This document provides an introduction and overview of strings in Python. It discusses that strings are a data type that can contain sequences of characters. The built-in string class is 'str' and strings can be defined using single, double, or triple quotes. Strings support various methods like indexing, slicing, concatenation, formatting and more. Common string methods are also described such as upper(), lower(), split(), join() which allow manipulating strings. The document also discusses comparing and slicing strings in Python.

1. Unit IV
Strings
Subject :PPS
Subject Teacher :Ms.Mhaske N.R.

2. Ms.Mhaske N.R. (PPS) 2
Introduction
● String is a data type in python
● String is a sequence made up of one or more individual characters
● Where characters could be a letter, digit,whitespace or any other
symbol
● Python has inbuild class name 'str' that has many useful feature
● We can declare and define a string by creating a variable of string type.
● String literal can be enclosed by either triple, double,or single quotes.
● Escape sequence with each type of string literal
● E,g. name=”India”
● Graduate='B.E.'

3. Ms.Mhaske N.R. (PPS) 3
Reading and Converting
• We prefer to read data
in using strings and
then parse and convert
the data as we need
• This gives us more
control over error
situations and/or bad
user input
• Raw input numbers
must be converted
from strings
>>> name = raw_input('Enter:')
Enter:Chuck
>>> print name
Chuck
>>> apple = raw_input('Enter:')
Enter:100
>>> x = apple – 10
Traceback (most recent call last):
File "<stdin>", line 1, in
<module>TypeError: unsupported
operand type(s) for -: 'str' and 'int'
>>> x = int(apple) – 10
>>> print x
90

4. Indexing
• We can get at any single
character in a string using
an index specified in square
brackets
• The index value must be an
integer and starts at zero
• The index value can be an
expression that is computed
>>> fruit = 'banana'
>>> letter = fruit[1]
>>> print letter
a
>>> n = 3
>>> w = fruit[n - 1]
>>> print w
n
00
bb
11
aa
22
nn
33
aa
44
nn
55
aa

5. Traversing a string
● A string can be traversed by accessing characters from one index to another
● E,g.
message=”hello”
Index=0
for i in message:
print(“message[“ , index,” ] ”= ”, i)
index += 1
OUTPUT: message[0]=h
message[1]=e
message[2]=l
message[3]=l
message[4]=o
●

6. Concatinating,Appending and Multiplaying
strings
● Concatenate :means to join together
● '+' opeartor is used for concating purpose.
● The * operator can be used to repeat the string for a given number of
times.
str1 = 'Hello'
str2 ='World!'
# using +
print('str1 + str2 = ', str1 + str2)
# using *
print('str1 * 3 =', str1 * 3)
OUTPUT:
tr1 + str2 = HelloWorld!
str1 * 3 = HelloHelloHello

7. ● Append a string :to add something at the end.
● In python you can add one string at the end of another string using the
' +=' operator
● E.g.
Str='hello'
name=raw_input(“n enter your name”)
str+=name
Str+=”.welcome to pune”
print(str)
Output:
enter your name:ashish
hello ashish.welcome to pune

8. ● Iterating Through String :Using for loop we can
iterate through a string. Here is an example to count
the number of 'l' in a string.
count = 0
for letter in 'Hello World':
if(letter == 'l'):
count += 1
print(count,'letters found')
OUTPUT:
3 letters found

9. String Membership Test
● We can test if a sub string exists within a string or not, using the
keyword in
>>> 'a' in 'program'
True
>>> 'at' not in 'battle'
False

10. String are Immutable
● Python string are immutable
● That is once created they cannot be changed.
● Whenever you try to modify an existing string variable, a new string is
created.
● Every object in python is stored in memory .
● You can find out whether two varibles are referring to the same object or not
by using the id().
● The id() returns the memory address of that object.
● As both str1 and str2 points to same memory location, they both point to the
same object

11. str1=”Hello”
print (“str1 is “,str1)
print(“id of str1 is”, id(str1))
str2=”world”
print(“str2 is”,str2)
print(“id of str2 is”,id(str2))
str1+=str2
print(“str1 after modification”,str1)
print(“id of str1 is”, id(str1))
str3=str1
print(“str3 is”,str3)
Print( “id of str3 is”, id(str3)
Output:
Str1 is Hello
id of str1 is 45093344
Str2 is world
id of str1 is 45093312
str1 after modification: Helloworld
Id of str1 is 43861792
Str3 Helloworld
Id of str3 is 43861792

12. Escape characters and their meaning
Escape character Meaning
a Bell or alert
b backspace
n New line
t Horizontal tab space
v Vertical tab space
r Enter button
x Character x
Display single

13. ● Raw String to ignore escape sequence
● Sometimes we may wish to ignore the escape sequences inside a string. To
do this we can place r or R in front of the string. This will imply that it is a raw
string and any escape sequence inside it will be ignored.
>>> print("This is x61 ngood example")
This is a
good example
>>> print(r"This is x61 ngood example")
This is x61 ngood example

14. Python String Formatting
● Escape Sequence
●
If we want to print a text like -He said, "What's there?"- we can neither use single quote or double
quotes.
●
An escape sequence starts with a backslash and is interpreted differently. If we use single quote to
represent a string, all the single quotes inside the string must be escaped. Similar is the case with
double quotes.
# using triple quotes
print('''He said, "What's there?"''')
# escaping single quotes
print('He said, "What's there?"')
# escaping double quotes
print("He said, "What's there?"")
OUTPUT :He said, "What's there?"
He said, "What's there?"
He said, "What's there?"

15. Formating symbol
Format symbol Purpose
%c character
%d or %i Signed decimal integer
%s string
%u Unsigned decimal integer
%o Octal integer
%x or %X Hexadecimal integer
%e or %E Exponential notation
%f Floating point number
%g or %G Short number in floating point or
exponential notation.

16. ● The syntax for string formatting opeartion is:
“<format>” % (<Values>)
● The statement began with a format string consisting of a sequence of characters and
conversion specifications.
● Conversion specification start with a % opeartor and can appear anywhere within the
string.
● Following the format string is a % sign and the a set value ,one per conversion
specification , seperated by commas and enclosed in parathesis. If ther is single value
then parenthesis is optional.
● E,g,
name=”ashish”
Age=8
print(“name=%s and age=%d” %(name,age))
print(“name=%s and age=%d”m, %(“ankita”,6))
Output:
name=ashish and age=8
name=ankita and age=6

17. ● Formatting a string means presenting the string in a clearly understandable
manner.
● The format method is used to format the string
● This methos is used as
'formatstring with replacement field'.format(value)
● E.g. id=10, name='shankar' sal=20000
1) Str='{},{},{}'.format(id,name,sal)
print(str) #output=10,shankar,20000
2) Str='{} - {} - {}'.format(id,name,sal)
print(str) #output=10 – shankar – 20000
3 )Str='id={}n name={}n sal={}'.format(id,name,sal)
print(str) #output=
id=10
name=shankar
sal=20000

18. Built-in string methods and functions
● String are an example of python objects.
● An object is an entity that contains both data(the actual string
itself) as well as functions to manupulate that data.
● These functions are availble to any instance(variable) of the object
● Python supports many built-in methods.
● A methods is just like a function.
● The only difference between function and tring methods is invoked
or called on an object.
● e.g. if the varible str is a string, then you can call the upper()
methods as str.upper() to convert all the characters of str in
uppercase.

19. Python has a set of built-in methods that you can use on
strings.
/media/admin1/HP x740w/PPS PPT/UNIT iv/in built function in string.pdf

20. String methods
Function Usage Example
Capitalize() To capitalize first letter of the string str=“hello”
print(str.capitalize())
O: Hello
Center(width,fillchar) Returns string with the original string
centered to a total of width columns and filled
with fillchar in columns that do not have
characters
Str=“hello”
print(str.center(10,’*’))
O: **hello***
Count(str,beg,end) Counts number of times str occurs in a
string.
msg=“my best friend”
print(msg.endswith(“end”,0,
len(msg)))
O: True
Endswith(suffix,beg,end) Checks if string ends with prefix msg=“my best friend”
print(msg.endswith(“end”,0,
len(msg)))
O: True
startswith(suffix,beg,end) Checks if string starts with prefix msg=“my best friend”
print(msg.startswith(“my”,0,
len(msg)))
O: True
Find(str,beg,end) Checks if str is present in string. msg=“my best friend”
print(msg.find(“best”,0, len(msg)))
O: 3
Index(str, beg, end) Same as find but raises an exception if str
is not found
msg=“my best friend”
print(msg.index(“mine”,0,
len(msg)))
O: valuerror:substring not found

21. String methods
Function Usage Example
Rfind(str,beg,end) Same as find but starts searching from
the end
msg=“my best friend”
print(msg.rfind(“best”,0, len(msg))
O: 3
Rindex(str,beg,end) Same as index but starts searching from
the end and but raises an exception if
str is not found
msg=“my best friend”
print(msg.rindex(“my”,0, len(msg))
O: 0
Isalnum() Returns True if string has at least one
character and every character is either
a number or an alphabet
msg=“jamesbond007”
print(msg.isalnum())
O: True
Isalpha() Returns True if string has at least one
character and every character is an
alphabet
msg=“jamesbond007”
print(msg.isalpha())
O: False
Isdigit() Returns True if string contains only
digits
msg=“007”
print(msg.isdigit())
O: True
Islower() Returns True if string has at least one
character and every character is a
lowercase alphabet
msg=“Hello”
print(msg.islower())
O: False
Isspace() Returns True if string contains only
whitespace characters
msg=“ “
print(msg.isspace())
O: True
Isupper() Returns True if string has at least one
character and every character is a
uppercase alphabet
msg=“HELLO”
print(msg.isupper())
O: True

22. String methods
Format Usage Example
len(string) Returns the length of the string str=“hello”
print(len(str))
O : 5
ljust(width,[fillchar]) Returns a string left justified to a total of width columns. Columns
without characters are padded with the character specified in the
fillchar argument
str=“hello”
print(ljust(10,”’*’))
O : hello*****
rjust(width,[fillchar]) Returns a string right justified to a total of width columns.
Columns without characters are padded with the character
specified in the fillchar argument
str=“hello”
print(rjust(10,”’*’))
O : *****hello
zfill(width) Returns string left padded with zeros to a total of width
characters.
str=“1234”
Print(str.zfill(10))
O:0000001234
lower() Converts all characters in the string into lowercase str=“Hello”
print(str.lower())
O : hello
upper() Converts all characters in the string into uppercase str=“hello”
print(str.upper())
O : HELLO
lstrip() Removes all leading whitespace in string str=“hello”
print(lstr.lstrip())
O : hello

23. String methods
Format Usage Example
rstrip() Removes all trailing whitespace in string str=“hello ”
print(rstr.lstrip())
O : hello
strip() Removes all leading and trailing whitespace in string str=“ hello ”
print(str.lstrip())
O : hello
max(str) Returns the highest alphabetical character (having highest
ASCII value) from the string str
str=“hello friendz”
print(max(str))
O : z
min(str) Returns the lowest alphabetical character (having lowest ASCII
value) from the string str
str=“hello friendz”
print(min(str))
O : d
replace(old,new[,
max])
Replaces all or max occurrences of old in string with new str=“hello hello hello”
Print(str.replace(“he”,”fo”))
O:follo follo follo
title() Return string in title case str=“python language”
Print(str.title())
O: Python Language
swapcase() Toggles the case of every character str=“python Language”
Print(str.swapcase())
O: PYTHON lANGUAGE

24. String methods
FormatFormat UsageUsage ExampleExample
split(delim) Returns a list of substrings seperated by the
specified delimiter. If no delimiter is specified
then by default it spilts strings on all whiespace
characters
str=“abc,def,ghi,jkl”
print(str.split(,))
O: [‘abc’,’’def’,’ghi’,’jkl’]
join(list) Joins a list of strings using the delimiter with
which the function invoked.
print(‘-’.join([‘abc’,’def’,’gh
i’,’jkl’])
O:abc-def-ghi-jkl
isidentifier() Returns true if the string is a valid identifier str=“hello”
print(str.isidentifier())
O: True
enumerate(str) Returns an enumerate object that lists the
index and value of all the characters in the
string as pairs
str=“hello”
print(list(enumerate(str))
O: [(0,’h’),(1,’e’),(2,’l’),
(3,’l’),(4,’o’)]

25. Ord() and chr() functions
● Ord() : function returns the ASCII code of the
character
● Chr() :function returns character represented by
ASCII number.
● e.g.
ch=='R'
ptint(ord(ch))
Output: 82
Print (chr(82))
Output: R

26. Comparing string
● Python allows you to compare strings using relational (or comparison)
operator such as >,<.<=,>=, etc.
● ASCII value for A-Z is 65-90 and a-z is 97-122
Operator Description example
== If two strings are equal , it return
true
>>>”AbC”==”AbC”
True
!= or <> If two strings are not equal , it returns
true
>>”abC”!=”Abc”
True
> If the first string is greater than
second,it returns true
>>>”abc”>”Abc”
True
< If the second string is greater than
first,it returns true
>>>”abC”<”abc”
True
>= If the first string is greater than or equal
to the second,it return true
>>>”aBC”>=”ABC”
True
<= If the second string is greater than or
equal to the first,it return true
>>>”Abc”<=”ABc”
True

27. Exmaples
>>>“TED” ==”ted”
False
>>>”tend”<=”tent”
True
>>>”main”<=”main”
● True

28. SLICE Operation
● A substring of a stirng is called a slice.
● The slice operation is used to refer to sub parts of sequence and
string
● You can take subset of a string from the original string by using [ ]
operator also known as slicing operator.
● e.g. string with indexing

29. ●
The syntax of slice opeartion is s[start:end]
● Where start specifies the beginning index of the substring and end-1 is the index of
the last chacters.
● Eg
str=”python”
print(“str[1:5]=”,str[1:5])
print(“str[:6]=”,str[:6])
print(“str[:]=”,str[:])
print(“str[1:20]=”,str[1:20])
● Output:
str[1:5]=ytho
Str[:6]=python
Str[:]=python
Str[1:20]=ython

30. ● Slice opration with negative index
str=”python”
print(“str[-1]=”,str[-1])
print(“str[-6]=”,str[-6])
print(“str[-2:]=”,str[-2:])
print(“str[:-2]=”,str[:-2])
print(“str[-5:-2]=”,str[-5:-2])
● Output:
str[-1]=N
str[-6]=p
str[-2:]=on
str[:-2]=Pyth
str[-5:-2]=yth
●

31. Specifying stride while slicing strings
● In the slice operation , you can specify a third argument as the stride.
● Which refers to the number of characters to move forward after the first character is retrieved from the string.
● The default value of stride is 1.
str=”welcome to the world of python”
print(“str[2:10]=”,str[2:10])
print(“str[2:10:1]=”,str[2:10:1])
print(“str[2:10:2]=”,str[2:10:2])
print(“str[2:13:4]=”,str[2:13:4])
Output:
str[2:10]=lcome to
Str[2:10:1]=lcome to
Str[2:10:2]=loet
str[2:13:4]=le

32. ● Program to demonstrate slice opration with just
last(neagtive)argument.
● i.e print string in reverse order
str=”welcome to learn python:
print(“str[:-1]=”,str[::-1])
Output:
Str[:::-1]=nohtyp nrael ot emoclew
● Print[::-3]=nyr c

33. The string module
● The string module consists of a number of useful constant, classes and function.
● These functions are used to manipulate strings
●
String constant:some constant define in string modules are:
– string.ascii_letters:combination of ascii_lowercase and ascii_uppercase constant
– string.ascii_lowercase:refers to all lowercase letters from a-z
– string.ascii_uppercase:refers to all uppercase letters A-Z
– string.digit:refers to digit from 0-9
– string.hexdigit:refers to hexadecimal digit from 0-9,a-f and A-F
– string.lowercase:a string that has all the characters that are considered lowercase letters.
– string.octdigits:refers to octal,0-7
– string.punctuation:string of ASCII characters that are considered uppercase letter
– string.printable: string of printable characters which include digit,letter,punctucation, and whitespce.
– string.upperrcase:a string that has all the characters that are considered uppercase letters
– string.lowercase:a string that has all the characters that are considered lowercase letters
– string.whitespace:a string that all characters that are considerd whitespace like space, tab etc.

34. Str=”welcome to the world of python”
print(“uppercase=”,str.upper())
print(“lowercase=”,str.lower())
print(split=”,str.split())
print(“join =”,'-'.join(str.split()))
print(“replace=”,str.replace(“python”,”java”))
print(“count of o”,str.count('o'))
print(“find of=”,str.find(“of”))
Output:
Uppercase=WELCOME TO THE WORLD OFPYTHON
lowercse=welcome to the world of python
Split =['wlcome','to', 'the',' world', 'of', 'python']
join=welcome- to- the- world -of -python
Replace-welcome to the world of java
Count of o -5
Find of-21

35. String module
To see the content of string module, use dir() with the modulename.
>>>dir(string)
To know the details of perticuler item, you can use the type command.
e.g.
Import string
Print(type(string.digits))
OUTPUT:
<class ‘str’>.
import string
print(string.digits)
OUTPUT:
0123456789
Working with constants in string module
import string
print(‘g’ in string.lowercase)
OUTPUT:
True

36. Programming Examples
1.Write a program to print the follewing pattern
A
AB
ABC
ABCD
ABCDE
ABCDEF
2. Write a program that takes users name and PAN card number as
input.Validate the information using isX function and print the details.
.3. Write a program that encrypts a message by adding a key value to every
character
If key=3, then add 3 to every character
4. Write a program to reverse a string.