Textbook - Dictionaries, Computer Science (Python), Class 11 PDF Download

 Page 1


227 
Chapter 3 
Dictionaries 
After studying this lesson, the students will be able to 
? understand the need of dictionaries; 
? solve problems by using dictionaries; 
? get clear idea about dictionaries functions; and 
? understand the difference between list and dictionary. 
What is dictionary? 
A dictionary is like a list, but more in general. In a list, index value is an integer, while 
in a dictionary index value can be any other data type and are called keys. The key will 
be used as a string as it is easy to recall. A dictionary is an extremely useful data storage 
construct for storing and retrieving all key value pairs, where each element is accessed 
(or indexed) by a unique key.  However, dictionary keys are not in sequences and hence 
maintain no left-to right order. 
Key-value pair 
We can refer to a dictionary as a mapping between a set of indices (which are called 
keys) and a set of values. Each key maps a value. The association of a key and a value is 
called a key-value pair. 
Syntax: 
 my_dict = {'key1': 'value1','key2': 'value2','key3': ' v al ue3' … ' ke yn': 'valuen'} 
Note: Dictionary is created by using curly brackets(ie. {}).  
Example 
 >>> A={1:"one",2:"two",3:"three"} 
 >>> print A 
 {1: 'one', 2: 'two', 3: 'three'} 
Page 2


227 
Chapter 3 
Dictionaries 
After studying this lesson, the students will be able to 
? understand the need of dictionaries; 
? solve problems by using dictionaries; 
? get clear idea about dictionaries functions; and 
? understand the difference between list and dictionary. 
What is dictionary? 
A dictionary is like a list, but more in general. In a list, index value is an integer, while 
in a dictionary index value can be any other data type and are called keys. The key will 
be used as a string as it is easy to recall. A dictionary is an extremely useful data storage 
construct for storing and retrieving all key value pairs, where each element is accessed 
(or indexed) by a unique key.  However, dictionary keys are not in sequences and hence 
maintain no left-to right order. 
Key-value pair 
We can refer to a dictionary as a mapping between a set of indices (which are called 
keys) and a set of values. Each key maps a value. The association of a key and a value is 
called a key-value pair. 
Syntax: 
 my_dict = {'key1': 'value1','key2': 'value2','key3': ' v al ue3' … ' ke yn': 'valuen'} 
Note: Dictionary is created by using curly brackets(ie. {}).  
Example 
 >>> A={1:"one",2:"two",3:"three"} 
 >>> print A 
 {1: 'one', 2: 'two', 3: 'three'} 
228 
In the above example, we have created a list that maps from numbers to English words, 
so the keys values are in numbers and values are in strings. 
  
 
                                          A     = 
 
 
 
 
                                                Map between keys and values 
Example 
 >>>computer={'input':'keybord','output':'mouse','language':'python','os':'windows-
8',} 
 >>> print computer 
 {'input': 'keyboard', 'os': 'windows-8', 'language': 'python', 'output': 'mouse'} 
 >>> 
In the above example, we have created a list that maps from computer related things 
with example, so here the keys and values are in strings. The order of the key-value 
pairs is not in same order (ie. input and output orders are not same). We can get 
different order of items in different computers.  Thus, the order of items in a dictionary 
is unpredictable. 
Example 
 >>> 
D={'sun':'Sunday','mon':'Monday','tue':'Tuesday','wed':'Wednesday','thu':'Thursda
y','fri':'Friday','sat':'Saturday'} 
 >>> print D 
 {'wed': 'Wednesday', 'sun': 'Sunday', 'thu': 'Thursday', 'tue': 'Tuesday', 'mon': 
'Monday', 'fri': 'Friday', 'sat': 'Saturday'} 
Creation, initializing and accessing the elements in a Dictionary 
The function dict ( ) is used to create a new dictionary with no items. This function is 
called built-in function. We can also create dictionary using {}. 
1             one 
 
2              two 
 
3             three 
Page 3


227 
Chapter 3 
Dictionaries 
After studying this lesson, the students will be able to 
? understand the need of dictionaries; 
? solve problems by using dictionaries; 
? get clear idea about dictionaries functions; and 
? understand the difference between list and dictionary. 
What is dictionary? 
A dictionary is like a list, but more in general. In a list, index value is an integer, while 
in a dictionary index value can be any other data type and are called keys. The key will 
be used as a string as it is easy to recall. A dictionary is an extremely useful data storage 
construct for storing and retrieving all key value pairs, where each element is accessed 
(or indexed) by a unique key.  However, dictionary keys are not in sequences and hence 
maintain no left-to right order. 
Key-value pair 
We can refer to a dictionary as a mapping between a set of indices (which are called 
keys) and a set of values. Each key maps a value. The association of a key and a value is 
called a key-value pair. 
Syntax: 
 my_dict = {'key1': 'value1','key2': 'value2','key3': ' v al ue3' … ' ke yn': 'valuen'} 
Note: Dictionary is created by using curly brackets(ie. {}).  
Example 
 >>> A={1:"one",2:"two",3:"three"} 
 >>> print A 
 {1: 'one', 2: 'two', 3: 'three'} 
228 
In the above example, we have created a list that maps from numbers to English words, 
so the keys values are in numbers and values are in strings. 
  
 
                                          A     = 
 
 
 
 
                                                Map between keys and values 
Example 
 >>>computer={'input':'keybord','output':'mouse','language':'python','os':'windows-
8',} 
 >>> print computer 
 {'input': 'keyboard', 'os': 'windows-8', 'language': 'python', 'output': 'mouse'} 
 >>> 
In the above example, we have created a list that maps from computer related things 
with example, so here the keys and values are in strings. The order of the key-value 
pairs is not in same order (ie. input and output orders are not same). We can get 
different order of items in different computers.  Thus, the order of items in a dictionary 
is unpredictable. 
Example 
 >>> 
D={'sun':'Sunday','mon':'Monday','tue':'Tuesday','wed':'Wednesday','thu':'Thursda
y','fri':'Friday','sat':'Saturday'} 
 >>> print D 
 {'wed': 'Wednesday', 'sun': 'Sunday', 'thu': 'Thursday', 'tue': 'Tuesday', 'mon': 
'Monday', 'fri': 'Friday', 'sat': 'Saturday'} 
Creation, initializing and accessing the elements in a Dictionary 
The function dict ( ) is used to create a new dictionary with no items. This function is 
called built-in function. We can also create dictionary using {}. 
1             one 
 
2              two 
 
3             three 
229 
>>> D=dict() 
>>> print D 
{} 
{} represents empty string. To add an item to the dictionary (empty string), we can use 
square brackets for accessing and initializing dictionary values. 
Example 
>>> H=dict() 
>>> H["one"]="keyboard" 
>>> H["two"]="Mouse" 
>>> H["three"]="printer" 
>>> H["Four"]="scanner" 
>>> print H 
{'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
>>> 
Traversing a dictionary 
Let us visit each element of the dictionary to display its values on screen. This can be 
done by using ‘for-loop’. 
Example 
Code 
 H={'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
 for i in H: 
 print i,":", H[i],"   ", 
Output 
 >>>  
 Four: scanner     one: keyboard     three: printer     two: Mouse     
 >>> 
Page 4


227 
Chapter 3 
Dictionaries 
After studying this lesson, the students will be able to 
? understand the need of dictionaries; 
? solve problems by using dictionaries; 
? get clear idea about dictionaries functions; and 
? understand the difference between list and dictionary. 
What is dictionary? 
A dictionary is like a list, but more in general. In a list, index value is an integer, while 
in a dictionary index value can be any other data type and are called keys. The key will 
be used as a string as it is easy to recall. A dictionary is an extremely useful data storage 
construct for storing and retrieving all key value pairs, where each element is accessed 
(or indexed) by a unique key.  However, dictionary keys are not in sequences and hence 
maintain no left-to right order. 
Key-value pair 
We can refer to a dictionary as a mapping between a set of indices (which are called 
keys) and a set of values. Each key maps a value. The association of a key and a value is 
called a key-value pair. 
Syntax: 
 my_dict = {'key1': 'value1','key2': 'value2','key3': ' v al ue3' … ' ke yn': 'valuen'} 
Note: Dictionary is created by using curly brackets(ie. {}).  
Example 
 >>> A={1:"one",2:"two",3:"three"} 
 >>> print A 
 {1: 'one', 2: 'two', 3: 'three'} 
228 
In the above example, we have created a list that maps from numbers to English words, 
so the keys values are in numbers and values are in strings. 
  
 
                                          A     = 
 
 
 
 
                                                Map between keys and values 
Example 
 >>>computer={'input':'keybord','output':'mouse','language':'python','os':'windows-
8',} 
 >>> print computer 
 {'input': 'keyboard', 'os': 'windows-8', 'language': 'python', 'output': 'mouse'} 
 >>> 
In the above example, we have created a list that maps from computer related things 
with example, so here the keys and values are in strings. The order of the key-value 
pairs is not in same order (ie. input and output orders are not same). We can get 
different order of items in different computers.  Thus, the order of items in a dictionary 
is unpredictable. 
Example 
 >>> 
D={'sun':'Sunday','mon':'Monday','tue':'Tuesday','wed':'Wednesday','thu':'Thursda
y','fri':'Friday','sat':'Saturday'} 
 >>> print D 
 {'wed': 'Wednesday', 'sun': 'Sunday', 'thu': 'Thursday', 'tue': 'Tuesday', 'mon': 
'Monday', 'fri': 'Friday', 'sat': 'Saturday'} 
Creation, initializing and accessing the elements in a Dictionary 
The function dict ( ) is used to create a new dictionary with no items. This function is 
called built-in function. We can also create dictionary using {}. 
1             one 
 
2              two 
 
3             three 
229 
>>> D=dict() 
>>> print D 
{} 
{} represents empty string. To add an item to the dictionary (empty string), we can use 
square brackets for accessing and initializing dictionary values. 
Example 
>>> H=dict() 
>>> H["one"]="keyboard" 
>>> H["two"]="Mouse" 
>>> H["three"]="printer" 
>>> H["Four"]="scanner" 
>>> print H 
{'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
>>> 
Traversing a dictionary 
Let us visit each element of the dictionary to display its values on screen. This can be 
done by using ‘for-loop’. 
Example 
Code 
 H={'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
 for i in H: 
 print i,":", H[i],"   ", 
Output 
 >>>  
 Four: scanner     one: keyboard     three: printer     two: Mouse     
 >>> 
230 
 OR 
Code 
 H = {'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
 print "i value","\t","H[i] value" 
 for i in H: 
 print i,"\t", H[i] 
Output 
 i value  H[i] value 
 Four  scanner 
 one  keyboard 
 three  printer 
 two  Mouse 
As said previously, the order of items in a dictionary is unpredictable.  
Creating, initializing values during run time (Dynamic allocation) 
We can create a dictionary during run time also by using dict () function. This way of 
creation is called dynamic allocation. Because, during the run time, memory keys and 
values are added to the dictionary. 
Example 
Write a program to input total number of sections and class teachers’ name in 11
th
 class 
and display all information on the output screen. 
Code 
 classxi=dict() 
 n=input("Enter total number of section in xi class") 
 i=1 
 while i<=n: 
 a=raw_input("enter section") 
Page 5


227 
Chapter 3 
Dictionaries 
After studying this lesson, the students will be able to 
? understand the need of dictionaries; 
? solve problems by using dictionaries; 
? get clear idea about dictionaries functions; and 
? understand the difference between list and dictionary. 
What is dictionary? 
A dictionary is like a list, but more in general. In a list, index value is an integer, while 
in a dictionary index value can be any other data type and are called keys. The key will 
be used as a string as it is easy to recall. A dictionary is an extremely useful data storage 
construct for storing and retrieving all key value pairs, where each element is accessed 
(or indexed) by a unique key.  However, dictionary keys are not in sequences and hence 
maintain no left-to right order. 
Key-value pair 
We can refer to a dictionary as a mapping between a set of indices (which are called 
keys) and a set of values. Each key maps a value. The association of a key and a value is 
called a key-value pair. 
Syntax: 
 my_dict = {'key1': 'value1','key2': 'value2','key3': ' v al ue3' … ' ke yn': 'valuen'} 
Note: Dictionary is created by using curly brackets(ie. {}).  
Example 
 >>> A={1:"one",2:"two",3:"three"} 
 >>> print A 
 {1: 'one', 2: 'two', 3: 'three'} 
228 
In the above example, we have created a list that maps from numbers to English words, 
so the keys values are in numbers and values are in strings. 
  
 
                                          A     = 
 
 
 
 
                                                Map between keys and values 
Example 
 >>>computer={'input':'keybord','output':'mouse','language':'python','os':'windows-
8',} 
 >>> print computer 
 {'input': 'keyboard', 'os': 'windows-8', 'language': 'python', 'output': 'mouse'} 
 >>> 
In the above example, we have created a list that maps from computer related things 
with example, so here the keys and values are in strings. The order of the key-value 
pairs is not in same order (ie. input and output orders are not same). We can get 
different order of items in different computers.  Thus, the order of items in a dictionary 
is unpredictable. 
Example 
 >>> 
D={'sun':'Sunday','mon':'Monday','tue':'Tuesday','wed':'Wednesday','thu':'Thursda
y','fri':'Friday','sat':'Saturday'} 
 >>> print D 
 {'wed': 'Wednesday', 'sun': 'Sunday', 'thu': 'Thursday', 'tue': 'Tuesday', 'mon': 
'Monday', 'fri': 'Friday', 'sat': 'Saturday'} 
Creation, initializing and accessing the elements in a Dictionary 
The function dict ( ) is used to create a new dictionary with no items. This function is 
called built-in function. We can also create dictionary using {}. 
1             one 
 
2              two 
 
3             three 
229 
>>> D=dict() 
>>> print D 
{} 
{} represents empty string. To add an item to the dictionary (empty string), we can use 
square brackets for accessing and initializing dictionary values. 
Example 
>>> H=dict() 
>>> H["one"]="keyboard" 
>>> H["two"]="Mouse" 
>>> H["three"]="printer" 
>>> H["Four"]="scanner" 
>>> print H 
{'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
>>> 
Traversing a dictionary 
Let us visit each element of the dictionary to display its values on screen. This can be 
done by using ‘for-loop’. 
Example 
Code 
 H={'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
 for i in H: 
 print i,":", H[i],"   ", 
Output 
 >>>  
 Four: scanner     one: keyboard     three: printer     two: Mouse     
 >>> 
230 
 OR 
Code 
 H = {'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'} 
 print "i value","\t","H[i] value" 
 for i in H: 
 print i,"\t", H[i] 
Output 
 i value  H[i] value 
 Four  scanner 
 one  keyboard 
 three  printer 
 two  Mouse 
As said previously, the order of items in a dictionary is unpredictable.  
Creating, initializing values during run time (Dynamic allocation) 
We can create a dictionary during run time also by using dict () function. This way of 
creation is called dynamic allocation. Because, during the run time, memory keys and 
values are added to the dictionary. 
Example 
Write a program to input total number of sections and class teachers’ name in 11
th
 class 
and display all information on the output screen. 
Code 
 classxi=dict() 
 n=input("Enter total number of section in xi class") 
 i=1 
 while i<=n: 
 a=raw_input("enter section") 
231 
 b=raw_input ("enter class teacher name") 
 classxi[a]=b 
 i=i+1 
 print "Class","\t","Section","\t","teacher name" 
 for i in classxi: 
 print "XI","\t",i,"\t",classxi[i] 
Output 
 >>>  
 Enter total number of section in xi class3 
 enter sectionA 
 enter class teacher nameLeena 
 enter sectionB 
 enter class teacher nameMadhu 
 enter sectionC 
 enter class teacher nameSurpreeth 
 Class  Section     teacher name 
 XI  A   Leena 
 XI  C   Surpreeth 
 XI  B   Madhu 
 >>>       
Appending values to the dictionary 
We can add new elements to the existing dictionary, extend it with single pair of values 
or join two dictionaries into one. If we want to add only one element to the dictionary, 
then we should use the following method. 
Syntax: 
 Dictionary name [key]=value