Computer Science - Python

1. NAVODAYA VIDYALAYA SAMITI
HYDERABAD REGION
E-CONTENT
COMPUTER SCIENCE CLASS XI

2. COMPUTATIONAL THINKING AND
PROGRAMMING-1
DICTIONARY IN PYTHON
SHIFA JAMES
PGT CS , JNV KADAPA

3. 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.

4. Learning Objectives
After completing this lesson, the students should be able to:
⮚ 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

5. 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.

6. 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

7. Internally stored as Mappings
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

8. 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>. . . }
•Example:
Teachers = {“Rajeev”:”Math”, “Amal”:”Physics”, ”Arun”:”Chemistry“ , “ SJ”:”CS” }
In above given example :
Key-value pair Key Value
“Rajeev”:”Math” “Rajeev” “Math”
“Amal”:”Physics” “Amal” “Physics”
“Arun”:”Chemistry” “Arun” “Chemistry”
“SJ”:”CS” “SJ” “CS”

9. 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
{}

10. 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}

11. 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]

12. 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

13. Characteristics of a Dictionary
▪ Keys must be unique
▪ Each key within dictionary must be unique. However two unique keys can have same values.
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

14. 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'

15. DICTIONARIES ARE MUTABLE
Add new item, Update an item , Delete an item

16. 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}
Note : The new item added will be at the end of the dictionary

17. 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}

18. 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.

19. 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.

20. 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

21. 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

22. 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

23. 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

24. Dictionary Methods and Built-in Functions
Method Description Example
len() Returns the length or number of
key: value pairs of the dictionary
passed as the argument
>>> dict1 = {'Mohan':95,'Ram':89,
'Suhel':92, 'Sangeeta':85}
>>> len(dict1)
4
dict() Creates a dictionary from a
sequence of key-value pairs
pair1 = [('Mohan',95),('Ram',89),
('Suhel',92),('Sangeeta',85)]
>>> pair1
[('Mohan', 95), ('Ram', 89), ('Suhel', 92), ('Sangeeta', 85)]
>>> dict1 = dict(pair1)
>>> dict1
{'Mohan': 95, 'Ram': 89, 'Suhel': 92, 'Sangeeta': 85}
get() Returns the value
corresponding to the key
passed as the argument
If the key is not present in the
dictionary it will return None
>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85}
>>> dict1.get('Sangeeta')
85
>>> dict1.get('Sohan')
>>>

25. Dictionary Methods and Built-in Functions
Method Description Example
keys() Returns a list of keys in
the dictionary
>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85}
>>> dict1.keys()
dict_keys(['Mohan', 'Ram', 'Suhel', 'Sangeeta'])
values() Returns a list of values in
the dictionary
>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85}
>>> dict1.values()
dict_values([95, 89, 92, 85])
items() Returns a list of tuples(key –
value) pair
>>> dict1 = {'Mohan':95, 'Ram':89, 'Suhel':92, 'Sangeeta':85}
>>> dict1.items()
dict_items([( 'Mohan', 95), ('Ram', 89), ('Suhel', 92),
('Sangeeta', 85)])
clear() Deletes or clear all the items of
the dictionary
>>> dict1 = {'Mohan':95,'Ram':89, 'Suhel':92, 'Sangeeta':85}
>>> dict1.clear()
>>> dict1
{ }

26. Dictionary Methods and Built-in Functions
Method Description Example
update() appends the key-value pair of the dictionary
passed as the argument to the key-value
pair of the given dictionary
>>> dict1 = {'Mohan':95, 'Ram':89,
'Suhel':92, 'Sangeeta':85}
>>> dict2 = {'Sohan':79,'Geeta':89}
>>> dict1.update(dict2)
>>> dict1
{'Mohan': 95, 'Ram': 89, 'Suhel': 92,
'Sangeeta': 85, 'Sohan': 79, 'Geeta': 89}
>>> dict2
{'Sohan': 79, 'Geeta': 89}
popitem() It will remove the last dictionary item and
return key,value. of the removed item
>>> dict1 = {'Mohan':95, 'Ram':89,
'Suhel':92, 'Sangeeta':85}
>>> k,v = dict1.popitem()
>>> print(k,”: “ v)
Sangeeta :85
copy() it will create a copy of dictionary. >>> dict1 = {'Mohan':95, 'Ram':89,
'Suhel':92, 'Sangeeta':85}
>>> dic2 = dict1.copy()

27. Dictionary Methods and Built-in Functions
Method Description Example
fromkeys() return new dictionary with
the given set of elements
as the keys of the
dictionary.
Note:
Set of elements should be
immutable type
>>>seq = ('name', 'age', 'sex')
>>>dict = dict.fromkeys(seq)
>>>print ("New Dictionary : “ , str(dict) )
New Dictionary : {'age': None, 'name': None, 'sex':
None}
>>>dict = dict.fromkeys(seq, 10)
>>>print ("New Dictionary : " , str(dict) )
New Dictionary : {'age': 10, 'name': 10, 'sex': 10}
setdefault() returns the value of the
item with the specified
key.
If the key does not exist,
insert the key, with the
specified value,
>>>car = { "brand": "Ford", “model": "Mustang",
"year": 1964 }
>>>x = car.setdefault("color", "white“)
>>>print(x)
white

28. Dictionary Methods and Built-in Functions
Method Description Example
sorted() this function is used to sort the key or
value of dictionary in either ascending or
descending order. By default it will sort
the keys.
To display in descending order
>>> Dict1 = {1: 111, 4: 444 , 3: 300, 5: 555 , 2: 222}
>>> print(sorted(Dict1))
{1: 111, 2: 222, 3: 300, 4: 444, 5: 555}
>>>print(sorted(Dict1, reverse = ‘True’))
{5:555,4:444, 3:333 , 2:222 , 1:111}
min() This function returns lowest value in
dictionary, this will work only if all the
values in dictionary is of numeric type
>>> Dict1 = {1: 111, 4: 444 , 3: 300, 5: 555 , 2: 222}
>>>print(min(Dict1.values()))
111
max() This function returns highest value in
dictionary, this will work only if all the
values in dictionary is of numeric type
>>> Dict1 = {1: 111, 4: 444 , 3: 300, 5: 555 , 2: 222}
>>>print(max(Dict1.values()))
555

29. SUGGESTED PROGRAMS

30. 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])
Output:
Enter a string: HelloWorld
H : 1
e : 1
l : 3
o : 2
W : 1
r : 1
d : 1

31. 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("nnEMPLOYEE_NAMEtSALARYn~~~~~~~~~~~~~~~~~")
for k in employee:
print(k,'tt',employee[k])

32. 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

33. Summary
❖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().

34. 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