib(n-2)

dict[n]=val

return dict[n]

n=int(input("enter the value of n"))

print("fib=",fib(n))

print("dict=",dict);

enter the value of n5

fib= 5

dict= {0: 0, 1: 1, 2: 1, 3: 2, 4: 3, 5: 5}

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Thank you