NCERT Textbook: Strings | Computer Science for Class 11 - Humanities/Arts PDF Download

 Page 1


8.1 Introduct Ion We have studied in Chapter 5, that a sequence is an 
orderly collection of items and each item is indexed by 
an integer. Following sequence data types in Python 
were also briefly introduced in Chapter 5.
• Strings
• Lists
• Tuples
Another data type ‘Dictionary’ was also introduced 
in chapter 5 which falls under the category of mapping. 
In this chapter, we will go through strings in detail. 
List will be covered in Chapter 9 whereas tuple and 
dictionary will be discussed in Chapter 10.
8.2 Str Ing S 
String is a sequence which is made up of one or more 
UNICODE characters. Here the character can be a letter, 
digit, whitespace or any other symbol. A string can be 
created by enclosing one or more characters in single, 
double or triple quote.
Example 8.1 
>>> str1 = 'Hello World!'
>>> str2 = "Hello World!"
>>> str3 = """Hello World!"""
>>> str4 = '''Hello World!''
str1, str2, str3, str4 are all string variables 
having the same value 'Hello World!'. Values stored in 
str3 and str4 can be extended to multiple lines using 
triple codes as can be seen in the following example:
>>> str3 = """Hello World!
          welcome to the world of Python"""
>>> str4 = '''Hello World!
          welcome to the world of Python'''
“The great thing about a 
computer notebook is that no 
matter how much you stuff 
into it, it doesn't get bigger or 
heavier. ”
– Bill Gates
Chapter 8
Strings
In this chapter
 » Introduction to 
Strings
 » String Operations
 » Traversing a String
 » Strings Methods 
and Built-in 
Functions
 » Handling Strings
Ch 8.indd   175 08-Apr-19   12:39:21 PM
2024-25
Page 2


8.1 Introduct Ion We have studied in Chapter 5, that a sequence is an 
orderly collection of items and each item is indexed by 
an integer. Following sequence data types in Python 
were also briefly introduced in Chapter 5.
• Strings
• Lists
• Tuples
Another data type ‘Dictionary’ was also introduced 
in chapter 5 which falls under the category of mapping. 
In this chapter, we will go through strings in detail. 
List will be covered in Chapter 9 whereas tuple and 
dictionary will be discussed in Chapter 10.
8.2 Str Ing S 
String is a sequence which is made up of one or more 
UNICODE characters. Here the character can be a letter, 
digit, whitespace or any other symbol. A string can be 
created by enclosing one or more characters in single, 
double or triple quote.
Example 8.1 
>>> str1 = 'Hello World!'
>>> str2 = "Hello World!"
>>> str3 = """Hello World!"""
>>> str4 = '''Hello World!''
str1, str2, str3, str4 are all string variables 
having the same value 'Hello World!'. Values stored in 
str3 and str4 can be extended to multiple lines using 
triple codes as can be seen in the following example:
>>> str3 = """Hello World!
          welcome to the world of Python"""
>>> str4 = '''Hello World!
          welcome to the world of Python'''
“The great thing about a 
computer notebook is that no 
matter how much you stuff 
into it, it doesn't get bigger or 
heavier. ”
– Bill Gates
Chapter 8
Strings
In this chapter
 » Introduction to 
Strings
 » String Operations
 » Traversing a String
 » Strings Methods 
and Built-in 
Functions
 » Handling Strings
Ch 8.indd   175 08-Apr-19   12:39:21 PM
2024-25
Computer SCien Ce – Cla SS xi 176
8.2.1 Accessing Characters in a String 
Each individual character in a string can be accessed 
using a technique called indexing. The index specifies 
the character to be accessed in the string and is written 
in square brackets ([ ]). The index of the first character 
(from left) in the string is 0 and the last character is n-1 
where n is the length of the string. If we give index value 
out of this range then we get an IndexError. The index 
must be an integer (positive, zero or negative).
#initializes a string str1
>>> str1 = 'Hello World!'   
#gives the first character of str1
>>> str1[0]                  
 'H'
#gives seventh character of str1
>>> str1[6]                  
'W'
#gives last character of str1
>>> str1[11]                 
'!'
#gives error as index is out of range
>>> str1[15]                 
IndexError: string index out of range
The index can also be an expression including 
variables and operators but the expression must 
evaluate to an integer.
#an expression resulting in an integer index 
#so gives 6
th
 character of str1
>>> str1[2+4]        
 'W'                
#gives error as index must be an integer
>>> str1[1.5]       
TypeError: string indices must be integers
Python allows an index value to be negative also. 
Negative indices are used when we want to access the 
characters of the string from right to left. Starting from 
right hand side, the first character has the index as -1 
and the last character has the index –n where n is the 
length of the string. Table 8.1 shows the indexing of 
characters in the string ‘Hello World!’ in both the cases, 
i.e., positive and negative indices.
>>> str1[-1] #gives first character from right
'!'
>>> str1[-12]#gives last character from right
'H'
Python does not have 
a character data type. 
String of length one is 
considered as character.
Ch 8.indd   176 08-Apr-19   12:39:21 PM
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Page 3


8.1 Introduct Ion We have studied in Chapter 5, that a sequence is an 
orderly collection of items and each item is indexed by 
an integer. Following sequence data types in Python 
were also briefly introduced in Chapter 5.
• Strings
• Lists
• Tuples
Another data type ‘Dictionary’ was also introduced 
in chapter 5 which falls under the category of mapping. 
In this chapter, we will go through strings in detail. 
List will be covered in Chapter 9 whereas tuple and 
dictionary will be discussed in Chapter 10.
8.2 Str Ing S 
String is a sequence which is made up of one or more 
UNICODE characters. Here the character can be a letter, 
digit, whitespace or any other symbol. A string can be 
created by enclosing one or more characters in single, 
double or triple quote.
Example 8.1 
>>> str1 = 'Hello World!'
>>> str2 = "Hello World!"
>>> str3 = """Hello World!"""
>>> str4 = '''Hello World!''
str1, str2, str3, str4 are all string variables 
having the same value 'Hello World!'. Values stored in 
str3 and str4 can be extended to multiple lines using 
triple codes as can be seen in the following example:
>>> str3 = """Hello World!
          welcome to the world of Python"""
>>> str4 = '''Hello World!
          welcome to the world of Python'''
“The great thing about a 
computer notebook is that no 
matter how much you stuff 
into it, it doesn't get bigger or 
heavier. ”
– Bill Gates
Chapter 8
Strings
In this chapter
 » Introduction to 
Strings
 » String Operations
 » Traversing a String
 » Strings Methods 
and Built-in 
Functions
 » Handling Strings
Ch 8.indd   175 08-Apr-19   12:39:21 PM
2024-25
Computer SCien Ce – Cla SS xi 176
8.2.1 Accessing Characters in a String 
Each individual character in a string can be accessed 
using a technique called indexing. The index specifies 
the character to be accessed in the string and is written 
in square brackets ([ ]). The index of the first character 
(from left) in the string is 0 and the last character is n-1 
where n is the length of the string. If we give index value 
out of this range then we get an IndexError. The index 
must be an integer (positive, zero or negative).
#initializes a string str1
>>> str1 = 'Hello World!'   
#gives the first character of str1
>>> str1[0]                  
 'H'
#gives seventh character of str1
>>> str1[6]                  
'W'
#gives last character of str1
>>> str1[11]                 
'!'
#gives error as index is out of range
>>> str1[15]                 
IndexError: string index out of range
The index can also be an expression including 
variables and operators but the expression must 
evaluate to an integer.
#an expression resulting in an integer index 
#so gives 6
th
 character of str1
>>> str1[2+4]        
 'W'                
#gives error as index must be an integer
>>> str1[1.5]       
TypeError: string indices must be integers
Python allows an index value to be negative also. 
Negative indices are used when we want to access the 
characters of the string from right to left. Starting from 
right hand side, the first character has the index as -1 
and the last character has the index –n where n is the 
length of the string. Table 8.1 shows the indexing of 
characters in the string ‘Hello World!’ in both the cases, 
i.e., positive and negative indices.
>>> str1[-1] #gives first character from right
'!'
>>> str1[-12]#gives last character from right
'H'
Python does not have 
a character data type. 
String of length one is 
considered as character.
Ch 8.indd   176 08-Apr-19   12:39:21 PM
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String S 177
An inbuilt function len() in Python returns the length 
of the string that is passed as parameter. For example, 
the length of string str1 = 'Hello World!' is 12.
#gives the length of the string str1
>>> len(str1)            
12
#length of the string is assigned to n
>>> n = len(str1)       
>>> print(n)
12
#gives the last character of the string
>>> str1[n-1]            
'!'
#gives the first character of the string
>>> str1[-n]             
'H'
8.2.2 String is Immutable
A string is an immutable data type. It means that 
the contents of the string cannot be changed after it 
has been created. An attempt to do this would lead to 
an error.
>>> str1 = "Hello World!"    
#if we try to replace character 'e' with 'a'
>>> str1[1] = 'a'  
TypeError: 'str' object does not support item 
assignment
8.3 Str Ing o perat Ion S
As we know that string is a sequence of characters. 
Python allows certain operations on string data type, 
such as concatenation, repetition, membership and 
slicing. These operations are explained in the following 
subsections with suitable examples.
8.3.1 Concatenation
To concatenate means to join. Python allows us to join 
two strings using concatenation operator plus which is 
denoted by symbol +.
Table 8.1 Indexing of characters in string 'Hello World!' 
Positive Indices 0 1 2 3 4 5 6 7 8 9 10 11
String H e l l o W o r l d !
Negative Indices -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
Ch 8.indd   177 08-Apr-19   12:39:21 PM
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Page 4


8.1 Introduct Ion We have studied in Chapter 5, that a sequence is an 
orderly collection of items and each item is indexed by 
an integer. Following sequence data types in Python 
were also briefly introduced in Chapter 5.
• Strings
• Lists
• Tuples
Another data type ‘Dictionary’ was also introduced 
in chapter 5 which falls under the category of mapping. 
In this chapter, we will go through strings in detail. 
List will be covered in Chapter 9 whereas tuple and 
dictionary will be discussed in Chapter 10.
8.2 Str Ing S 
String is a sequence which is made up of one or more 
UNICODE characters. Here the character can be a letter, 
digit, whitespace or any other symbol. A string can be 
created by enclosing one or more characters in single, 
double or triple quote.
Example 8.1 
>>> str1 = 'Hello World!'
>>> str2 = "Hello World!"
>>> str3 = """Hello World!"""
>>> str4 = '''Hello World!''
str1, str2, str3, str4 are all string variables 
having the same value 'Hello World!'. Values stored in 
str3 and str4 can be extended to multiple lines using 
triple codes as can be seen in the following example:
>>> str3 = """Hello World!
          welcome to the world of Python"""
>>> str4 = '''Hello World!
          welcome to the world of Python'''
“The great thing about a 
computer notebook is that no 
matter how much you stuff 
into it, it doesn't get bigger or 
heavier. ”
– Bill Gates
Chapter 8
Strings
In this chapter
 » Introduction to 
Strings
 » String Operations
 » Traversing a String
 » Strings Methods 
and Built-in 
Functions
 » Handling Strings
Ch 8.indd   175 08-Apr-19   12:39:21 PM
2024-25
Computer SCien Ce – Cla SS xi 176
8.2.1 Accessing Characters in a String 
Each individual character in a string can be accessed 
using a technique called indexing. The index specifies 
the character to be accessed in the string and is written 
in square brackets ([ ]). The index of the first character 
(from left) in the string is 0 and the last character is n-1 
where n is the length of the string. If we give index value 
out of this range then we get an IndexError. The index 
must be an integer (positive, zero or negative).
#initializes a string str1
>>> str1 = 'Hello World!'   
#gives the first character of str1
>>> str1[0]                  
 'H'
#gives seventh character of str1
>>> str1[6]                  
'W'
#gives last character of str1
>>> str1[11]                 
'!'
#gives error as index is out of range
>>> str1[15]                 
IndexError: string index out of range
The index can also be an expression including 
variables and operators but the expression must 
evaluate to an integer.
#an expression resulting in an integer index 
#so gives 6
th
 character of str1
>>> str1[2+4]        
 'W'                
#gives error as index must be an integer
>>> str1[1.5]       
TypeError: string indices must be integers
Python allows an index value to be negative also. 
Negative indices are used when we want to access the 
characters of the string from right to left. Starting from 
right hand side, the first character has the index as -1 
and the last character has the index –n where n is the 
length of the string. Table 8.1 shows the indexing of 
characters in the string ‘Hello World!’ in both the cases, 
i.e., positive and negative indices.
>>> str1[-1] #gives first character from right
'!'
>>> str1[-12]#gives last character from right
'H'
Python does not have 
a character data type. 
String of length one is 
considered as character.
Ch 8.indd   176 08-Apr-19   12:39:21 PM
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String S 177
An inbuilt function len() in Python returns the length 
of the string that is passed as parameter. For example, 
the length of string str1 = 'Hello World!' is 12.
#gives the length of the string str1
>>> len(str1)            
12
#length of the string is assigned to n
>>> n = len(str1)       
>>> print(n)
12
#gives the last character of the string
>>> str1[n-1]            
'!'
#gives the first character of the string
>>> str1[-n]             
'H'
8.2.2 String is Immutable
A string is an immutable data type. It means that 
the contents of the string cannot be changed after it 
has been created. An attempt to do this would lead to 
an error.
>>> str1 = "Hello World!"    
#if we try to replace character 'e' with 'a'
>>> str1[1] = 'a'  
TypeError: 'str' object does not support item 
assignment
8.3 Str Ing o perat Ion S
As we know that string is a sequence of characters. 
Python allows certain operations on string data type, 
such as concatenation, repetition, membership and 
slicing. These operations are explained in the following 
subsections with suitable examples.
8.3.1 Concatenation
To concatenate means to join. Python allows us to join 
two strings using concatenation operator plus which is 
denoted by symbol +.
Table 8.1 Indexing of characters in string 'Hello World!' 
Positive Indices 0 1 2 3 4 5 6 7 8 9 10 11
String H e l l o W o r l d !
Negative Indices -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
Ch 8.indd   177 08-Apr-19   12:39:21 PM
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Computer SCien Ce – Cla SS xi 178
>>> str1 = 'Hello'      #First string
>>> str2 = 'World!'     #Second string
>>> str1 + str2         #Concatenated strings
'HelloWorld!'
    #str1 and str2 remain same  
>>> str1  #after this operation.
'Hello'
>>> str2
'World!'
8.3.2 Repetition
Python allows us to repeat the given string using 
repetition operator which is denoted by symbol *.
#assign string 'Hello' to str1
>>> str1 = 'Hello'        
#repeat the value of str1 2 times
>>> str1 * 2              
'HelloHello'
#repeat the value of str1 5 times
>>> str1 * 5              
'HelloHelloHelloHelloHello'
Note: str1 still remains the same after the use of 
repetition operator.
8.3.3 Membership
Python has two membership operators 'in' and 'not 
in'. The 'in' operator takes two strings and returns 
True if the first string appears as a substring in the 
second string, otherwise it returns False.
>>> str1 = 'Hello World!'
>>> 'W' in str1
True
>>> 'Wor' in str1
True
>>> 'My' in str1
False
The 'not in' operator also takes two strings and 
returns True if the first string does not appear as a 
substring in the second string, otherwise returns False.
>>> str1 = 'Hello World!'
>>> 'My' not in str1    
True
>>> 'Hello' not in str1
False
8.3.4 Slicing
In Python, to access some part of a string or substring, 
we use a method called slicing. This can be done by 
Ch 8.indd   178 08-Apr-19   12:39:21 PM
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Page 5


8.1 Introduct Ion We have studied in Chapter 5, that a sequence is an 
orderly collection of items and each item is indexed by 
an integer. Following sequence data types in Python 
were also briefly introduced in Chapter 5.
• Strings
• Lists
• Tuples
Another data type ‘Dictionary’ was also introduced 
in chapter 5 which falls under the category of mapping. 
In this chapter, we will go through strings in detail. 
List will be covered in Chapter 9 whereas tuple and 
dictionary will be discussed in Chapter 10.
8.2 Str Ing S 
String is a sequence which is made up of one or more 
UNICODE characters. Here the character can be a letter, 
digit, whitespace or any other symbol. A string can be 
created by enclosing one or more characters in single, 
double or triple quote.
Example 8.1 
>>> str1 = 'Hello World!'
>>> str2 = "Hello World!"
>>> str3 = """Hello World!"""
>>> str4 = '''Hello World!''
str1, str2, str3, str4 are all string variables 
having the same value 'Hello World!'. Values stored in 
str3 and str4 can be extended to multiple lines using 
triple codes as can be seen in the following example:
>>> str3 = """Hello World!
          welcome to the world of Python"""
>>> str4 = '''Hello World!
          welcome to the world of Python'''
“The great thing about a 
computer notebook is that no 
matter how much you stuff 
into it, it doesn't get bigger or 
heavier. ”
– Bill Gates
Chapter 8
Strings
In this chapter
 » Introduction to 
Strings
 » String Operations
 » Traversing a String
 » Strings Methods 
and Built-in 
Functions
 » Handling Strings
Ch 8.indd   175 08-Apr-19   12:39:21 PM
2024-25
Computer SCien Ce – Cla SS xi 176
8.2.1 Accessing Characters in a String 
Each individual character in a string can be accessed 
using a technique called indexing. The index specifies 
the character to be accessed in the string and is written 
in square brackets ([ ]). The index of the first character 
(from left) in the string is 0 and the last character is n-1 
where n is the length of the string. If we give index value 
out of this range then we get an IndexError. The index 
must be an integer (positive, zero or negative).
#initializes a string str1
>>> str1 = 'Hello World!'   
#gives the first character of str1
>>> str1[0]                  
 'H'
#gives seventh character of str1
>>> str1[6]                  
'W'
#gives last character of str1
>>> str1[11]                 
'!'
#gives error as index is out of range
>>> str1[15]                 
IndexError: string index out of range
The index can also be an expression including 
variables and operators but the expression must 
evaluate to an integer.
#an expression resulting in an integer index 
#so gives 6
th
 character of str1
>>> str1[2+4]        
 'W'                
#gives error as index must be an integer
>>> str1[1.5]       
TypeError: string indices must be integers
Python allows an index value to be negative also. 
Negative indices are used when we want to access the 
characters of the string from right to left. Starting from 
right hand side, the first character has the index as -1 
and the last character has the index –n where n is the 
length of the string. Table 8.1 shows the indexing of 
characters in the string ‘Hello World!’ in both the cases, 
i.e., positive and negative indices.
>>> str1[-1] #gives first character from right
'!'
>>> str1[-12]#gives last character from right
'H'
Python does not have 
a character data type. 
String of length one is 
considered as character.
Ch 8.indd   176 08-Apr-19   12:39:21 PM
2024-25
String S 177
An inbuilt function len() in Python returns the length 
of the string that is passed as parameter. For example, 
the length of string str1 = 'Hello World!' is 12.
#gives the length of the string str1
>>> len(str1)            
12
#length of the string is assigned to n
>>> n = len(str1)       
>>> print(n)
12
#gives the last character of the string
>>> str1[n-1]            
'!'
#gives the first character of the string
>>> str1[-n]             
'H'
8.2.2 String is Immutable
A string is an immutable data type. It means that 
the contents of the string cannot be changed after it 
has been created. An attempt to do this would lead to 
an error.
>>> str1 = "Hello World!"    
#if we try to replace character 'e' with 'a'
>>> str1[1] = 'a'  
TypeError: 'str' object does not support item 
assignment
8.3 Str Ing o perat Ion S
As we know that string is a sequence of characters. 
Python allows certain operations on string data type, 
such as concatenation, repetition, membership and 
slicing. These operations are explained in the following 
subsections with suitable examples.
8.3.1 Concatenation
To concatenate means to join. Python allows us to join 
two strings using concatenation operator plus which is 
denoted by symbol +.
Table 8.1 Indexing of characters in string 'Hello World!' 
Positive Indices 0 1 2 3 4 5 6 7 8 9 10 11
String H e l l o W o r l d !
Negative Indices -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
Ch 8.indd   177 08-Apr-19   12:39:21 PM
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Computer SCien Ce – Cla SS xi 178
>>> str1 = 'Hello'      #First string
>>> str2 = 'World!'     #Second string
>>> str1 + str2         #Concatenated strings
'HelloWorld!'
    #str1 and str2 remain same  
>>> str1  #after this operation.
'Hello'
>>> str2
'World!'
8.3.2 Repetition
Python allows us to repeat the given string using 
repetition operator which is denoted by symbol *.
#assign string 'Hello' to str1
>>> str1 = 'Hello'        
#repeat the value of str1 2 times
>>> str1 * 2              
'HelloHello'
#repeat the value of str1 5 times
>>> str1 * 5              
'HelloHelloHelloHelloHello'
Note: str1 still remains the same after the use of 
repetition operator.
8.3.3 Membership
Python has two membership operators 'in' and 'not 
in'. The 'in' operator takes two strings and returns 
True if the first string appears as a substring in the 
second string, otherwise it returns False.
>>> str1 = 'Hello World!'
>>> 'W' in str1
True
>>> 'Wor' in str1
True
>>> 'My' in str1
False
The 'not in' operator also takes two strings and 
returns True if the first string does not appear as a 
substring in the second string, otherwise returns False.
>>> str1 = 'Hello World!'
>>> 'My' not in str1    
True
>>> 'Hello' not in str1
False
8.3.4 Slicing
In Python, to access some part of a string or substring, 
we use a method called slicing. This can be done by 
Ch 8.indd   178 08-Apr-19   12:39:21 PM
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String S 179
specifying an index range. Given a string str1, the 
slice operation str1[n:m] returns the part of the string 
str1 starting from index n (inclusive) and ending at m 
(exclusive). In other words, we can say that str1[n:m] 
returns all the characters starting from str1[n] till 
str1[m-1]. The numbers of characters in the substring 
will always be equal to difference of two indices m and 
n, i.e., (m-n).
>>> str1 = 'Hello World!'
#gives substring starting from index 1 to 4
>>> str1[1:5]  
'ello'
#gives substring starting from 7 to 9
>>> str1[7:10]  
'orl'
#index that is too big is truncated down to
#the end of the string
>>> str1[3:20]    
'lo World!'   
#first index > second index results in an 
#empty '' string
 >>> str1[7:2]
If the first index is not mentioned, the slice starts 
from index. 
#gives substring from index 0 to 4
>>> str1[:5]
'Hello'
If the second index is not mentioned, the slicing is 
done till the length of the string.
#gives substring from index 6 to end
>>> str1[6:]   
'World!'
The slice operation can also take a third index that 
specifies the ‘step size’. For example, str1[n:m:k], 
means every k
th
 character has to be extracted from 
the string str1 starting from n and ending at m-1. By 
default, the step size is one.
>>> str1[0:10:2]
'HloWr'
>>> str1[0:10:3]
'HlWl'
Negative indexes can also be used for slicing.
   #characters at index -6,-5,-4,-3 and -2 are   
  #sliced
>>> str1[-6:-1]   
Ch 8.indd   179 21-May-19   12:26:40 PM
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