NAVODAYA VIDYALAYA SAMITI� HYDERABAD REGION

E-CONTENT

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COMPUTER SCIENCE CLASS XI

COMPUTATIONAL THINKING AND PROGRAMMING-1

DICTIONARY IN PYTHON

SHIFA JAMES

PGT CS , JNV KADAPA

CONTENTS

• Definition, Creation, Accessing elements of a dictionary
• Add an item, modify an item in a dictionary; Traversal
• Functions/methods – len(), dict(), keys(), values(), items(), get(), update(), del(), del, clear(), fromkeys(), copy(), pop(), popitem(), setdefault(), max(), min(), count(), sorted() , copy();
• Suggested programs :
• count the number of times a character appears in a given string using a dictionary,
• create a dictionary with names of employees, their salary and access them.

Learning Objectives

After completing this lesson, the students should be able to:

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• Understand the need of a Dictionary
• Understand how to implement it in python.
• Gain knowledge about Different types Operations in Dictionary
• Understand about Different Methods and Built-in Functions in Dictionary
• Implementing Dictionary in Program

INTRODUCTION

• Have you used a Telephone Directory ?
• How are the contents arranged in a Telephone Directory ?
• Subscribers number gets stored according to his name and address.
• Here name becomes a key and phone number becomes a value.
• In dictionary, we have a concept of Key – Value pair.

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Dictionary - Definition

A Dictionary is an unordered collection of items where each item is a key-value pair.

It is a mapping between a set of keys and a set of values. The key-value pair is called an item.

Elements of a Dictionary are enclosed in curly brackets { } and are separated by commas.

Dictionaries are mutable data types in the form of key:value , where key is used to access the corresponding Value .

Keys are unique within a dictionary and are of immutable data types whereas Values are of mutable types

Internally stored as Mappings

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Internally, the key:value pair are associated with one another with some internal function(called hash function). This way of linking is called as mapping

Creating a Dictionary

• To create a dictionary, it is needed to collect pairs of key:value in “{ }”.
• <dictionary-name> = { <key1>:<value1>, <key2>:<value2>, <key3>:<value3>. . . }

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• Example:

Teachers = {“Rajeev”:”Math”, “Amal”:”Physics”, ”Arun”:”Chemistry“ , “ SJ”:”CS” }

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In above given example :

Different methods of creating a Dictionary

1) dict1 is an empty Dictionary created . Curly braces are used for empty dictionary

Example :

>>> dict1 = {}

>>> dict1

{}

2. dict2 is an empty dictionary created using built-in function

>>> dict2 = dict()

>>> dict2

{}

Different methods of creating a Dictionary

3) dict3 is the dictionary that maps names of the students to respective marks in percentage

>>> dict3 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> dict3

{'Mohan': 95, 'Ram': 89, 'Suhel': 92, 'Sangeeta': 85}

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Accessing keys and values simultaneously

• To access only keys of a dicionary , we can use the function keys()
• >>> mydict={'empno':1,'name':‘Arjun','dept':'sales','salary':20000}
• >>>mydict.keys()
• dict_keys(['empno', 'name', 'dept', 'salary'])
• To access only values of a dicionary , we can use the function values()
• >>>mydict.values()
• dict_values([1, ‘Arjun', 'sales', 20000])
• We can convert the sequence returned by keys() and values() by using list() as shown below:
• >>> list(mydict.keys())
• ['empno', 'name', 'dept', 'salary']
• >>> list(mydict.values()) [1, ‘Arjun', 'sales', 20000]

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Characteristics of a Dictionary

• Unordered set
• A dictionary is a unordered set of key:value pair
• Not a sequence
• Unlike a string, tuple, and list, a dictionary is not a sequence because it is unordered set of elements. The sequences are indexed by a range of ordinal numbers. Hence they are ordered but a dictionary is an unordered collection
• Indexed by Keys, Not Numbers
• Dictionaries are indexed by keys. Keys are immutable type

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Characteristics of a Dictionary

• Keys must be unique
• Each key within dictionary must be unique. However two unique keys can have same values.

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For example

Data = {1:100, 2:200,3:300,4:200}

• Mutable
• Like lists, dictionary are also mutable. We can change the value of a certain “key” in place
• Data[3]=400

So, to change value of dictionary the format is :

DictionaryName[“key” / key ]=new_value

You can even add new key:value pair using the same format:

DictionaryName[“key” / key ]=new_value

Accessing Elements in a Dictionary

• The items of a dictionary are accessed via the keys rather than via their relative positions or indices. Each key serves as the index and maps to a value.

For example :

>>> dict3 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> dict3['Ram']

89

>>> dict3['Sangeeta']

85

>>> dict3['Shyam'] #the key does not exist

KeyError: 'Shyam'

DICTIONARIES ARE MUTABLE

Add new item, Update an item , Delete an item

Adding elements to Dictionary

We can add a new item to the dictionary as shown in the following example:

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> dict1['Meena'] = 78

>>> dict1

{'Mohan': 95, 'Ram': 89, 'Suhel': 92, 'Sangeeta': 85, 'Meena': 78}

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Note : The new item added will be at the end of the dictionary

Updating elements in Dictionary

• The existing dictionary can be modified by just overwriting the key-value pair. Example to modify a given item in the dictionary:

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

#Marks of Suhel changed to 93.5

>>> dict1['Suhel'] = 93.5

>>> dict1

{'Mohan': 95, 'Ram': 89, 'Suhel': 93.5, 'Sangeeta': 85}

Deleting elements from Dictionary

Method 1 - Using del command

• Syntax :

del dictionaryName[“Key”]

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> del dict1[‘Ram’]

>>> dict1

{'Mohan':95, 'Suhel':92, 'Sangeeta':85}

Note :

If you try to remove the item whose key does not exists, the python runtime error occurs.

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Deleting elements from Dictionary

Method 2 - Using pop() function

Syntax :

dictionaryName.pop([“Key”])

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> dict1.pop(‘Suhel’)

>>>dict1

{'Mohan':95,'Ram':89, 'Sangeeta':85}

Note:

if key passed to pop() doesn’t exists then python will raise an exception.

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Traversing a Dictionary

We can access each item of the dictionary or traverse a dictionary using for loop.

Method 1

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

for key in dict1:

print(key , ':‘ , dict1[key])

Output

Mohan: 95

Ram: 89

Suhel: 92

Sangeeta: 85

Traversing a Dictionary

• Method 2
• >>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

for key,value in dict1.items():

print(key,':',value)

Output

Mohan: 95

Ram: 89

Suhel: 92

Sangeeta: 85

Dictionary Operations – �Membership operator – in

The membership operator in checks if the key is present in the dictionary and returns True, else it returns False.

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> 'Suhel' in dict1

True

Dictionary Operations – �Membership operator – not in

The not in operator returns True if the key is not present in the dictionary, else it returns False.

>>> dict1 = {'Mohan':95,'Ram':89,'Suhel':92, 'Sangeeta':85}

>>> 'Suhel' not in dict1

False

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Dictionary Methods and Built-in Functions

Dictionary Methods and Built-in Functions

Dictionary Methods and Built-in Functions

Dictionary Methods and Built-in Functions

Dictionary Methods and Built-in Functions

SUGGESTED PROGRAMS

Program 1

#Count the number of times a character appears in a given string

st = input("Enter a string: ")

dic = {} #creates an empty dictionary

for ch in st:

if ch in dic: #if next character is already in the dictionary

dic[ch] += 1

else:

dic[ch] = 1 #if ch appears for the first time

for key in dic:

print(key,':',dic[key])

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Output:

Enter a string: HelloWorld

H : 1

e : 1

l : 3

o : 2

W : 1

r : 1

d : 1

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Program 2

#Program to create a dictionary which stores names of the employee and their salary

num = int(input("Enter the number of employees whose data to be stored: "))

count = 1

employee = dict() #create an empty dictionary

while count <= num:

name = input("Enter the name of the Employee: ")

salary = int(input("Enter the salary: "))

employee[name] = salary

count += 1

print (‘\nDetails of Employees ‘)

print("\n\nEMPLOYEE_NAME\tSALARY\n~~~~~~~~~~~~~~~~~")

for k in employee:

print(k,'\t\t',employee[k])

Output

Enter the number of employees to be stored: 5

Enter the name of the Employee: 'Tarun'

Enter the salary: 12000

Enter the name of the Employee: 'Amina'

Enter the salary: 34000

Enter the name of the Employee: 'Joseph'

Enter the salary: 24000

Enter the name of the Employee: 'Rahul'

Enter the salary: 30000

Enter the name of the Employee: 'Zoya‘

Enter the salary: 25000

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Details of Employees

EMPLOYEE_NAME SALARY

~~~~~~~~~~~~~~~~~~~~~~~~~~

'Tarun' 12000

'Amina' 34000

'Joseph' 24000

'Rahul' 30000

'Zoya' 25000

Summary

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• Dictionary is a mapping (non-scalar) data type.
• It is an unordered collection of key-value pair; keyvalue pair are put inside curly braces.
• Each key is separated from its value by a colon.
• Keys are unique and act as the index.
• Keys are of immutable type but values can be mutable.
• We can add an item, modify an item in a dictionary;
• Functions/methods used in Dictionary are – len(), dict(), keys(), values(), items(), get(), update(), del(), del, clear(), fromkeys(), copy(), pop(), popitem(), setdefault(), max(), min(), sorted().

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BIBLIOGRAPHY:

1. Computer Science Textbook for class XI

by NCERT

2. Computer Science with Python

by Sumita Arora

3. Computer Science with Python

by Preeti Arora