Strings in Python Class 11 Notes

Teachers and Examiners (CBSESkillEduction) collaborated to create the Strings in Python Class 11 Notes. All the important Information are taken from the NCERT Textbook Computer Science (083) class 11.

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Strings

A string is a group of one or more UNICODE characters in sequence. A letter, number, space, or any other sign may be used as the character in this situation. One or more characters can be enclosed in a single, double, or triple quote to produce a string.

Example –

>>> str1 = ‘Hello World!’
>>> str2 = “Hello World!”
>>> str3 = “””Hello World!”””
>>> str4 = ”’Hello World!”

Note – Python accepts single (‘), double (“), triple (”’) or triple(“””) quotes to denote string literals. Single quoted strings and double quoted strings are equal. Triple quotes are used for contain special characters like TAB, or NEWLINES.

Strings in Python Class 11 Notes

Accessing Characters in a String

A method known as indexing can be used to retrieve each individual character in a string. The character to be retrieved in the string is specified by the index, which is enclosed in square brackets ([ ]). The string has an index of 0 for the first character (counted from the left) and n-1 for the last character, where n is the string’s length. We receive an IndexError if we provide an index value outside of this range. A number must make up the index (positive, zero or negative).

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

Strings in Python Class 11 Notes

String Operations

A string is a group of letters and numbers. Concatenation, repetition, membership, and slicing are just a few of the operations Python supports on the string data type. The following subsections provide explanations of these procedures along with pertinent examples.

Concatenation

To concatenate means to join. Python allows us to join two strings using concatenation operator plus which is denoted by symbol +.

>>> str1 = ‘Hello’
>>> str2 = ‘World!’
>>> str1 + str2

Output:
Hello World!

Repetition

Python allows us to repeat the given string using repetition operator which is denoted by symbol *.

>>> str1 = ‘Hello’
>>> str1 * 2

Output:
>>> str1 * 5
‘HelloHelloHelloHelloHello’

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.

In

>>> str1 = ‘Hello World!’
>>> ‘W’ in str1
True
>>> ‘Wor’ in str1
True
>>> ‘My’ in str1
False

Not In

>>> str1 = ‘Hello World!’
>>> ‘My’ not in str1
True
>>> ‘Hello’ not in str1
False

Slicing

In Python, to access some part of a string or substring, we use a method called slicing. This can be done by specifying an index range. Given a string str1, the slice operation str1[n:m].

>>> str1 = ‘Hello World!’
>>> str1[1:5]
‘ello’
>>> str1[7:10]
‘orl’
>>> str1[3:20]
‘lo World!’
>>> str1[7:2]

Strings in Python Class 11 Notes

Traversing a String

We can access each character of a string or traverse a string using for loop and while loop.

String Traversal Using for Loop:

>>> str1 = ‘Hello World!’
>>> for ch in str1:
print(ch,end = ”)
Hello World!

In the above code, the loop starts from the first character of the string str1 and automatically ends when the last character is accessed.

String Traversal Using while Loop:

>>> str1 = ‘Hello World!’
>>> index = 0>>> while index < len(str1):
print(str1[index],end = ”)
index += 1
Hello World!

Here while loop runs till the condition index < len(str) is True, where index varies from 0 to len(str1) -1.

Strings in Python Class 11 Notes

String Methods and Built-in Functions

There are numerous built-in functions in Python that let us operate with strings. a few of the most popular built-in functions for manipulating strings.

len()

Returns the length of the given string

>>> str1 = ‘Hello World!’
>>> len(str1)
12

title()

Returns the string with first letter of every word in the string in uppercase and rest in lowercase

>>> str1 = ‘hello WORLD!’
>>> str1.title()
‘Hello World!’

lower()

Returns the string with all uppercase letters converted to lowercase

>>> str1 = ‘hello WORLD!’
>>> str1.lower()
‘hello world!’

upper()

Returns the string with all lowercase letters converted to uppercase

>>> str1 = ‘hello WORLD!’
>>> str1.upper()
‘HELLO WORLD!’

count(str, start, end)

Returns number of times substring str occurs in the given string. If we do not give start index and end index then searching starts from index 0 and ends at length of the string.

>>> str1 = ‘Hello World! Hello
Hello’
>>> str1.count(‘Hello’,12,25)
2
>>> str1.count(‘Hello’)
3

find(str, start, end)

Returns the first occurrence of index of substring str occurring in the given string. If we do not give start and end then searching starts from index 0 and ends at length of the string. If the substring is not present in the given string, then the function returns -1

>>> str1 = ‘Hello World! Hello Hello’
>>> str1.find(‘Hello’,10,20)
13
>>> str1.find(‘Hello’,15,25)
19
>>> str1.find(‘Hello’)
0
>>> str1.find(‘Hee’)
-1

index(str, start, end)

Same as find() but raises an exception if the substring is not present in the given string

>>> str1 = ‘Hello World! Hello
Hello’
>>> str1.index(‘Hello’)
0
>>> str1.index(‘Hee’)
ValueError: substring not found

Strings in Python Class 11 Notes

endswith()

Returns True if the given string ends with the supplied substring otherwise returns False

>>> str1 = ‘Hello World!’
>>> str1.endswith(‘World!’)
True
>>> str1.endswith(‘!’)
True
>>> str1.endswith(‘lde’)
False

startswith()

Returns True if the given string starts with the supplied substring otherwise returns False

>>> str1 = ‘Hello World!’
>>> str1.startswith(‘He’)
True
>>> str1.startswith(‘Hee’)
False

isalnum()

Returns True if characters of the given string are either alphabets or numeric. If whitespace or special symbols are part of the given string or the string is empty it returns False

>>> str1 = ‘HelloWorld’
>>> str1.isalnum()
True
>>> str1 = ‘HelloWorld2’
>>> str1.isalnum()
True
>>> str1 = ‘HelloWorld!!’
>>> str1.isalnum()
False

islower()

Returns True if the string is non-empty and has all lowercase alphabets, or has at least one character as lowercase alphabet and rest are non-alphabet characters

>>> str1 = ‘hello world!’
>>> str1.islower()
True
>>> str1 = ‘hello 1234’
>>> str1.islower()
True
>>> str1 = ‘hello ??’
>>> str1.islower()
True
>>> str1 = ‘1234’
>>> str1.islower()
False
>>> str1 = ‘Hello World!’
>>> str1.islower()
False

isupper()

Returns True if the string is non-empty and has all uppercase alphabets, or has at least one character as uppercase character and rest are non-alphabet characters

>>> str1 = ‘HELLO WORLD!’
>>> str1.isupper()
True
>>> str1 = ‘HELLO 1234’
>>> str1.isupper()
True
>>> str1 = ‘HELLO ??’
>>> str1.isupper()
True
>>> str1 = ‘1234’
>>> str1.isupper()
False
>>> str1 = ‘Hello World!’
>>> str1.isupper()
False

isspace()

Returns True if the string is non-empty and all characters are white spaces (blank, tab, newline, carriage return)

>>> str1 = ‘ \n \t \r’
>>> str1.isspace()
True
>>> str1 = ‘Hello \n’
>>> str1.isspace()
False

istitle()

Returns True if the string is non-empty and title case, i.e., the first letter of every word in the string in uppercase and rest in lowercase

>>> str1 = ‘Hello World!’
>>> str1.istitle()
True
>>> str1 = ‘hello World!’
>>> str1.istitle()
False

lstrip()

Returns the string after removing the spaces only on the left of the string

>>> str1 = ‘ Hello World!’
>>> str1.lstrip()
‘Hello World!

Strings in Python Class 11 Notes

rstrip()

Returns the string after removing the spaces only on the right of the string

>>> str1 = ‘ Hello World!’
>>> str1.rstrip()
‘ Hello World!’

strip()

Returns the string after removing the spaces both on the left and the right of the string

>>> str1 = ‘ Hello World!’
>>> str1.strip()
‘Hello World!’

replace(oldstr, newstr)

Replaces all occurrences of old string with the new string

>>> str1 = ‘Hello World!’
>>> str1.replace(‘o’,’*’)
‘Hell* W*rld!’
>>> str1 = ‘Hello World!’
>>> str1.replace(‘World’,’Country’)
‘Hello Country!’
>>> str1 = ‘Hello World! Hello’
>>> str1.replace(‘Hello’,’Bye’)
‘Bye World! Bye’

join()

Returns a string in which the characters in the string have been joined by a separator

>>> str1 = (‘HelloWorld!’)
>>> str2 = ‘-‘ #separator
>>> str2.join(str1)
‘H-e-l-l-o-W-o-r-l-d-!’

partition()

Partitions the given string at the first occurrence of the substring (separator) and returns the string partitioned into three parts.
1. Substring before the separator
2. Separator
3. Substring after the separator
If the separator is not found in the string, it returns the whole string itself and two empty strings

>>> str1 = ‘India is a Great Country’
>>> str1.partition(‘is’)
(‘India ‘, ‘is’, ‘ a GreatCountry’)
>>> str1.partition(‘are’)
(‘India is a Great Country’,’ ‘,”)

split()

Returns a list of words delimited by the specified substring. If no delimiter is given then words are separated by space.

>>> str1 = ‘India is a Great Country’
>>> str1.split()
[‘India’,’is’,’a’,’Great’, ‘Country’]
>>> str1 = ‘India is a Great Country’
>>> str1.split(‘a’)
[‘Indi’, ‘ is ‘, ‘ Gre’, ‘t Country’]

Strings in Python Class 11 Notes

Handling Strings

In this section, we’ll discover how to use user-defined functions in Python to manipulate strings in various ways.

Q. Write a program with a user defined function to count the number of times a character (passed as argument) occurs in the given string.

#Program
#Function to count the number of times a character occurs in a
def charCount(ch,st):
count = 0
for character in st:
if character == ch:
count += 1
return count
st = input(“Enter a string: “)
ch = input(“Enter the character to be searched: “)
count = charCount(ch,st)
print(“Number of times character”,ch,”occurs in the string is:”,count)

Output:
Enter a string: Today is a Holiday
Enter the character to be searched: a
Number of times character a occurs in the string is: 3

Strings in Python Class 11 Notes

Q. Write a program with a user defined function with string as a parameter which replaces all vowels in the string with ‘*’.

#Program
#Function to replace all vowels in the string with ‘*’
def replaceVowel(st):
newstr = ”
for character in st:
if character in ‘aeiouAEIOU’:
newstr += ‘*’
else:
newstr += character
return newstr
st = input(“Enter a String: “)
st1 = replaceVowel(st)
print(“The original String is:”,st)
print(“The modified String is:”,st1)

Output:
Enter a String: Hello World
The original String is: Hello World
The modified String is: H*ll* W*rld

Q. Write a program to input a string from the user and print it in the reverse order without creating a new string.

#Program
#Program to display string in reverse order
st = input(“Enter a string: “)
for i in range(-1,-len(st)-1,-1):
print(st[i],end=”)

Output:
Enter a string: Hello World
dlroW olleH

Q. Write a program which reverses a string passed as parameter and stores the reversed string in a new string. Use a user defined function for reversing the string.

#Program
#Function to reverse a string
def reverseString(st):
newstr = ” #create a new string
length = len(st)
for i in range(-1,-length-1,-1):
newstr += st[i]
return newstr
#end of function
st = input(“Enter a String: “)
st1 = reverseString(st)
print(“The original String is:”,st)
print(“The reversed String is:”,st1)

Output:
Enter a String: Hello World
The original String is: Hello World
The reversed String is: dlroW olleH

Strings in Python Class 11 Notes

Q. Write a program using a user defined function to check if a string is a palindrome or not. (A string is called palindrome if it reads same backwards as forward. For example, Kanak is a palindrome.)

#Program
#Function to check if a string is palindrome or not
def checkPalin(st):
i = 0
j = len(st) – 1
while(i <= j):
if(st[i] != st[j]):
return False
i += 1
j -= 1
return True
#end of function
st = input(“Enter a String: “)
result = checkPalin(st)
if result == True:
print(“The given string”,st,”is a palindrome”)
else:
print(“The given string”,st,”is not a palindrome”)

Output 1:
Enter a String: kanak
The given string kanak is a palindrome
Output 2:
Enter a String: computer
The given string computer is not a palindrome

Computer Science Class 11 Notes

Unit 1 : Basic Computer Organisation
Unit 1 : Encoding Schemes and Number System
Unit 2 : Introduction to problem solving
Unit 2 : Getting Started with Python
Unit 2 : Conditional statement and Iterative statements in Python
Unit 2 : Function in Python
Unit 2 : String in Python
Unit 2 : Lists in Python
Unit 2 : Tuples in Python
Unit 2 : Dictionary in Python
Unit 3 : Society, Law and Ethics

Computer Science Class 11 MCQ

Computer Science Class 11 NCERT Solutions

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