str Datatype

What is String?

Any sequence of characters within either single quotes or double quotes is considered as a String.

Syntax:

s='Python'

s="Python"

Note:  

In most of other languages like C, C++, Java, a single

character with in single quotes is treated as char data type

value . But in Python we are not having char data type.

Hence it is treated as string only.

Eg:

ch='a'

type(ch)

<class 'str'>

How to define multi-line String literals:

We can define multi-line String literals by using triple

quotes.

Example:

s= ‘’’Hello World!
Welcome to my YouTube channel !’’’

We can also use triple quotes to use single quotes or double quotes as symbol inside string literal.

 Example 1:

s = ‘’’ MV’s Code Guide ‘’’

 If your string literal has single or double quote then it’s better to go for triple quotes to save such string literal.

 Or else you can use backslash (\) before the quotes in the literal values.

Example:

s= “MV\’s Code Guide”

Example 2:

s= ‘’’Learn Python ‘easily’ and “efficiently” ’’’

in the above example both single and double quotes are present in the string literal, in this case if you want to save the string literal in single or double quotes then using backslash is must.

S=”Learn Python \‘easily\’ and \“efficiently\” ”

S=’ Learn Python \‘easily\’ and \“efficiently\” ’

To avoid this extra effort of putting backslash before each and every single quote and double quote, you can use triple quotes.

Also, if you are importing data from some remote file then it’s not in possible for you to take care of all the data and put backslashes in the huge data.

So, triple quotes can be useful for that situation.

How to access characters of a String:

We can access characters of a string by using the following ways.

1. By using index

2. By using slice operator

 

Write a program to accept some string from the keyboard and display its characters index wise (both positive and negative index)

s=input("Enter Some String:")

i=0

for x in s:

print("The character present at positive index {} and at negative index {} is {}".format(i,i-len(s),x))

i=i+1

Accessing characters by using slice operator:

Syntax:  s[beginindex : endindex : step]

beginindex: From where we have to consider slice(substring)

endindex:  We have to terminate the slice(substring) at endindex-1

step :          incremented value

Note:

·       If we are not specifying begin index then it will consider from beginning of the string.

·       If we are not specifying end index then it will consider up to end of the string

·       The default value for step is 1

 

Comparison of Strings:

We can use comparison operators (<,<=,>,>=) and equality operators(==,!=) for strings.

Comparison will be performed based on alphabetical order.

 

s1=input("Enter first string:")

s2=input("Enter Second string:")

if s1==s2:

print("Equal Strings")

elif s1>s2:

print("s1 is greater than s2")

else:

print("s2 is greater than s1")

Removing spaces from the string:

We can use the following 3 methods

1. rstrip()---->To remove spaces at right hand side

2. lstrip()----->To remove spaces at left hand side

3. strip() ----->To remove spaces both sides

 

Finding Sub Strings:

We can use the following 4 methods:

For forward direction:

find()

index()

For backward direction:

rfind()

rindex()

1. find():    s.find(substring)

Returns index of first occurrence of the given sub string. If it is not available then we will get -1

s="Learning Python is very easy"

print(s.find("Python")) ---------> 9

print(s.find("Java")) -----------> -1

print(s.find("r"))---------------->  3

print(s.rfind("r"))---------------> 21

Note: By default find() method can search total string. We can also specify the boundaries to search.

s.find(substring , begin ,end)

It will always search from begin index to end-1 index

Eg:

s="Learning Python is very easy"

 s.find('a',0,15)

2

s.find('r',4,20)

-1

s.find('r',4,22)

21

s.find('a',4,22)

-1

index() method: 

 index() method is exactly same as find() method except that if the specified substring is not available then we will get ValueError

 

Counting substring in the given String:

count():  to count the number of occurrences of substring in original string

1. s.count(sub_string) :  It will search through out the string

2. s.count(sub_string, begin, end) : It will search from begin index to end-1 index

Replacing a string with another string:

s.replace(oldstring,newstring): to replace every occurrence of oldstring with newstring inside the string s and return a new string with these changes

Splitting of Strings:

We can split the given string according to specified seperator by using split() method.

l=s.split(seperator)

The default seperator is space. The return type of split() method is List

 

Joining of Strings:

We can join a group of strings(list or tuple) with respect to the given separator.

s=separator.join(group of strings)

l=['apple','boy','cat','dog']
>>> s="#".join(l)
>>> print(s)
apple#boy#cat#dog 

Changing case of a String:

We can change case of a string by using the following 4 methods.

1. upper()---->To convert all characters to upper case

2. lower() ----->To convert all characters to lower case

3. swapcase()-----> converts all lower case characters to upper case and all upper case characters to lower case

4. title() -----> To convert all character to title case. i.e. first character in every word should be upper case and all remaining characters should be in lower case.

5. capitalize() ----> Only first character will be converted to upper case and all remaining characters can be converted to lower case

 

Checking starting and ending part of the string:

Python contains the following methods for this purpose

1. s.startswith(substring)

2. s.endswith(substring)

 

To check type of characters present in a string:

Python contains the following methods for this purpose.

1) isalnum(): Returns True if all characters are alphanumeric( a to z , A to Z ,0 to9 )

2) isalpha(): Returns True if all characters are only alphabet symbols(a to z,A to Z)

3) isdigit(): Returns True if all characters are digits only( 0 to 9)

4) islower(): Returns True if all characters are lower case alphabet symbols

5) isupper(): Returns True if all characters are upper case alphabet symbols

6) istitle(): Returns True if string is in title case

7) isspace(): Returns True if string contains only spaces

Formatting the Strings:

We can format the strings with variable values by using replacement operator {} and format() method.

Eg:

name='Ram'

salary=10000

age=42

print("{} 's salary is {} and his age is {}".format(name,salary,age)) print("{0} 's salary is {1} and his age is {2}".format(name,salary,age))

print("{x} 's salary is {y} and his age is {z}" .format(z=age , y=salary,x=name))

Output:  

Ram's salary is 10000 and his age is 48 

Ram's salary is 10000 and his age is 48

Ram 's salary is 10000 and his age is 48