STRING IN PYTHON

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• Python Quiz – 01

In this tutorial we are going learn more about Python strings. We’ll learn how to create, edit and remove lines, also learn about associate operations and string functions.Now lets start from beginning, what are string?

Strings are one of most popular aspects of python which can be create by adding single or double quotes around characters. Strings in python are created using single (‘ ‘), double (” “) or triple (”’/ “””) quotes terminated by corresponding quotes. Triple-quoted strings are useful when the contents of a string literal span multiple lines of text.

>>> corona = 'stay home ! stay safe '

In the example above, we have declared a variable corona and defined it as string value ‘stay home ! stay safe ‘ using single quote. Similarly, we can use double quote as well.

>>> corona = "I will stay home "

Imagine, if you want to change I will stay at home to I’ll stay at home . Now you will run into problems because string will ‘I’ starts and end here. And all the character after the second apostrophe python interpreter does not know what they are.

>>> corona = 'I'll stay home'
output : SyntaxError: invalid syntax

So to solve this problem, we are going to use double quotes to define our string and we have single quote in middle of our string. Example is shown below:

>>> corona = "I'll stay home"
>>> print (corona)
output: 
    "I'll stay home"

What will you do to print I stay at “home”? For this kind of problem, we are going to use single quote to define our string and use double quotes in the middle of string. Let’s see the example below:

>>> quarantine = 'I stay at "home"'
>>> print(quarantine)
 output : 'I stay at "home"'

So far we only deal with single and double quotes, what if you wanted to define multiple line strings. For example, what if you want to define a string for the message that we send in an email. In that case we are going to use triple quotes to define our string. Let see the example below:

Slicing

Slicing operator [] can be used with string. Strings are stored as sequences of characters indexed by integers, starting at zero. To extract a single character, use the indexing operator s[i] like this:

>>> s="sawid"
>>> print (s[1])

output : a

Here output is ‘a’ since s[0]=’s’, s[1]=’a’, and similarly s[4]=’d’. As square brackets can be used to access elements of the string, we can access the range of characters using slicing. To obtain certain part of characters we specify starting and ending index separated by colon ‘:’.

+---+---+---+---+---+---+ 
| P | y | t | h | o | n | 
+---+---+---+---+---+---+ 
  0   1   2   3    4   5 
 -6  -5  -4  -3   -2  -1  

>>> a = 'Python'
>>> c = a[2:4]
>>> print (c)
output : th

Here, we have assigned the starting index 2 i.e ‘t’ and ending index 4 i.e 0. But character of index 4 is ignored and only 2nd and 3rd indexed characters are shown. This extracts all characters from s[i:j] whose index is in the range i<=k<j. If either index is omitted, the beginning or end of the string is assumed, respectively: Similarly, we can use negative slicing as follows:

>>> c = a[-4:-2]
>>> print (c)
output : h

In negative slicing, starting and ending index are reverse and negative sign(-) is introduced before them.

In slicing, start is always included, and the end always excluded

Concatenation of string

>>> a = " I "
>>> b = " Love "
>>> print ( a + b + " Python")
output : I Love Python

Strings in python can be concatenated (glued together) using plus (+) operator as shown in example above. Strings aren’t interpreted as numeric data. These numeric data acts as string and can be concatenated.

>>> a = "32"
>>> print (" Output is " + a)
output: Output is 32

To perform numerical operation, string data types must be converted into Numbers using int() or float () function. Similarly, non-strings data can be converted into string using str() function. Let’s take an example

>>> a = "24"
>>> c = int(a)  # converting string to integer
>>> print ( 26+ c)
output: 50

Here string a is converted into integer using int() function and value is stored in variable c. Hence on addition operation on a, output is obtained as 50.

Two or morestring literals (i.e. the ones enclosed between quotes) next to each other are automatically concatenated. This feature is particularly useful when you want to break long strings. This only works with two literals though, not with variables or expressions.

>>> a = ‘All is ‘ ‘ well’
>>> print (a)
output : ‘All is well’

Typecasting in String

>>> a = 32
>>> c = str(a)
>>> print (type(c))
output: <class 'str'>

In the above example, integer is changed into string using str() function. str()and repr() both are used to create strings.

>>> a = 5.225
>>> print(" Using str() : ", str(a))
>>> print(" using repr() : ", repr(a))

output:
  Using str() :  5.225
  using repr() :  5.225

Formatted Strings

Formatted string in python which is used to generate dynamic text from the variables. The curly bracket in formatted string defines place holder and when we run code, these place holder gets filled with value of previously defined variables.

Here ,we define formatted string with ‘f’ and curly bracket

>>> first_name = "Diwas"
>>> last_name = "Pandey"
>>> msg = f'{first_name} {last_name} is a Ai expert'
>>> print(msg)

output:
   Diwas Pandey is a Ai expert

Let a = ‘ I love Python 123’

METHODS	FUNCTION	EXAMPLE
capitalize()	Converts the first character to upper case	a.capitalize()
casefold()	Converts string into lower case	a.casefold()
count()	Returns the number of times a specified value occurs in a string	a.count(‘r’)
endswith()	Returns true if the string ends with the specified value	a.endswith(‘n’)
find()	Searches the string for a specified value and returns the position of where it was found	a.find(‘o’)
index()	Searches the string for a specified value and returns the position of where it was found	a.index(‘o’)
isalnum()	Returns True if all characters in the string are alphanumeric	a.isalnum()
isalpha()	Returns True if all characters in the string are in the alphabet	a.isalpha()
isdecimal()	Returns True if all characters in the string are decimals	a.isdecimal()
isdigit()	Returns True if all characters in the string are digits	a.isdigit()
islower()	Returns True if all characters in the string are lower case	a.islower()
isnumeric()	Returns True if all characters in the string are numeric	a.isnumeric()
isspace()	Returns True if all characters in the string are whitespaces	a.isspace()
isupper()	Returns True if all characters in the string are upper case	a.isupper()
lower()	Converts a string into lower case	a.lower()
len()	Returns length of string	len(a)
replace()	Returns a string where a specified value is replaced with a specified value	a.replace(‘python’,’ai’)
startswith()	Returns true if the string starts with the specified value	a.startswith(‘I’)
strip()	Returns a trimmed version of the string	a.strip(“a”)
swapcase()	Swaps cases, lower case becomes upper case and vice versa	a.swapcase()
title()	Converts the first character of each word to upper case	a.title()
upper()	Converts a string into upper case	a.upper()

The index() method is almost the same as the find() method, the only difference is that the find() method returns -1 if the value is not found.

index() generates error sub string not found

If we need to insert characters that are illegal as string, we use ‘\’ followed by the character ( example: \@ ). Other escape characters used in python are:

SPECIAL CHARACTER	FUNCTION
\’	Single Quote
\\	\
\n	New Line
\r	Carriage Return
\b	Backspace
\f	Form Feed
\ooo	Octal value
\xhh	Hex value

>>> print("i love \' programming")

output:
     i love ' programming 

>>> print("I love  programming \n I love python")

output:
     I love  programming 
     I love python