Python programming : Strings

Strings And Characters
Creating Strings
Example-1 s = 'Welcome to Python'
Example-2 s = "Welcome to Python"
Example-3 s = """
Welcome to Python
"""
Example-4 s = '''
Welcome to Python
'''
Example-5 s = "Welcome to 'Core' Python"
Example-6 s = 'Welcome to "Core" Python'
Example-7 s = "Welcome totCorenPython"
Example-8 s = r"Welcome totCorenPython"

Length of a String
len() Is used to find the length of the string
Example str = "Core Python"
n = len(str)
print("Len: ", n)

Indexing the Strings
str = "Core Python"
#Method-1: Access each character using
while loop
n = len(str)
i = 0
while i < n:
print(str[i], end=' ')
i += 1
#Method-2: Using for loop
for i in str:
print(i, end=' ')
#Method-3: Using slicing operator
for i in str[::]:
print(i, end='')
print()
#Method-4: Using slicing operator
#Take sthe step size as -1
for i in str[: : -1]:
print(i, end='')

Both positive and Negative indexing is possible in Python

Slicing the Strings
str = "Core Python"
1 str[: :]
Prints all
2 str[0: 9: 1]
Access the string from 0th to 8th element
3 str[0: 9: 2]
Access the string in the step size of 2
4 str[2: 3: 1]
Access the string from 2nd to 3rd Character
5 str[: : 2] Access the entire string in the step size of 2
6 str[: 4: ] Access the string from 0th to 3rd location in steps of 1
7 str[-4: -1: ] Access from str[-4] to str[-2] from left to right
8 str[-6: :] Access from -6 till the end of the string
9 str[-1: -4: -1] When stepsize is negative, then the items are counted from right to
left
10 str[-1: : -1] Retrieve items from str[-1] till the first element from right to left

Repeating the Strings

The repetition operator * is used for repeating the strings
Example-1 str = "Core Python"
print(str * 2)
Example-2
print(str[5: 7] * 2)

Concatenation of Strings

+ is used as a concatenation operator
Example-1 s1 = "Core"
s2 = "Python"
s3 = s1 + s2

Membership Operator

We can check, if a string or a character is a member of another string or not using
'in' or 'not in' operator

'in' or 'not in' makes case sensitive comaprisons
Example-1 str = input("Enter the first string: ")
sub = input("Enter the second string: ")
if sub in str:
print(sub+" is found in main string")
else:
print(sub+" is not found in main string")

Removing Spaces
str = " Ram Ravi "
lstrip() #Removes spaces from the left side
print(str.lstrip())
rstrip() #Removes spaces from the right side
print(str.rstrip())
strip() #Removes spaces from the both sides
print(str.strip())

Finding the Sub-Strings

Methods useful for finding the strings in the main string

- find()

- rfind()

- index()

- rindex()

find(), index() will search for the sub-string from the begining

rfind(), rindex() will search for the sub-string from the end

find(): Returns -1, if sub-string is not found

index(): Returns 'ValueError' if the sub-string is not found

Syntax mainstring.find(substring, beg, end)
Example str = input("Enter the main string:")
sub = input("Enter the sub string:")
#Search for the sub-string
n = str.find(sub, 0, len(str))
if n == -1:
print("Sub string not found")
else:
print("Sub string found @: ", n + 1)

Syntax mainstring.index(substring, beg, end)
Example str = input("Enter the main string:")
sub = input("Enter the sub string:")
try:
n = str.index(sub, 0, len(str))
except ValueError:
print("Sub string not found")
else:
print("Sub string found @: ", n + 1)

Finding the Sub-Strings: Exercise
1 To display all positions of a sub-string in a given main string

Counting Sub-Strings in a String
count() To count the number of occurrences of a sub-string in a main string
Syntax stringname.count(substring, beg, end)
Example-1 str = “New Delhi”
n = str.count(‘Delhi’)
n = str.count(‘e’, 0, 3)
n = str.count(‘e’, 0, len(str))

Strings are Immutable

Immutable object is an object whose content cannot be changed
Immutable Numbers, Strings, Tuples
Mutable Lists, Sets, Dictionaries
Reasons: Why strings are made immutable in Python
Performance Takes less time to allocate the memory for the Immutable objects, since
their memory size is fixed
Security Any attempt to modify the string will lead to the creation of new object in
memory and hence ID changes which can be tracked easily

Strings are Immutable

Immutable object is an object whose content cannot be changed

Example:

s1 = “one”

s2 = “two”

S2 = s1
one two
S1 S2
one two
S1 S2

Replacing String with another String
replace() To replace the sub-string with another sub-string
Syntax stringname.replace(old, new)
Example str = "Ram is good boy"
str1 = str.replace("good", "handsome")
print(str1)

Splitting And Joining Strings
join() - Groups into one sring
Syntax separator.join(str)
- separator: Represents the character to be used between two strings
- str: Represents tuple or list of strings
Example str = ("one", "two", "three")
str1 = "-".join(str)
split() - Used to brake the strings
- Pieces are returned as a list
Syntax stringname.split(‘character’)
Example str = "one,two,three"
lst = str.split(',')

Changing the Case of the Strings
Methods upper()
lower()
swapcase()
title()
str = "Python is the future"
upper() print(str.upper()) PYTHON IS THE FUTURE
lower() print(str.lower()) python is the future
swapcase() print(str.swapcase()) pYTHON IS THE FUTURE
title() print(str.title()) Python Is The Future

Check: Starting & Ending of Strings
Methods startswith()
endswith()
str = "This is a Python"
startswith() print(str.startswith("This")) True
endswith() print(str.endswith("This")) False

String Testing Methods
isalnum() Returns True, if all characters in the string are alphanumeric(A – Z, a – z, 0
– 9) and there is atleast one character
isalpha() Returns True, if the string has atleast one character and all characters are
alphabets(A - Z, a – z)
isdigit() Returns True if the string contains only numeric digits(0-9) and False
otherwise
islower() Returns True if the string contains at least one letter and all characters are
in lower case; otherwise it returns False
isupper() Returns True if the string contains at least one letter and all characters are
in upper case; otherwise it returns False
istitle() Returns True if each word of the string starts with a capital letter and there
at least one character in the string; otherwise it returns False
isspace() Returns True if the string contains only spaces; otherwise, it returns False

Formatting the strings
format() Presenting the string in the clearly understandable manner
Syntax
"format string with replacement fields". format(values)
id = 10
name = "Ram"
sal = 19000.45
print("{}, {}, {}". format(id, name, sal))
print("{}-{}-{}". format(id, name, sal))
print("ID: {0}tName: {1}tSal: {2}n". format(id, name, sal))
print("ID: {2}tName: {0}tSal: {1}n". format(id, name, sal))
print("ID: {two}tName: {zero}tSal: {one}n". format(zero=id, one=name, two=sal))
print("ID: {:d}tName: {:s}tSal: {:10.2f}n". format(id, name, sal))

Formatting the strings
format() Presenting the string in the clearly understandable manner
Syntax
"format string with replacement fields". format(values)
n = 5000
print("{:*>15d}". format(num))
print("{:*^15d}". format(num))

Exercise
1. To know the type of character entered by the user
2. To sort the strings in alphabetical order
3. To search for the position for a string in agiven group of strings
4. To find the number of words in a given strings
5. To insert the sub-string into a main string in a particular position

Python programming : Strings

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