VTU 6 SEM EC 18EC646

1. PYTHON APPLICATION PROGRAMMING -18EC646
MODULE 2
STRINGS
Prof. Krishnananda L
Department of ECE
Govt SKSJTI
Bengaluru

2. STRINGS:
 A string is a group/ a sequence of characters. Since Python has no provision
for arrays, we simply use strings. We use a pair of single or double quotes to
declare a string.
 Strings is “sequence data type” in python: meaning – its ordered, elements
can be accessed using positional index
 String may contain alphabets, numerals, special characters or a combination
of all these.
 String is a sequence of characters represented in Unicode. Each character of
a string corresponds to an index number, starting with zero.
 Strings are “immutable” i.e., once defined, the string literals cannot be
reassigned directly.
 Python supports positive indexing of string starting from the beginning of the
string as well as negative indexing of string starting from the end of the
string .
 Python has a vast library having built-in methods to operate on strings
2

3. STRING EXAMPLE PROGRAM:
 Example1
 INPUT:
>>> print(‘Hello world’)
 OUTPUT:
Hello world
 INPUT:
>>> print(“Hello world@@”)
 OUTPUT:
Hello world@@
 Example 2
 INPUT:
>>>a = 'Hello world’
>>>print(a)
 OUTPUT:
Hello world
 INPUT:
>>>a = 'Hello world’
>>> a
 OUTPUT:
‘Hello world’
 Example 3
 INPUT:
>>>a = 'Hello world’
>>>a[0]=‘M’
 OUTPUT:
 Traceback (most recent call last):
File "<pyshell#19>", line 1, in
<module>
a[0]='M'
TypeError: 'str' object does not
support item assignment
Note: String is “immutable” Not
possible to change the elements of the
string through assignment operator
3

4. POSITIVEAND NEGATIVE INDEXING OF A STRING:
 Positive indexing:
 INPUT:
>>>a = ‘Hello world’
print(a[1])
 OUTPUT:
e
 Negative indexing:
 INPUT:
>>> a = 'Hello World’
>>> print(a[-1])
 OUTPUT:
d
>>>a = "Hello, World!"
>>>print(a[7])
W
4

5. MULTILINE STRINGS:
 using three quotes:
 INPUT:
a = “””Python is an interpreted high-level
general-purpose programming language. Python's
design philosophy emphasizes code readability
with significant indentation.”””
print(a)
 OUTPUT:
Python is an interpreted high-level general-
purpose programming language. Python's design
philosophy emphasizes code readability with
significant indentation.
 Using three single quotes:
 INPUT:
a = ‘’’Python is an interpreted high-level
general-purpose programming language. Python's
design philosophy emphasizes code readability
with significant indentation.’’’
print(a)
 OUTPUT:
Python is an interpreted high-level general-
purpose programming language. Python's design
philosophy emphasizes code readability with
significant indentation. 5

6. LOOPING THROUGHA STRING:
 For loop:
 INPUT:
for x in “hello":
print(x)
 OUTPUT:
h
e
l
l
o
 While loop:
 INPUT:
st=“hello"
i=0
while i<len(st):
print(st[i], end=‘ ’))
i=i+1
 OUTPUT:
hello
str = "welcome to Python class "
for k in str:
if k == 'P':
break
print(k, end=' ')
Output:
welcome to
str = "welcome to Python class "
for k in str:
print(k, end=' ')
if k == 'P':
break
Output:
welcome to P
6

7. LOOPING CONTD…
## to print names of the list
print ("illustration of for loop--- printing names
from list")
names=["anand", "arun", "akshata", "anusha"]
for i in names:
print ("hello " , i)
print ("COMPLETED ---")
# prints all letters except 'a' and 'k'
## illustration of continue
## illustration of logical operators
i = 0
str1 = ‘all is well'
while i < len(str1):
if str1[i] == ‘s' or str1[i] == ‘l':
i += 1
continue
print('Current Letter :', str1[i])
i += 1
Current Letter : a
Current Letter :
Current Letter : i
Current Letter :
Current Letter : w
Current Letter : e 7

8. LOOPING CONTD..
""" program to illustrate for loop and multiple if statements
with string to count the occurrence of each vowel """
txt='HELLO how are you? I am fine'
counta, counte, counti, counto, countu=0,0,0,0,0
for k in txt:
if k=='a' or k=='A':
counta+=1
if k=='e' or k=='E':
counte+=1
if k=='i' or k==‘’I’:
counti+=1
if k=='o' or k =='O':
counto+=1
if k=='u' or k=='U':
countu+=1
print ("Given string is %s " %(txt))
print ("total 'A' or 'a' in the string is: %d " %(counta))
print ("total 'E' or 'e' in the string is: %d " %(counte))
print ("total 'I' or 'i' in the string is: %d " %(counti))
print ("total 'O' or '0' in the string is: %d " %(counto))
print ("total 'U' or 'u' in the string is: %d " %(countu))
Output:
Given string is HELLO how are you? I am fine
total 'A' or 'a' in the string is: 2
total 'E' or 'e' in the string is: 3
total 'I' or 'i' in the string is: 2
total 'O' or '0' in the string is: 3
total 'U' or 'u' in the string is: 1
8

9. CHECK STRING (PHRASES IN STRINGS):
 Normal:
 INPUT:
txt = ‘happiness is free“
print("free" in txt)
 OUTPUT:
True
 INPUT:
txt = “happiness is free”
print(“hello” in txt)
 OUTPUT:
False
 With if statement:
 INPUT:
txt = “happiness is free”
if “free” in txt:
print("Yes, 'free' is present.")
 OUTPUT:
Yes, 'free' is present.
txt = "The quick brown fox jumps over
the lazy dog!"
if "fox" in txt:
print("Yes, 'fox' is present.")
else:
print ("No, ‘fox’is not present")
## to check for a phrase in a string
>>>txt = "The best things in life are free!"
>>>print(“life" in txt)
True 9

10. CHECK STRING :
 Normal:
 INPUT:
txt = ‘welcome to python’
print(‘world’ not in txt)
 OUTPUT:
True
 With if statement:
 INPUT:
txt = “welcome to python“
if “hello" not in txt:
print("Yes, ‘hello' is NOT present.")
 OUTPUT:
Yes, ‘hello' is NOT present.
10

11. STRING LENGTH AND SLICING:
 String length:
 INPUT:
a = "Hello World”
print(len(a))
 OUTPUT:
11
 INPUT:
a = “Welcome to Python programming @@@“
print (len(a))
 OUTPUT:
33
 String slicing:
 INPUT:
b = "Hello World“
print(b[2:5]) ## from index 2 to index4
 OUTPUT:
llo
 INPUT:
b = "Hello World“
print(b[:5]) ## from beginning 5 characters
 OUTPUT:
Hello
11

12. STRING SLICING METHODS:
 From the Start:
 INPUT:
>>>b = ‘’python is high level
programming language“
>>>print(b[:10])
 OUTPUT:
python is
## consider from the
beginning 10 characters and
print
 Till the End:
 INPUT:
>>>b = "python is high level
programming language“
>>>print(b[10:])
 OUTPUT:
high level programming
language
## skip first 10 characters
from the beginning and print
remaining till end of string
 With Negative indexing
 INPUT:
>>>b = "python is high level
programming language“
>>>print (b[:-5])
OUTPUT:
python is high level programming lan
>>>print (b[-5:])
OUTPUT:
guage 12

13. STRING SLICING CONTD…
## illustration of slicing
## get a range of characters from the complete string
>>>txt = "Govt SKSJTI Bengaluru"
# print characters in position 5 to 10. (position 11 not
included)
>>>print(txt[5:11])
SKSJTI
## slicing from the beginning without initial index
>>>txt = "Govt SKSJTI Bengaluru"
>>>print(txt[:4]) # from start to position 3
Govt
### slicing till the end without last index
>>>txt = "Govt SKSJTI Bengaluru"
>>>print(txt[7:]) ## from position 7 to last character
SJTI Bengaluru
### Negative indexing
>>>txt = "Govt SKSJTI Bengaluru"
>>> print(txt[-16:-8]) ## position -8 not
included
SKSJTI B
>>>txt = "Govt SKSJTI Bengaluru"
>>> print(txt[:-16])
Govt
>>>txt = "Govt SKSJTI Bengaluru"
>>> print(txt[-9:])
Bengaluru 13

14. BUILT-IN STRING METHODS IN PYTHON:
 Upper case:
 INPUT:
>>> a = "Hello World”
>>>print(a.upper())
 OUTPUT:
HELLO WORLD
 Lower case:
 INPUT:
a = "Hello World“
print(a.lower())
 OUTPUT:
hello world
 capitalize:
 INPUT:
>>>a = “hello world”
>>> print(a.capitalize())
 OUTPUT:
Hello world
 Misc:
 INPUT:
>>>a = “hello world”
>>> print(a.islower())
 OUTPUT:
True
>>>a = “hello world”
>>> print(a.isupper())
 OUTPUT:
False
Note:String methods do not modify the original string.
14

15. BUILT-IN STRING METHODS IN PYTHON
 Remove Whitespace(strip):
 INPUT:
>>> a = " Hello World "
>>>print(a.strip())
 OUTPUT:
Hello World
## remove white spaces at the
beginning and end of the string
>>> a = " our first python prog"
>>>print(a.lstrip(‘our’))
OUTPUT:
first python prog
 Splitting string:
 INPUT:
>>> a = " hello and welcome”
>>>print(a.split())
 OUTPUT:
['hello', 'and', 'welcome']
#list with 3 elements
>>>a = " hello,how,are,you”
>>>print(a.split())
 OUTPUT:
['hello,how,are,you']
## list with single element
 INPUT:
>>>a = " hello,how,are,you”
>>>print(a.split(‘,’))
 OUTPUT:
['hello', 'how', 'are', 'you']
## list with 4 elements
## splits the string at the specified
separator and returns a list. Default
separator white space
>>>a = “but put cut”
>>>print(a.split(‘t’))
 OUTPUT:
['bu', ' pu', ' cu', '']
15

16.  Count occurrence:
 INPUT:
a = "Hello World“
print(a.count(‘l’)
 OUTPUT:
3
 To find position
(index):
 INPUT:
>>>a = “Hello world”
>>> print(a.indix(‘w’))
OUTPUT:
6
BUILT-IN STRING METHODS IN PYTHON
 Check for digits:
 INPUT:
a = "Hello 23“
print(a.isdigit())
 OUTPUT:
False
a = “2345’
print(a.isdigit())
 OUTPUT:
True
 Check for alphabets:
 INPUT:
a = "Hello 23”
print(a.isalpha())
 OUTPUT:
False
>>>a = “hello "
>>>print(a.isaplpha())
 OUTPUT:
False
a = “hello" print(a.isaplpha())
 OUTPUT:
False
16

17.  Concatenation:
 INPUT:
a = "Hello“
b = "World“
c = a + b
d = a + “, " + b
print(c)
print(d)
 OUTPUT:
HelloWorld
Hello,World
BUILT-IN STRING METHODS IN PYTHON
Replace a character or string:
INPUT:
>>>a = “Geeks for Geeks“
print(a.replace(‘G’, ‘W’))
OUTPUT:
Weeks for Weeks
INPUT:
a = “Hello welcome to Python“
print(a.replace(‘Hello’, ‘Krishna’)
OUTPUT:
Krishna welcome to Python
17

18. LOOPING TO COUNT:
 INPUT:
word='python program’
count=0
for letter in word:
if letter=='p’:
count=count+1
print(count)
 OUTPUT:
2
 INPUT:
s = "peanut butter“
count = 0
for char in s:
if char == "t":
count = count + 1
print(count)
 OUTPUT:
3
18

19. PARSINGAND FORMAT STRINGS:
 Parsing:
 INPUT:
st="From mamatha.a@gmail.com"
pos=st.find('@’)
print('Position of @ is', pos)
 OUTPUT:
Position of @ is 14
 Format:
 INPUT:
print ('My name is %s and age is %d ’% (‘Arun', 21))
 OUTPUT:
My name is Arun and age is 21
19

20. STRING FORMATTING
 Without built in function:
 INPUT:
age = 36
print( "My name is John, I am ", + age)
 OUTPUT:
My name is John,I am 36
 With built in function:
 INPUT:
age = 36
print( "My name is John, I am {} ".format(age))
 OUTPUT:
My name is John, I am 36
20

21. MULTIPLE PLACE HOLDERS IN STRING FORMAT
 Without index numbers:
 INPUT:
quantity = 3
itemno = 5
price = 49.95
myorder = “I want {} pieces of item {} for {}
dollars.“
print(myorder.format(quantity, itemno, price))
 OUTPUT:
I want 3 pieces of item 5 for 49.95 dollars.
 With index numbers:
 INPUT:
quantity = 3
itemno = 5
price = 49.95
myorder = "I want to pay {2} dollars for {0}
pieces of item {1}.“
print(myorder.format(quantity, itemno, price))
 OUTPUT:
I want to pay 49.95 dollars for 3 pieces of item 5.
21

22. ESCAPE SEQUENCES
 To insert characters that are illegal in a string, we use an
escape character.
 Example:
 INPUT:
txt = "We are the so-called "Vikings" from the north."
print(txt)
 OUTPUT:
We are the so-called "Vikings" from the north.
22

23. 23

24. STRING COMPARISON:
 Normal:
 INPUT:
>>> print(‘python’==‘python’)
>>> print(’Python’< ‘python’)
>>> print (’PYTHON’==‘python’)
 OUTPUT:
True
True
False
.
 Using is operator:
 >>>str1=‘python’
 >>>str2=‘Python’
 >>>str3=str1
 >>> str1 is str2
False
>>> str3 is str1
True
>>> str2 is not str1
True
>>>str1+=‘s’
>>> id(str1)
1918301558320
>>> id(str2)
1918291321200
>>> id(str3)
1918301558320
>>>id(str1)
1918301822256
The relational operators compare
the Unicode values of the characters
of the strings from the zeroth index
till the end of the string. It then
returns a boolean value according to
the result of value comparison
is and is not operators check
whether both the operands
refer to the same object or
not. If object ids are same, it
returns True
24

25. STRING SPECIAL OPERATORS:
25

26. SUMMARY OF BUILT-IN STRING METHODS:
Method Description
capitalize() Converts the first character
to upper case
casefold() Converts string into lower
case
center() Returns a centered string
count() Returns the number of
times a specified value
occurs in a string
encode() Returns an encoded
version of the string
endswith() Returns true if the string
ends with the specified
value
find() Searches the string for a specified value and
returns the position of where it was found
format() Formats specified values in a string
format_map() Formats specified values in a string
index() Searches the string for a specified value and
returns the position of where it was found
isalnum() Returns True if all characters in the string are
alphanumeric
isalpha() Returns True if all characters in the string are
in the alphabet
isdecimal() Returns True if all characters in the string are
decimals
26

27. CONTD..
isdigit() Returns True if all characters in the string
are digits
isidentifi
er()
Returns True if the string is an identifier
islower() Returns True if all characters in the string
are lower case
isnumeri
c()
Returns True if all characters in the string
are numeric
isprintab
le()
Returns True if all characters in the string
are printable
isspace() Returns True if all characters in the string
are whitespaces
istitle() Returns True if the string follows the
isupper() ReturnsTrue if all characters in the
string are uppercase
join() Joins the elements of an iterable to the end
of the string
ljust() Returns a left justified version of the string
lower() Converts a string into lower case
lstrip() Returns a left trim version of the string
maketrans() Returns a translation table to be used in
translations
27

28. CONTD..
partition() Returns a tuple where the string is parted
into three parts
replace() Returns a string where a specified value is
replaced with a specified value
rfind() Searches the string for a specified value
and returns the last position of where it
was found
rindex() Searches the string for a specified value
and returns the last position of where it
was found
rjust() Returns a right justified version of the
string
rpartition() Returns a tuple where the string is parted
into three parts
rsplit() Splits the string at the specified
separator, and returnsa list
rstrip() Returns a right trim version of the
string
split() Splits the string at the specified
separator, and returnsa list
splitlines() Splits the string at line breaks and
returns a list
startswith() Returns true if the string starts with
the specified value
strip() Returns a trimmed version of the
string
28