NCERT Textbook: Working with Lists and Dictionaries | Informatics Practices for Class 11 - Humanities/Arts PDF Download

 Page 1


“Computer Science is a science of 
abstraction 
– 
creating the right model for 
a problem and devising the appropriate 
mechanizable techniques to solve it.” 
— A. Aho and J. Ullman
4.1 Introduct Ion to LIst The data type list is an ordered sequence which is 
mutable and made up of one or more elements. Unlike a 
string which consists of only characters, a list can have 
elements of different data types such as integer, float, 
string, tuple or even another list. A list is very useful to 
group elements of mixed data types. Elements of a list 
are enclosed in square brackets and are separated by 
comma. 
Example 4.1  
#list1 is the list of six even numbers
>>> list1 = [2,4,6,8,10,12]
>>> print(list1)
[2, 4, 6, 8, 10, 12]
Working with 
Lists and 
Dictionaries
Chapter 
4 
In this chapter
 » Introduction to List
 » List Operations
 » Traversing a List
 » List Methods and Built-
in Functions
 » List Manipulation
 » Introduction to 
Dictionaries
 » Traversing a Dictionary
 » Dictionary Methods and 
Built-in Functions
 » Manipulating 
Dictionaries
Chap 4.indd   55 19-Jul-19   3:31:20 PM
2024-25
Page 2


“Computer Science is a science of 
abstraction 
– 
creating the right model for 
a problem and devising the appropriate 
mechanizable techniques to solve it.” 
— A. Aho and J. Ullman
4.1 Introduct Ion to LIst The data type list is an ordered sequence which is 
mutable and made up of one or more elements. Unlike a 
string which consists of only characters, a list can have 
elements of different data types such as integer, float, 
string, tuple or even another list. A list is very useful to 
group elements of mixed data types. Elements of a list 
are enclosed in square brackets and are separated by 
comma. 
Example 4.1  
#list1 is the list of six even numbers
>>> list1 = [2,4,6,8,10,12]
>>> print(list1)
[2, 4, 6, 8, 10, 12]
Working with 
Lists and 
Dictionaries
Chapter 
4 
In this chapter
 » Introduction to List
 » List Operations
 » Traversing a List
 » List Methods and Built-
in Functions
 » List Manipulation
 » Introduction to 
Dictionaries
 » Traversing a Dictionary
 » Dictionary Methods and 
Built-in Functions
 » Manipulating 
Dictionaries
Chap 4.indd   55 19-Jul-19   3:31:20 PM
2024-25
56
Informat Ics Pract Ices – c lass XI
#list2 is the list of vowels
>>> list2 = ['a','e','i','o','u']
>>> print(list2)
['a', 'e', 'i', 'o', 'u']
#list3 is the list of mixed data types
>>> list3 = [100,23.5,'Hello']
>>> print(list3)
[100, 23.5, 'Hello']
#list4 is the list of lists called nested 
#list
>>> list4 =[['Physics',101],['Chemistry',202],                    
            ['Mathematics',303]] 
>>> print(list4)
[['Physics', 101], ['Chemistry', 202],
 ['Mathematics', 303]]
4.1.1 Accessing Elements in a List
Each element in list is accessed using value called index. 
The fist index value is 0, the second index is 1 and so 
on. Elements in the list are assigned index values in 
increasing order sterling from 0. 
 To access an element, use square brackets with 
the index [] value of that element. We may also use 
negative index value to access elements starting from 
the last element in the list, having index value -0.
#initialing a list named list1 
>>> list1 = [2,4,6,8,10,12]    
>>> list1[0] #returns first element of list1
2
>>> list1[3] #returns fourth element of list1
8
#Out of range index value for the list returns error
>>> list1[15] 
IndexError: list index out of range
#an expression resulting in an integer index 
>>> list1[1+4] 
12
>>> list1[-1] #return first element from right
12
#length of the list1 is assigned to n
>>> n = len(list1) 
>>> print(n)
6
#Get the last element of the list1 
>>> list1[n-1]  
12
n otes Chap 4.indd   56 19-Jul-19   3:31:20 PM
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Page 3


“Computer Science is a science of 
abstraction 
– 
creating the right model for 
a problem and devising the appropriate 
mechanizable techniques to solve it.” 
— A. Aho and J. Ullman
4.1 Introduct Ion to LIst The data type list is an ordered sequence which is 
mutable and made up of one or more elements. Unlike a 
string which consists of only characters, a list can have 
elements of different data types such as integer, float, 
string, tuple or even another list. A list is very useful to 
group elements of mixed data types. Elements of a list 
are enclosed in square brackets and are separated by 
comma. 
Example 4.1  
#list1 is the list of six even numbers
>>> list1 = [2,4,6,8,10,12]
>>> print(list1)
[2, 4, 6, 8, 10, 12]
Working with 
Lists and 
Dictionaries
Chapter 
4 
In this chapter
 » Introduction to List
 » List Operations
 » Traversing a List
 » List Methods and Built-
in Functions
 » List Manipulation
 » Introduction to 
Dictionaries
 » Traversing a Dictionary
 » Dictionary Methods and 
Built-in Functions
 » Manipulating 
Dictionaries
Chap 4.indd   55 19-Jul-19   3:31:20 PM
2024-25
56
Informat Ics Pract Ices – c lass XI
#list2 is the list of vowels
>>> list2 = ['a','e','i','o','u']
>>> print(list2)
['a', 'e', 'i', 'o', 'u']
#list3 is the list of mixed data types
>>> list3 = [100,23.5,'Hello']
>>> print(list3)
[100, 23.5, 'Hello']
#list4 is the list of lists called nested 
#list
>>> list4 =[['Physics',101],['Chemistry',202],                    
            ['Mathematics',303]] 
>>> print(list4)
[['Physics', 101], ['Chemistry', 202],
 ['Mathematics', 303]]
4.1.1 Accessing Elements in a List
Each element in list is accessed using value called index. 
The fist index value is 0, the second index is 1 and so 
on. Elements in the list are assigned index values in 
increasing order sterling from 0. 
 To access an element, use square brackets with 
the index [] value of that element. We may also use 
negative index value to access elements starting from 
the last element in the list, having index value -0.
#initialing a list named list1 
>>> list1 = [2,4,6,8,10,12]    
>>> list1[0] #returns first element of list1
2
>>> list1[3] #returns fourth element of list1
8
#Out of range index value for the list returns error
>>> list1[15] 
IndexError: list index out of range
#an expression resulting in an integer index 
>>> list1[1+4] 
12
>>> list1[-1] #return first element from right
12
#length of the list1 is assigned to n
>>> n = len(list1) 
>>> print(n)
6
#Get the last element of the list1 
>>> list1[n-1]  
12
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Working With Lists and d ictionaries 57
#Get the first element of list1 
>>> list1[-n] 
2
4.1.2 Lists are Mutable
In Python, lists are mutable. It means that the contents 
of the list can be changed after it has been created. 
#List list1 of colors 
 >>> list1 = ['Red','Green','Blue','Orange']   
#change/override the fourth element of list1 
 >>> list1[3] = 'Black'               
 >>> list1  #print the modified list list1
['Red', 'Green', 'Blue', 'Black']
4.2 LIst o perat Ions The data type list allows manipulation of its contents 
through various operations as shown below.
4.2.1 Concatenation
Python allows us to join two or more lists using 
concatenation operator using symbol +.
#list1 is list of first five odd integers 
>>> list1 = [1,3,5,7,9]  
#list2 is list of first five even integers 
>>> list2 = [2,4,6,8,10]  
#Get elements of list1 followed by list2 
>>> list1 + list2            
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
>>> list3 = ['Red','Green','Blue']
>>> list4 = ['Cyan', 'Magenta', 'Yellow' 
,'Black']
>>> list3 + list4
['Red','Green','Blue','Cyan','Magenta',     
 'Yellow','Black']
Note that, there is no change in original lists i.e., 
list1, list2, list3, list4 remain the same after 
concatenation operation. If we want to use the result of 
two concatenated lists, we should use an assignment 
operator. 
For example, 
#Join list 2 at the end of list 
>>> new List = list 1 + list 2
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
>> new list The concatenation operator '+’ requires that 
the operands should be of list type only. If we try to 
concatenate a list with elements of some other data 
type, TypeError occurs.
Concatenation is the 
merging of two or 
more values. Example: 
we can concatenate 
strings together.  
Chap 4.indd   57 19-Jul-19   3:31:20 PM
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Page 4


“Computer Science is a science of 
abstraction 
– 
creating the right model for 
a problem and devising the appropriate 
mechanizable techniques to solve it.” 
— A. Aho and J. Ullman
4.1 Introduct Ion to LIst The data type list is an ordered sequence which is 
mutable and made up of one or more elements. Unlike a 
string which consists of only characters, a list can have 
elements of different data types such as integer, float, 
string, tuple or even another list. A list is very useful to 
group elements of mixed data types. Elements of a list 
are enclosed in square brackets and are separated by 
comma. 
Example 4.1  
#list1 is the list of six even numbers
>>> list1 = [2,4,6,8,10,12]
>>> print(list1)
[2, 4, 6, 8, 10, 12]
Working with 
Lists and 
Dictionaries
Chapter 
4 
In this chapter
 » Introduction to List
 » List Operations
 » Traversing a List
 » List Methods and Built-
in Functions
 » List Manipulation
 » Introduction to 
Dictionaries
 » Traversing a Dictionary
 » Dictionary Methods and 
Built-in Functions
 » Manipulating 
Dictionaries
Chap 4.indd   55 19-Jul-19   3:31:20 PM
2024-25
56
Informat Ics Pract Ices – c lass XI
#list2 is the list of vowels
>>> list2 = ['a','e','i','o','u']
>>> print(list2)
['a', 'e', 'i', 'o', 'u']
#list3 is the list of mixed data types
>>> list3 = [100,23.5,'Hello']
>>> print(list3)
[100, 23.5, 'Hello']
#list4 is the list of lists called nested 
#list
>>> list4 =[['Physics',101],['Chemistry',202],                    
            ['Mathematics',303]] 
>>> print(list4)
[['Physics', 101], ['Chemistry', 202],
 ['Mathematics', 303]]
4.1.1 Accessing Elements in a List
Each element in list is accessed using value called index. 
The fist index value is 0, the second index is 1 and so 
on. Elements in the list are assigned index values in 
increasing order sterling from 0. 
 To access an element, use square brackets with 
the index [] value of that element. We may also use 
negative index value to access elements starting from 
the last element in the list, having index value -0.
#initialing a list named list1 
>>> list1 = [2,4,6,8,10,12]    
>>> list1[0] #returns first element of list1
2
>>> list1[3] #returns fourth element of list1
8
#Out of range index value for the list returns error
>>> list1[15] 
IndexError: list index out of range
#an expression resulting in an integer index 
>>> list1[1+4] 
12
>>> list1[-1] #return first element from right
12
#length of the list1 is assigned to n
>>> n = len(list1) 
>>> print(n)
6
#Get the last element of the list1 
>>> list1[n-1]  
12
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Working With Lists and d ictionaries 57
#Get the first element of list1 
>>> list1[-n] 
2
4.1.2 Lists are Mutable
In Python, lists are mutable. It means that the contents 
of the list can be changed after it has been created. 
#List list1 of colors 
 >>> list1 = ['Red','Green','Blue','Orange']   
#change/override the fourth element of list1 
 >>> list1[3] = 'Black'               
 >>> list1  #print the modified list list1
['Red', 'Green', 'Blue', 'Black']
4.2 LIst o perat Ions The data type list allows manipulation of its contents 
through various operations as shown below.
4.2.1 Concatenation
Python allows us to join two or more lists using 
concatenation operator using symbol +.
#list1 is list of first five odd integers 
>>> list1 = [1,3,5,7,9]  
#list2 is list of first five even integers 
>>> list2 = [2,4,6,8,10]  
#Get elements of list1 followed by list2 
>>> list1 + list2            
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
>>> list3 = ['Red','Green','Blue']
>>> list4 = ['Cyan', 'Magenta', 'Yellow' 
,'Black']
>>> list3 + list4
['Red','Green','Blue','Cyan','Magenta',     
 'Yellow','Black']
Note that, there is no change in original lists i.e., 
list1, list2, list3, list4 remain the same after 
concatenation operation. If we want to use the result of 
two concatenated lists, we should use an assignment 
operator. 
For example, 
#Join list 2 at the end of list 
>>> new List = list 1 + list 2
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
>> new list The concatenation operator '+’ requires that 
the operands should be of list type only. If we try to 
concatenate a list with elements of some other data 
type, TypeError occurs.
Concatenation is the 
merging of two or 
more values. Example: 
we can concatenate 
strings together.  
Chap 4.indd   57 19-Jul-19   3:31:20 PM
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58
Informat Ics Pract Ices – c lass XI
>>> list1 = [1,2,3]
>>> str1 = "abc"
>>> list1 + str1
TypeError: can only concatenate list (not 
"str") to list
4.2.2 Repetition
Python allows us to replicate the contents of a list using 
repetition operator depicted by symbol *.
>>> list1 = ['Hello']
#elements of list1 repeated 4 times
>>> list1 * 4   
['Hello', 'Hello', 'Hello', 'Hello']
4.2.3 Membership
The membership operator in checks if the element 
is present in the list and returns True, else returns 
False. 
>>> list1 = ['Red','Green','Blue']
>>> 'Green' in list1
True
>>> 'Cyan' in list1
False
The Operator not in transpose returns True if the 
element is not present in the list, else it returns False.
>>> list1 = ['Red','Green','Blue']
>>> 'Cyan' not in list1
True
>>> 'Green' not in list1
False
4.2.4 Slicing
Slicing operations allow us to create new list by taking 
out elements from an existing list. 
>>> list1 =['Red','Green','Blue','Cyan', 
'Magenta','Yellow','Black']
#subject from indexes 2 to 5 of list 1
>>> list1[2:6]
['Blue', 'Cyan', 'Magenta', 'Yellow'] 
 #list1 is truncated to the end of the list
>>> list1[2:20] #second index is out of range   
['Blue', 'Cyan', 'Magenta', 'Yellow', 
'Black']
 
>>> list1[7:2]  #first index > second index
[]    #results in an empty list
n otes Chap 4.indd   58 19-Jul-19   3:31:20 PM
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Page 5


“Computer Science is a science of 
abstraction 
– 
creating the right model for 
a problem and devising the appropriate 
mechanizable techniques to solve it.” 
— A. Aho and J. Ullman
4.1 Introduct Ion to LIst The data type list is an ordered sequence which is 
mutable and made up of one or more elements. Unlike a 
string which consists of only characters, a list can have 
elements of different data types such as integer, float, 
string, tuple or even another list. A list is very useful to 
group elements of mixed data types. Elements of a list 
are enclosed in square brackets and are separated by 
comma. 
Example 4.1  
#list1 is the list of six even numbers
>>> list1 = [2,4,6,8,10,12]
>>> print(list1)
[2, 4, 6, 8, 10, 12]
Working with 
Lists and 
Dictionaries
Chapter 
4 
In this chapter
 » Introduction to List
 » List Operations
 » Traversing a List
 » List Methods and Built-
in Functions
 » List Manipulation
 » Introduction to 
Dictionaries
 » Traversing a Dictionary
 » Dictionary Methods and 
Built-in Functions
 » Manipulating 
Dictionaries
Chap 4.indd   55 19-Jul-19   3:31:20 PM
2024-25
56
Informat Ics Pract Ices – c lass XI
#list2 is the list of vowels
>>> list2 = ['a','e','i','o','u']
>>> print(list2)
['a', 'e', 'i', 'o', 'u']
#list3 is the list of mixed data types
>>> list3 = [100,23.5,'Hello']
>>> print(list3)
[100, 23.5, 'Hello']
#list4 is the list of lists called nested 
#list
>>> list4 =[['Physics',101],['Chemistry',202],                    
            ['Mathematics',303]] 
>>> print(list4)
[['Physics', 101], ['Chemistry', 202],
 ['Mathematics', 303]]
4.1.1 Accessing Elements in a List
Each element in list is accessed using value called index. 
The fist index value is 0, the second index is 1 and so 
on. Elements in the list are assigned index values in 
increasing order sterling from 0. 
 To access an element, use square brackets with 
the index [] value of that element. We may also use 
negative index value to access elements starting from 
the last element in the list, having index value -0.
#initialing a list named list1 
>>> list1 = [2,4,6,8,10,12]    
>>> list1[0] #returns first element of list1
2
>>> list1[3] #returns fourth element of list1
8
#Out of range index value for the list returns error
>>> list1[15] 
IndexError: list index out of range
#an expression resulting in an integer index 
>>> list1[1+4] 
12
>>> list1[-1] #return first element from right
12
#length of the list1 is assigned to n
>>> n = len(list1) 
>>> print(n)
6
#Get the last element of the list1 
>>> list1[n-1]  
12
n otes Chap 4.indd   56 19-Jul-19   3:31:20 PM
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Working With Lists and d ictionaries 57
#Get the first element of list1 
>>> list1[-n] 
2
4.1.2 Lists are Mutable
In Python, lists are mutable. It means that the contents 
of the list can be changed after it has been created. 
#List list1 of colors 
 >>> list1 = ['Red','Green','Blue','Orange']   
#change/override the fourth element of list1 
 >>> list1[3] = 'Black'               
 >>> list1  #print the modified list list1
['Red', 'Green', 'Blue', 'Black']
4.2 LIst o perat Ions The data type list allows manipulation of its contents 
through various operations as shown below.
4.2.1 Concatenation
Python allows us to join two or more lists using 
concatenation operator using symbol +.
#list1 is list of first five odd integers 
>>> list1 = [1,3,5,7,9]  
#list2 is list of first five even integers 
>>> list2 = [2,4,6,8,10]  
#Get elements of list1 followed by list2 
>>> list1 + list2            
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
>>> list3 = ['Red','Green','Blue']
>>> list4 = ['Cyan', 'Magenta', 'Yellow' 
,'Black']
>>> list3 + list4
['Red','Green','Blue','Cyan','Magenta',     
 'Yellow','Black']
Note that, there is no change in original lists i.e., 
list1, list2, list3, list4 remain the same after 
concatenation operation. If we want to use the result of 
two concatenated lists, we should use an assignment 
operator. 
For example, 
#Join list 2 at the end of list 
>>> new List = list 1 + list 2
[1, 3, 5, 7, 9, 2, 4, 6, 8, 10]
>> new list The concatenation operator '+’ requires that 
the operands should be of list type only. If we try to 
concatenate a list with elements of some other data 
type, TypeError occurs.
Concatenation is the 
merging of two or 
more values. Example: 
we can concatenate 
strings together.  
Chap 4.indd   57 19-Jul-19   3:31:20 PM
2024-25
58
Informat Ics Pract Ices – c lass XI
>>> list1 = [1,2,3]
>>> str1 = "abc"
>>> list1 + str1
TypeError: can only concatenate list (not 
"str") to list
4.2.2 Repetition
Python allows us to replicate the contents of a list using 
repetition operator depicted by symbol *.
>>> list1 = ['Hello']
#elements of list1 repeated 4 times
>>> list1 * 4   
['Hello', 'Hello', 'Hello', 'Hello']
4.2.3 Membership
The membership operator in checks if the element 
is present in the list and returns True, else returns 
False. 
>>> list1 = ['Red','Green','Blue']
>>> 'Green' in list1
True
>>> 'Cyan' in list1
False
The Operator not in transpose returns True if the 
element is not present in the list, else it returns False.
>>> list1 = ['Red','Green','Blue']
>>> 'Cyan' not in list1
True
>>> 'Green' not in list1
False
4.2.4 Slicing
Slicing operations allow us to create new list by taking 
out elements from an existing list. 
>>> list1 =['Red','Green','Blue','Cyan', 
'Magenta','Yellow','Black']
#subject from indexes 2 to 5 of list 1
>>> list1[2:6]
['Blue', 'Cyan', 'Magenta', 'Yellow'] 
 #list1 is truncated to the end of the list
>>> list1[2:20] #second index is out of range   
['Blue', 'Cyan', 'Magenta', 'Yellow', 
'Black']
 
>>> list1[7:2]  #first index > second index
[]    #results in an empty list
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Working With Lists and d ictionaries 59
#return sublist from index 0 to 4 
>>> list1[:5] #first index missing 
['Red','Green','Blue','Cyan','Magenta'] 
#slicing with a given step size 
>>> list1[0:6:2]          
['Red','Blue','Magenta']
#negative indexes
#elements at index -6,-5,-4,-3 are sliced 
>>> list1[-6:-2]    
['Green','Blue','Cyan','Magenta'] 
#both first and last index missing
>>> list1[::2] #step size 2 on entire list
['Red','Blue','Magenta','Black']
#Access list in the reverse order using 
negative step size
>>> list1[::-1] 
['Black','Yellow','Magenta','Cyan','Blue', 
'Green','Red']
4.3 t ravers Ing a LIst We can access each element of the list or traverse a list 
using a for loop or a while loop.
(A) List traversal using for loop: 
>>> list1 = ['Red','Green','Blue','Yellow',  
      'Black']
>>> for item in list1:
         print(item)
 Output:
Red
Green
Blue
Yellow
Black
Another way of accessing the elements of the list is 
using range() and len() functions:
>>> for i in range(len(list1)):
         print(list1[i])
Output:
Red
Green
Blue
Yellow
Black
len (list1) returns 
the length or total 
number of elements of 
list1.
range(n) returns a 
sequence of numbers 
starting from 0, 
increases by 1 and ends 
at n-1 (one number 
less than the specified 
number i.e. is)
Chap 4.indd   59 19-Jul-19   3:31:20 PM
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