Computer Science
Class -XI
Code No. 083
2023-24




CLASS-XI
SUBJECT: COMPUTER SCIENCE (083) – PYTHON
INDEX
NO. CHAPTER NAME 
1 INTRODUCTION TO PYTHON 
2 PYTHON FUNDAMENTALS 
3 DATA HANDLING 
4 FLOW OF CONTROL 
5 FUNCTIONS IN PYTHON 
6 STRING IN PYTHON 






CHAPTER-6
STRING IN PYTHON
6.1 Introduction:
Definition: Sequence of characters enclosed in single, 
double or triple quotation marks.
Basics of String:
 Strings are immutable in python. It means it is 
unchangeable. At the same memory
address, the new value cannot be stored.
 Each character has its index or can be accessed 
using its index.
 String in python has two-way index for each location. 
(0, 1, 2, ……. In the forward
direction and -1, -2, -3, …….. in the backward direction.)
Example:
0 1 2 3 4 5 6 7
k e n d r i y a
-8 -7 -6 -5 -4 -3 -2 -1
 The index of string in forward direction starts 
from 0 and in backward direction starts
from -1.
 The size of string is total number of characters
present in the string. (If there are n
characters in the string, then last index in forward 
direction would be n-1 and last index
in backward direction would be –n.)
 String are stored each character in contiguous location.
 The character assignment is not supported in string 
because strings are immutable.
Example :
str = “kendriya”
str[2] = ‘y’ # it is invalid. Individual letter 
assignment not allowed in python
6.2 Traversing a String:
Access the elements of string, one character at a time.
str = “kendriya”
for ch in str :
print(ch, end= ‘ ‘)
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ii. String repetition Operator: It is also known as 
String replication operator. It requires
two types of operands- a string and an integer number.
Example:
>>>”you” * 3
‘youyouyou’
>>>3*”you”
‘youyouyou’
Note:You cannot have strings as n=both the operands 
with * operator.
Example:
>>>”you” * “you” # can't multiply sequence by 
non-int of type 'str'
It is invalid and generates an error.
b. Membership Operators:
in – Returns True if a character or a substring 
exists in the given string; otherwise False
not in - Returns True if a character or a substring 
does not exist in the given string; otherwise False
Example:
>>> "ken" in "Kendriya Vidyalaya"
False
>>> "Ken" in "Kendriya Vidyalaya"
True
>>>"ya V" in "Kendriya Vidyalaya"
True
>>>"8765" not in "9876543"
False
c. Comparison Operators: These operators compare 
two strings character by character
according to their ASCII value.
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Characters ASCII (Ordinal) Value
‘0’ to ‘9’ 48 to 57
‘A’ to ‘Z’ 65 to 90
‘a’ to ‘z’ 97 to 122
Example:
>>> 'abc'>'abcD'
False
>>> 'ABC'<'abc'
True
>>> 'abcd'>'aBcD'
True
>>> 'aBcD'<='abCd'
True
6.4 Finding the Ordinal or Unicode value of a character:
Function Description
ord() Returns ordinal value of a character
chr() Returns the corresponding character
Example:
>>> ord('b')
98
>>> chr(65)
'A'
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Program: Write a program to display ASCII code of 
a character and vice versa.
var=True
while var:
choice=int(input("Press-1 to find the ordinal value 
\n Press-2 to find a character of a value\n"))
if choice==1:
ch=input("Enter a character : ")
print(ord(ch))
elif choice==2:
val=int(input("Enter an integer value: "))
print(chr(val))
else:
print("You entered wrong choice")
print("Do you want to continue? Y/N")
option=input()
if option=='y' or option=='Y':
var=True
else:
var=False
6.5 Slice operator with Strings:
The slice operator slices a string using a range of indices.
Syntax:
string-name[start:end]
where start and end are integer indices. It returns 
a string from the index start to end-1.
0 1 2 3 4 5 6 7 8 9 10 11 12 13
d a t a s t r u c t u r e
-14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
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Example:
>>> str="data structure"
>>> str[0:14]
'data structure'
>>> str[0:6]
'data s'
>>> str[2:7]
'ta st'
>>> str[-13:-6]
'ata str'
>>> str[-5:-11]
' ' #returns empty string
>>> str[:14] # Missing index before colon is considered as 0.
'data structure'
>>> str[0:] # Missing index after colon is considered as 14.
(length of string)
'data structure'
>>> str[7:]
'ructure'
>>> str[4:]+str[:4]
' structuredata'
>>> str[:4]+str[4:] #for any index str[:n]+str[n:] returns 
original string
'data structure'
>>> str[8:]+str[:8]
'ucturedata str'
>>> str[8:], str[:8]
('ucture', 'data str')
Slice operator with step index:
Slice operator with strings may have third index. 
Which is known as step. It is optional.
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Syntax:
string-name[start:end:step]
Example:
>>> str="data structure"
>>> str[2:9:2]
't tu'
>>> str[-11:-3:3]
'atc'
>>> str[: : -1] # reverses a string
'erutcurts atad'
Interesting Fact: Index out of bounds causes error 
with strings but slicing a string outside the
index does not cause an error.
Example:
>>>str[14]
IndexError: string index out of range
>>> str[14:20] # both indices are outside the bounds
' ' # returns empty string
>>> str[10:16]
'ture'
Reason: When you use an index, you are accessing 
a particular character of a string, thus the
index must be valid and out of bounds index causes 
an error as there is no character to return
from the given index.
But slicing always returns a substring or empty string, 
which is valid sequence.
6.6 Built-in functions of string:
Example:
str=”data structure”
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s1= “hello365”
s2= “python”
s3 = ‘4567’
s4 = ‘ ‘
s5= ‘comp34%@’
S. No. Function Description Example
1 len( ) Returns the length of a string >>>print(len(str))
14
2 capitalize( ) Returns a string with its first character
capitalized.
>>> str.capitalize()
'Data structure'
3 find(sub,start,end) Returns the lowest index in 
the string where the
substring sub is found within the slice range.
Returns -1 if sub is not found.
>>> str.find("ruct",5,13)
7
>>> str.find("ruct",8,13)
-1
4 isalnum( ) Returns True if the characters in the string are
alphabets or numbers. False otherwise
>>>s1.isalnum( )
True
>>>s2.isalnum( )
True
>>>s3.isalnum( )
True
>>>s4.isalnum( )
False
>>>s5.isalnum( )
False
5 isalpha( ) Returns True if all characters in the string are
alphabetic. False otherwise.
>>>s1.isalpha( )
False
>>>s2.isalpha( )
True
>>>s3.isalpha( )
False
>>>s4.isalpha( )
False
>>>s5.isalpha( )
False
6 isdigit( ) Returns True if all the characters in the string are
digits. False otherwise.
>>>s1.isdigit( )
False
>>>s2.isdigit( )
False
>>>s3.isdigit( )
True
>>>s4.isdigit( )
False
>>>s5.isdigit( )
False
7 islower( ) Returns True if all the characters in the string are
lowercase. False otherwise.
>>> s1.islower()
True
>>> s2.islower()
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True
>>> s3.islower()
False
>>> s4.islower()
False
>>> s5.islower()
True
8 isupper( ) Returns True if all the characters in the string are
uppercase. False otherwise.
>>> s1.isupper()
False
>>> s2.isupper()
False
>>> s3.isupper()
False
>>> s4.isupper()
False
>>> s5.isupper()
False
9 isspace( ) Returns True if there are only whitespace
characters in the string. False otherwise.
>>> " ".isspace()
True
>>> "".isspace()
False
10 lower( ) Converts a string in lowercase characters. 
>>> "HeLlo".lower()
'hello'
11 upper( ) Converts a string in uppercase characters. 
>>> "hello".upper()
'HELLO'
12 lstrip( ) Returns a string after removing the leading
characters. (Left side).
if used without any argument, it removes the
leading whitespaces.
>>> str="data structure"
>>> str.lstrip('dat')
' structure'
>>> str.lstrip('data')
' structure'
>>> str.lstrip('at')
'data structure'
>>> str.lstrip('adt')
' structure'
>>> str.lstrip('tad')
' structure'
13 rstrip( ) Returns a string after removing the trailing
characters. (Right side).
if used without any argument, it removes the
trailing whitespaces.
>>> str.rstrip('eur')
'data struct'
>>> str.rstrip('rut')
'data structure'
>>> str.rstrip('tucers')
'data '
14 split( ) breaks a string into words and creates 
a list out of it >>> str="Data Structure"
>>> str.split( )
['Data', 'Structure']
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Programs related to Strings:
1. Write a program that takes a string with multiple 
words and then capitalize the first
letter of each word and forms a new string out of it.
Solution:
s1=input("Enter a string : ")
length=len(s1)
a=0
end=length
s2="" #empty string
while a