PYTHON STRINGS

PYTHON STRINGS

Python string is a built-in type text sequence. It is used to handle textual data in python. Python Strings are immutable sequences of Unicode points. Creating Strings are simplest and easy to use in Python.

We can simply create Python String by enclosing a text in single as well as double quotes. Python treat both single and double quotes statements same.

Accessing Python Strings
In Python, Strings are stored as individual characters in a contiguous memory location.
The benefit of using String is that it can be accessed from both the directions (forward and backward).
Both forward as well as backward indexing are provided using Strings in Python.
Forward indexing starts with 0,1,2,3,....
Backward indexing starts with -1,-2,-3,-4,....

Example



str[0]='P'=str[-6] ,
str[1]='Y' = str[-5]  ,
str[2] = 'T' = str[-4]  , 
str[3] = 'H' = str[-3] 
str[4] = 'O' = str[-2]  , 
str[5] = 'N' = str[-1]. 

Python String Example
Here, we are creating a simple program to retrieve String in reverse as well as normal form.

name="Rajat" 
length=len(name) 
i=0 
for n in range(-1,(-length-1),-1): 
    print name[i],"\t",name[n] 
    i+=1 
Output:
>>>
R       t
a       a
j       j
a       a
t       R
>>>

Python Strings Operators
To perform operation on string, Python provides basically 3 types of Operators that are given below.

Basic Operators.
Membership Operators.
Relational Operators.

Python String Basic Operators
There are two types of basic operators in String "+" and "*".

String Concatenation Operator (+)
The concatenation operator (+) concatenates two Strings and creates a new String.

Python String Concatenation Example
>>> "ratan" + "jaiswal"
Output:
'ratanjaiswal'
>>>


Expression
Output
'10' + '20'
'1020'
"s" + "007"
's007'
'abcd123' + 'xyz4'
'abcd123xyz4'

NOTE: Both the operands passed for concatenation must be of same type, else it will show an error.

Eg:
'abc' + 3
>>>
output:
Traceback (most recent call last):
  File "", line 1, in
    'abc' + 3
TypeError: cannot concatenate 'str' and 'int' objects
>>>

Python String Replication Operator (*)
Replication operator uses two parameters for operation, One is the integer value and the other one is the String argument.

The Replication operator is used to repeat a string number of times. The string will be repeated the number of times which is given by the integer value.

Python String Replication Example
>>> 5*"Vimal" 
Output:
'VimalVimalVimalVimalVimal'


Expression
Output
"soono"*2
'soonosoono'
3*'1'
'111'
'$'*5
'$$$$$'

NOTE: We can use Replication operator in any way i.e., int * string or string * int. Both the parameters passed cannot be of same type.

Python String Membership Operators
Membership Operators are already discussed in the Operators section. Let see with context of String.

There are two types of Membership operators
1) in:"in" operator returns true if a character or the entire substring is present in the specified string, otherwise false.

2) not in:"not in" operator returns true if a character or entire substring does not exist in the specified string, otherwise false.

Python String membership operator Example

>>> str1="javatpoint" 
>>> str2='sssit' 
>>> str3="seomount" 
>>> str4='java' 
>>> st5="it" 
>>> str6="seo" 
>>> str4 in str1 
True 
>>> str5 in str2 
>>> st5 in str2 
True 
>>> str6 in str3 
True 
>>> str4 not in str1 
False 
>>> str1 not in str4 
True 

Python Relational Operators
All the comparison (relational) operators i.e., (<,><=,>=,==,!=,<>) are also applicable for strings. The Strings are compared based on the ASCII value or Unicode(i.e., dictionary Order).

Python Relational Operators Example
>>> "RAJAT"=="RAJAT" 
True 
>>> "afsha">='Afsha' 
True 
>>> "Z"<>"z" 
True 

Explanation:
The ASCII value of a is 97, b is 98, c is 99 and so on. The ASCII value of A is 65,B is 66,C is 67 and so on. The comparison between strings are done on the basis on ASCII value.

Python String Slice Notation
Python String slice can be defined as a substring which is the part of the string. Therefore further substring can be obtained from a string.

There can be many forms to slice a string, as string can be accessed or indexed from both the direction and hence string can also be sliced from both the directions.

Python String Slice Syntax
<string_name>[startIndex:endIndex], 
<string_name>[:endIndex], 
<string_name>[startIndex:] 

Python String Slice Example 1
>>> str="Nikhil" 
>>> str[0:6] 
'Nikhil' 
>>> str[0:3] 
'Nik' 
>>> str[2:5] 
'khi' 
>>> str[:6] 
'Nikhil' 
>>> str[3:] 
'hil' 

Note: startIndex in String slice is inclusive whereas endIndex is exclusive.

String slice can also be used with Concatenation operator to get whole string.

Python String Slice Example 2
>>> str="Mahesh" 
>>> str[:6]+str[6:] 
'Mahesh' 
//here 6 is the length of the string.

Python String Functions and Methods
Python provides various predefined or built-in string functions. They are as follows:

capitalize()
It capitalizes the first character of the String.
count(string,begin,end)
It Counts number of times substring occurs in a String between begin and end index.
endswith(suffix ,begin=0,end=n)
It returns a Boolean value if the string terminates with given suffix between begin and end.
find(substring ,beginIndex, endIndex)
It returns the index value of the string where substring is found between begin index and end index.
index(subsring, beginIndex, endIndex)
It throws an exception if string is not found and works same as find() method.
isalnum()
It returns True if characters in the string are alphanumeric i.e., alphabets or numbers and there is at least 1 character. Otherwise it returns False.
isalpha()
It returns True when all the characters are alphabets and there is at least one character, otherwise False.
isdigit()
It returns True if all the characters are digit and there is at least one character, otherwise False.
islower()
It returns True if the characters of a string are in lower case, otherwise False.
isupper()
It returns False if characters of a string are in Upper case, otherwise False.
isspace()
It returns True if the characters of a string are whitespace, otherwise false.
len(string)
It returns the length of a string.
lower()
It converts all the characters of a string to Lower case.
upper()
It converts all the characters of a string to Upper Case.
startswith(str ,begin=0,end=n)
It returns a Boolean value if the string starts with given str between begin and end.
swapcase()
It inverts case of all characters in a string.
lstrip()
It removes all leading whitespace of a string and can also be used to remove particular character from leading.
rstrip()
It removes all trailing whitespace of a string and can also be used to remove particular character from trailing.

Python String capitalize() Method Example
This method capitalizes the first character of the String.

1.    >>> 'abc'.capitalize() 
Output:
'Abc'

Python String count(string) Method Example
This method counts number of times substring occurs in a String between begin and end index.

msg = "welcome to sssit"; 
substr1 = "o"; 
print  msg.count(substr1, 4, 16) 
substr2 = "t"; 
print  msg.count(substr2) 
Output:
>>>
2
2
>>>

Python String endswith(string) Method Example
This method returns a Boolean value if the string terminates with given suffix between begin and end.

string1="Welcome to SSSIT"; 
substring1="SSSIT"; 
substring2="to"; 
substring3="of"; 
print string1.endswith(substring1); 
print string1.endswith(substring2,2,16); 
print string1.endswith(substring3,2,19); 
print string1.endswith(substring3); 
Output:
>>>
True
False
False
False
>>>

Python String find(string) Method Example
This method returns the index value of the string where substring is found between begin index and end index.

str="Welcome to SSSIT"; 
substr1="come"; 
substr2="to"; 
print str.find(substr1); 
print str.find(substr2); 
print str.find(substr1,3,10); 
print str.find(substr2,19); 

Output:
>>>
3
8
3
-1
>>>

Python String index() Method Example
This method returns the index value of the string where substring is found between begin index and end index.

str="Welcome to world of SSSIT"; 
substr1="come"; 
substr2="of"; 
print str.index(substr1); 
print str.index(substr2); 
print str.index(substr1,3,10); 
print str.index(substr2,19); 

Output:
>>>
3
17
3
Traceback (most recent call last):
  File "C:/Python27/fin.py", line 7, in
    print str.index(substr2,19);
ValueError: substring not found
>>>

Python String isalnum() Method Example
This method returns True if characters in the string are alphanumeric i.e., alphabets or numbers and there is at least 1 character. Otherwise it returns False.

str="Welcome to sssit"; 
    print str.isalnum(); 
str1="Python47"; 
print str1.isalnum(); 

Output:
>>>
False
True
>>>

Python String isalpha() Method Example
It returns True when all the characters are alphabets and there is at least one character, otherwise False.

string1="HelloPython";  
 # Even space is not allowed 
print string1.isalpha(); 
string2="This is Python2.7.4" 
print string2.isalpha(); 

Output:
>>>
True
False
>>>

Python String isdigit() Method Example
This method returns True if all the characters are digit and there is at least one character, otherwise False.

string1="HelloPython";  
print string1.isdigit(); 
string2="98564738" 
print string2.isdigit(); 

Output:
>>>
False
True
>>>

Python String islower() Method Example
This method returns True if the characters of a string are in lower case, otherwise False.

string1="Hello Python";  
print string1.islower(); 
string2="welcome to " 
print string2.islower(); 

Output:
>>>
False
True
>>>

Python String isupper() Method Example
This method returns False if characters of a string are in Upper case, otherwise False.

string1="Hello Python";  
print string1.isupper(); 
string2="WELCOME TO" 
print string2.isupper(); 

Output:
>>>
False
True
>>>

Python String isspace() Method Example
This method returns True if the characters of a string are whitespace, otherwise false.

string1="    ";  
print string1.isspace(); 
string2="WELCOME TO WORLD OF PYT" 
print string2.isspace(); 

Output:
>>>
True
False
>>>

Python String len(string) Method Example
This method returns the length of a string.

string1="    ";  
print len(string1); 
string2="WELCOME TO SSSIT" 
print len(string2); 

Output:
>>>
4
16
>>>

Python String lower() Method Example
It converts all the characters of a string to Lower case.

string1="Hello Python";  
print string1.lower(); 
string2="WELCOME TO SSSIT" 
print string2.lower(); 

Output:
>>>
hello python
welcome to sssit
>>>

Python String upper() Method Example
This method converts all the characters of a string to upper case.

string1="Hello Python";  
print string1.upper(); 
string2="welcome to SSSIT" 
print string2.upper(); 

Output:
>>>
HELLO PYTHON
WELCOME TO SSSIT
>>>

Python String startswith(string) Method Example
This method returns a Boolean value if the string starts with given str between begin and end.

string1="Hello Python";  
print string1.startswith('Hello'); 
string2="welcome to SSSIT" 
print string2.startswith('come',3,7); 

Output:
>>>
True
True
>>>

Python String swapcase() Method Example
It inverts case of all characters in a string.

string1="Hello Python";  
print string1.swapcase(); 
string2="welcome to SSSIT" 
print string2.swapcase(); 

Output:
>>>
hELLO pYTHON
WELCOME TO sssit
>>>

Python String lstrip() Method Example
It removes all leading whitespace of a string and can also be used to remove particular character from leading.

string1="    Hello Python";  
print string1.lstrip(); 
string2="@@@@@@@@welcome to SSSIT" 
print string2.lstrip('@'); 

Output:
>>>
Hello Python
welcome to world to SSSIT
>>>

Python String rstrip() Method Example
It removes all trailing whitespace of a string and can also be used to remove particular character from trailing.

string1="    Hello Python     ";  
print string1.rstrip(); 
string2="@welcome to SSSIT!!!" 
print string2.rstrip('!'); 

Output:
>>>
          Hello Python
@welcome to SSSIT
>>>