DEV Community: Mohit Raj

4.3 String slicing

Mohit Raj — Wed, 01 Jan 2020 10:15:36 +0000

4.2 index() and find() method - Python

Mohit Raj — Mon, 23 Dec 2019 20:13:15 +0000

Indexing in Python string

Strings are ordered sequences of character data. Indexing allows you to access individual characters in a string directly by using a numeric value. String indexing is zero-based: the first character in the string has index 0, the next is 1, and so on. In this post, I’ll show you string indexing syntax and practice with several examples

From the above figure-

first character in the string has index 0, the next is 1, and so on.
If start indexing from the last character then you have to start with -1 and as you come near to first character negative value increases by 1 each time. this is also called negative indexing.

How to access a character in string:

Simple answer is by using the index of all the characters in the string.

Example:

s='Hello'
#if i want to access 'H' in Hello then 
print("First character is : "s[0])      # indexing stated from 0

# if i want to access 'o' in s
print(" Last character using positive indexing : ",s[4])

#another way
print("last charcater using negative indexing : ",s[-1])    # using negative indexing

Output is:

First character is : H
Last character using positive indexing : o
last charcater using negative indexing : o

If we pass a index value which not exist for that string then it will return an error

s='python'
print("Third character is : ",s[2])
print("Seven character is : ",s[7])    # which is not possible for given string

Output gives an error for the second value

Third character is : t
Seven character is : 
IndexError: string index out of range

Python String index() Method

The index() method finds the first occurrence of the specified value.
The index() method raises an exception if the value is not found.

Syntax

     string.index(value, start, end)

value(Required.) -The string to be searched for.

start(Optional.)- This function specifies the position
from where search has to be started. Default is 0

end(Optional.) - This function specifies the
position from where search has to end. Default is to the end of the string

Return the starting index number of the passed valued according to the start and end value passed.

By default start value is 0 and end value is string length.

Example

sentence='hello world and hello python'
value='h'
x=sentence.index(value)   # start and end is default value
print(x)

y=sentence.index(value,2)  # end value is default
print(y)

z=sentence.index(value,6,8)   # give error because in between 6 and 8 'h' not exist.
print(z)

value='and'
a=sentence.index(value,6,20)
print(a)

Output is :

0
16
ValueError: substring not found
12

Python String find() Method

The find() method finds the first occurrence of the specified value.
The find() method returns -1 if the value is not found.
The find() method is almost the same as the index() method, the only difference is that the index() method raises an exception if the value is not found.
If value is not found find() method return -1.

Syntax

string.find(value, start, end)

value(Required.) -The string to be searched for.

start(Optional.)- This function specifies the position
from where search has to be started. Default is 0

end(Optional.) - This function specifies the
position from where search has to end. Default is to the end of the string

Example:

sentence='hello world and hello python'
value='h'
x=sentence.index(value)   # start and end is default value
print(x)

y=sentence.index(value,2)  # end value is default
print(y)

z=sentence.index(value,6,8)   #  Retuen -1 because value not found between 6 and 8.
print(z)

value='and'
a=sentence.index(value,6,20)
print(a)

Output is :

4.1 String formatting in python Part-2

Mohit Raj — Sun, 22 Dec 2019 15:17:35 +0000

The new f-strings in Python 3 :-

One of the many goodies packed into the new release are formatted string literals, or simply called “f-strings”.

First, they are called f-strings because you need to prefix a string with the letter “f” in order to get an f-string, similar to how you create a raw string by prefixing it with “r”, or you can use the prefixes “b” and “u” to designate byte strings and unicode strings.

The letter “f” also indicates that these strings are used for formatting.
Now Python already provides several ways for formatting strings, so you may wonder why the Python gods introduced yet another way.

people who overlook the Zen of Python states that simple is better than complex and practicality beats purity - and yes, f-strings are really the most simple and practical way for formatting strings.

To create an f-string, prefix the string with the letter “ f ”. The string itself can be formatted in much the same way that you would with str.format(). F-strings provide a concise and convenient way to embed python expressions inside string literals for formatting.

For example:

name='Mohit'
age=20
sentence=f"my name is {name} and i am {age} years old."
print(sentence)

Output is:

my name is Mohit and i am 20 years old.

As you see, this works pretty much like the .format() method, however you can directly insert the names from the current scope in the format string. This is much simpler than the old way, and avoids duplication:

name='Mohit'
age=20
sentence='He said his name is {name} and he is {age} years old.'.format(name=name, age=age)
print(sentence)

He said his name is Fred and he is 42 years old.

Sure, you can omit the names inside the curly braces since Python 3.1, like this:

name='Mohit'
age=20
sentence='He said his name is {} and he is {} years old.'.format(name,age)
print(sentence)

Same output appers:

He said his name is Fred and he is 42 years old.

But still, this is longer and not as readable as the f-string notation.

And it gets even better than this, as f-strings also support any Python expressions inside the curly braces. You can also write triple-quoted f-strings that span multiple lines, like so:

name='mohit'
seven=7
f'''He said his name is {name.upper()} 
and he is {6 * seven} years old.'''

Output is:

'He said his name is MOHIT\n and he is 42 years old.'

=>> F-strings are faster than the two most commonly used string formatting mechanisms, which are % formatting and str.format().

4.1 String formatting in python Part-1

Mohit Raj — Fri, 20 Dec 2019 07:28:31 +0000

In this tutorial, you’ll learn the four main approaches to string formatting in Python, as well as their strengths and weaknesses. You’ll also get a simple rule of thumb for how to pick the best general purpose string formatting approach in your own programs.

Let’s jump right in, as we’ve got a lot to cover. In order to have a simple toy example for experimentation, let’s assume you’ve got the following variables (or constants, really) to work with:

errno = 50159747054
name = 'dev'

Based on these variables, you’d like to generate an output string containing a simple error message:

'Hii dev, there is a 0xbadc0ffee error!'

That error could really spoil a dev’s Monday morning… But we’re here to discuss string formatting. So let’s get to work.

1. “Old Style” String Formatting (% Operator)

a) Strings in Python have a unique built-in operation that can be accessed with the % operator.

b) This lets you do simple positional formatting very easily. If you’ve ever worked with a printf-style function in C, you’ll recognize how this works instantly.

Here’s a simple example:

print("Hello, %s" % name)

Output is :

hello, dev

c) I’m using the %s format specifier here to tell Python where to substitute the value of name, represented as a string.

d) There are other format specifiers available that let you control the output format. For example, it’s possible to convert numbers to hexadecimal notation or add whitespace padding to generate nicely formatted tables and reports.

e) Here, you can use the %x format specifier to convert an int value to a string and to represent it as a hexadecimal number:

print('%x' % errno)   # first convert int into string and then hexadecimal value

Output is :

'badc0ffee'

f) The “old style” string formatting syntax changes slightly if you want to make multiple substitutions in a single string. Because the % operator takes only one argument, you need to wrap the right-hand side in a tuple, like so:

print('Hey %s, there is a 0x%x error!' % (name, errno))

Output is :

'Hey dev, there is a 0xbadc0ffee error!'

I’m sure you’ve been wondering why this printf-style formatting is called “old style” string formatting. It was technically superseded by “new style” formatting in Python 3, which we’re going to talk about next.

2. “New Style” String Formatting (str.format)

a) Python 3 introduced a new way to do string formatting that was also later back-ported to Python 2.7. This “new style” string formatting gets rid of the %-operator special syntax and makes the syntax for string formatting more regular.
b) Formatting is now handled by calling .format() on a string object.

Introduction:

Python’s str.format() method of the string class allows you to do variable substitutions and value formatting. This lets you concatenate elements together within a string through positional formatting.

This tutorial will guide you through some of the common uses of formatters in Python, which can help make your code and program more readable and user friendly.

Using formatters:

Formatters work by putting in one or more replacement fields or placeholders — defined by a pair of curly braces {} — into a string and calling the str.format() method.

You’ll pass into the method the value you want to concatenate with the string. This value will be passed through in the same place that your placeholder is positioned when you run the program.

Let’s print out a string that uses a formatter:

print('my age is {}'.format(20))

Output is :

my age is 20

a. In the example above, we constructed a string with a pair of curly braces as a placeholder: 'my age is {}'
b. We then added the str.format() method and passed the value of the integer 20 to that method. This places the value of 20 into the string where the curly braces were: my age is 20

We can also assign a variable to be equal to the value of a string that has formatter placeholders:

age='I am {} years old.'
print(age.format('twenty'))

Output is:

I am twenty years old

In this example, we concatenated the string "twenty" with the larger string, replacing the curly braces in the original string.

Using Formatters with Multiple Placeholders :

You can use multiple pairs of curly braces when using formatters. If we’d like to add another variable substitution to the sentence above, we can do so by adding a second pair of curly braces and passing a second value into the method:

strings="my name is {} and i am {} years old"
print(strings.format("DEV","twenty"))

Output is :

my name is DEV and i am twenty years old

To add another substitution, we added a second pair of curly braces into the original string. Then, we passed two strings into the str.format() method, separating them by a comma.

Reordering Formatters with Positional and Keyword Arguments :

When we leave curly braces empty without any parameters, Python will replace the values passed through the str.format() method in order.

As we have seen, so far, a formatter construction with two empty curly braces with two values passed through will look like this:

strings="my name is {} and i am {} years old"
print(strings.format("DEV","twenty"))

Output is :

my name is DEV and i am twenty years old

=>> The first pair of curly braces is substituted with the string value of "DEV", and the second pair is substituted with the string value of "twenty".
The values that exist within the method look like this: ('DEV','twenty')

=>> They are essentially the tuple data type and each individual value contained in the tuple can be called by its index number, which starts with the index number 0.

We can pass these index numbers into the curly braces that serve as the placeholders in the original string:

strings="my name is {0} and i am {1} years old"
print(strings.format("DEV","twenty"))

In the above example, the output will be what we get without passing index numbers into the braces as we are calling the values in the tuple in order:
Output is :

my name is DEV and i am twenty years old

=>> But, if we reverse the index numbers with the parameters of the placeholders we can reverse the values being passed into the string:

strings="my name is {1} and i am {0} years old"
print(strings.format("DEV","twenty"))

my name is twenty and i am DEV years old

=>>If you call an index number of 2 in a tuple that has values at index positions 0 and 1, then you are calling on a value that is out of range. When you call an index number that is out of range, you’ll receive an error message:

strings="my name is {2} and i am {0} years old"
print(strings.format("DEV","twenty"))

IndexError: tuple index out of range

The error message we see refers to the tuple only having values at index numbers 0 and 1, therefore placing index number 2 out of range.

Using Variables :

name='DEV'
age='twenty'
strings="my name is {} and i am {} years old"
print(strings.format(name,age)   # passing the variable defined

Output

my name is DEV and i am twenty years old

Conclusion

Using formatters for variable substitution can be effective way to concatenate strings and organize values and data. Formatters represent a simple but non-descriptive way for passing variable substitutions into a string, and are useful for making sure output is readable and user friendly.

Thank You

3.6 Exercise for you

Mohit Raj — Thu, 19 Dec 2019 12:23:58 +0000

Now its time to first exercise.

Q1. Ask user to input 5 numbers and you have to print the average of these five number. take all the input in one line.

Q2. You have few email id's :'abc@gmail.com' , ram@yahoo.com' , 'dev@gmail.com' , '123@gmail.com'
you have to print a list for each email id which contain two value one is values befor @ and one which is after @.

Q3. Take two number as an input (in one line) and print their sum,difference and multiplication value as an output.

Note : You can post your answer in the comment box. answer of these question covered in the next post.

Thank you

3.5 split() method in Python

Mohit Raj — Thu, 19 Dec 2019 12:07:56 +0000

3.4 More about variables

Mohit Raj ・ Dec 7 '19

#python #beginners #webdev

split() method

Split a string into a list where each word is a list item:
It breaks the given input by the specified separator.
If separator is not provided then any white space is a separator.
Generally, user use a split() method to split a Python string but one can used it in taking multiple input.

Syntax :

     input().split(separator, maxsplit)

a) seperator (optional): separator (optional)- The is a delimiter. The string splits at the specified separator.If the separator is not specified, any whitespace (space, newline etc.) string is a separator.
b) maxsplit (optional): The maxsplit defines the maximum number of splits.
The default value of maxsplit is -1, meaning, no limit on the number of splits.

Return Value from split():
The split() breaks the string at the separator and returns a list of strings.

Example 1: when no seperator is specified-

x='my name is abc'
print(x.split())

y='hii welcome'
print(y.split())

Run the above code and your output is look like

['my','name','is','abc']
['hii','welcome']

Example 2: when there is a seperator specified-

x='my,name,is,abc'
print(x.split())

y='my name,is,abc and,your name,is'
print(y.split())

when you run the above code and your output look like

['my','name','is','abc']
['my name','is','abc and','your name','is']

Example 3: when maxsplit is specified

grocery = 'Milk, Chicken, Bread, Butter'
print(grocery.split(', ', 2))
print(grocery.split(', ', 1))
print(grocery.split(', ', 0))
print(grocery.split(', ', 5))

['Milk', 'Chicken', 'Bread, Butter']
['Milk', 'Chicken, Bread, Butter']
['Milk, Chicken, Bread, Butter']
['Milk', 'Chicken', 'Bread', 'Butter']

Multiple input using split() method

Developer often wants a user to enter multiple values or inputs in one line. In C++/C user can take multiple inputs in one line using scanf but in Python user can take multiple values or inputs in one line by two methods.

Using split() method
Using List comprehension (discussed later)

a>. This function helps in getting a multiple inputs from user .
b>. It breaks the given input by the specified separator.
c>. If separator is not provided then any white space is a separator.
d>. Generally, user use a split() method to split a Python string but one can used it in taking multiple input

Examples

x, y = input("Enter two value: ").split()   # here seperator is space
print("first value is :",x) 
print("second value is :",y)

print()       # to break line

x, y = input("Enter a two value: ").split(",")   # here seperator is comma
print("first value is :",x) 
print("second value is :",y)

when you run the above code output is look like this

Enter two value:5 10
first value is :5
second value is :10

Enter two value:5,10
first value is :5
second value is :10

We can also take a list as an input as because split() method return a list as output.

Example

x=input("Enter the elements of the list by comma seperated :").split(",")
print(x)
print(type(x))

output looks like

Enter the elements of the list by comma seperated : a,b,c,d,e,123,name
['a','b','c','d','e','123','name']
<class list>

Thank you

3.4 More about variables

Mohit Raj — Sat, 07 Dec 2019 17:33:56 +0000

3.3 Type casting in Python

Mohit Raj ・ Dec 7 '19

#python #beginners #webdev

Declaring more than one variable in single line:

=>> yes we can declaring more than one variable in a single line and assign it different value.

for example

x,y=1,2
print(x)
print(y)

output is

1
2

==>> we can assign any variable with and data type of values in single line
For example

a,b,c=1,'mohit','123as'
print(a,type(a))
print(b,type(b))
print(c,type(c))

output is

1 <class int>
mohit <class str>
123as <class str>

=>> We can also assign a single value to more than one variable in single line.
For example

x=y=z=12
print(x)
print(y)
print(z)
print(x,y,z)
print(x+y+z)

Output is

Thank you and if you have any problem regarding this please let me know in the comment box

3.3 Type casting in Python

Mohit Raj — Sat, 07 Dec 2019 17:06:16 +0000

3.2 How to take User input in python

Mohit Raj ・ Nov 6 '19

#python #beginners #webdev

Type casting:

In computer science, type conversion, type casting, type coercion, and type juggling are different ways of changing an expression from one data type to another.
In Python input() function the input taken by the user is a string.
If we want to take a integer as an input then we have to convert the input into the integer. for this we can use int() function in python.

int function:

The int() function converts the specified value into an integer number.
for example,
i have to string variable and we want to add together then it will generally concatenate both the value .
Now if we want to add them because 12 and 13 is simple an integer value and after adding it 12+13=25. here we can use int() function to convert '12'(string 12) into 12(integer 12).
Syntax : int(x)

x='12'
print(type(x))

Output- <class str>

For converting into an int value

int_value=int(x)
print(type(int_value))

output - <class int>

Similarly you can convert y into an integer value.
Now add both and print

This is generally known as type cast (converting an expression of one data type into other)

float() function:

The float() method is used to return a floating point number from a number or a string.
As we convert a variable into int similarly we can type cast a variable into float.
Syntax: float(x)
floating numbers are like 12.23 ,34.00,123.9 etc means all number which contain decimal For example

x=12
print(type(x)
y=float(x)
print(type(y)
print(x," is " ,type(x)," value")
print(y," is ",type(y)," value")

Output

<class int>
<class float>
12 is <class int> value
12.0 is <class float> value

There are too many inbuilt function for type casting in python like str(),list(),dict(),tuple(),set() etc i will discussed this all in upcoming tutorials.

If you have any problem then put it in the comment box.

Thank you

Sorry for delay in the tutorial

Mohit Raj — Sat, 07 Dec 2019 15:51:49 +0000

Hey guys,
Sorry for teh delay in posting Python for Beginners tutorials.
Last tutorial that i post :

3.2 How to take User input in python

Mohit Raj ・ Nov 6 '19 ・ 2 min read

#python #beginners #webdev

on the "Python for Beginners series".
This is because of my semester exam on head that time. Now i am free from that and ready to post next tutorials.
Next part is coming as soon as possible and it will continue regularly one or 2 post a day.

Thanks for the patience.

3.2 How to take User input in python

Mohit Raj — Wed, 06 Nov 2019 16:05:49 +0000

3.1 String Concatenation

Mohit Raj ・ Nov 4 '19 ・ 2 min read

#python #beginners #webdev

2.3 Variables in Python

Mohit Raj ・ Nov 3 '19 ・ 2 min read

#python #beginners #webdev

User input

Developers often have a need to interact with users, either to get data or to provide some sort of result.
When you write a program sometime we need to give input from keyboard or any input device.
Python provides us with two inbuilt functions to read the input from the keyboard.

a). raw_input(prompt)
b). input(prompt)

raw_input() function

This function works in older version (like Python 2.x). This function takes exactly what is typed from the keyboard, convert it to string and then return it to the variable in which we want to store.
If you entered any number then these numbers also converted into string.

input() function

This function first takes the input from the user and then evaluates the expression, which means Python automatically identifies whether user entered a string or a number or list.
If the input provided is not correct then either syntax error or exception is raised by python.
Example : x=input("What is your name: ") Here, 'x' is a variable which will get the value, typed by user during the execution of program.
command written in the double or single quotes inside the input function is printed as it is on the screen as shown below.Here you can see that program will print 'What is your name:' and waiting for the user to input the value.
Program execuation will stop until user not given any input Here user enter the name. And this name value is stored in the variable 'x'.
After giving the input hit enter button so that program continue their execution.
Let's check whether x assigned the same value (user input) or not for this we use print function. Example : print(x) and we get the same output that the user input.

This is this simple way to take input from the user directly. you may take a list as an input. these all are discussed in upcoming posts.

If you have any problem regarding any topic till discussed in this series please let me known in the comment box.

Thank You.....

3.1 String Concatenation

Mohit Raj — Mon, 04 Nov 2019 06:18:14 +0000

Previous post in this series:

2.3 Variables in Python

Mohit Raj ・ Nov 3 '19 ・ 2 min read

#python #beginners #webdev

2.2 Python as a calculator

Mohit Raj ・ Nov 2 '19 ・ 2 min read

#python #beginners #webdev #github

What is string ?

Strings are the most popular types in Python.
We can create them simply by enclosing characters in quotes.
Python treats single quotes the same as double quotes.
Creating strings is as simple as assigning a value to a variable. For example −

var1 = 'Hello World!'
var2 = "Python Programming"

{here we can say that var1 and var2 are two string variable.for proofing this we use type() function.}

=>> you can convert a number into a string by putting it into the single or double quotes.
Example:
x=2 (here 2 is treated as a number)
x='2' or x="2" (here 2 is string. known this by using type function)

Type() function

type() method returns class type of the argument(object) passed as parameter. (more details in upcoming posts)
Synatx : type(paramenter)

Here we have two variable var1 and var2 so we use type function to know its data types we write
type(var1)
type(var2) , and then print it.

String concatenation

combining two or more string in a single string.
In order to merge two strings into a single object/string, you may use the “+” operator. When writing code, that would look like this:

str1="hello"
str2="World"
str3=str1+str2 , output is helloWorld.
This concatenation is not possible with integers.

String is only concatenation with string. if you try to concatenate one string with a number the it will show error. 2. Use of (*) in concatenation a. If you write str1="hello"*2 and print it then the output is hellohello Its meaning is similar as for the numbers when you write 4*2 it means you add 4 two times. similarly here when we write "hello"*2 it add "hello" two time ("hello"+"hello")=hellohello.

2.3 Variables in Python

Mohit Raj — Sun, 03 Nov 2019 12:09:07 +0000

2.2 Python as a calculator

Mohit Raj ・ Nov 2 '19 ・ 2 min read

#python #beginners #webdev #github

2.1 Python basics

Mohit Raj ・ Nov 2 '19 ・ 3 min read

#python #beginners #webdev

variables :

Variables are nothing but reserved memory locations to store values. It means that when you create a variable, you reserve some space in the memory.
Based on the data type of a variable, the interpreter allocates memory and decides what can be stored in the reserved memory.
There are different types of data types in python:
a. Numbers
b. String
c. List
d. Tuple
e. Dictionary
Note: I will discuss these all in separate section
A variable can hold different types of values.
for example if you assign a string to a variable then that variable belong to string data types,and if you assign number to the same variable the its belong to number data types.
Means to say that you can store any type of variable in any variable,that variable behaves according to that particular data type. need not to declare data type before the variable name (as done in language like C and C++).

How to declare variable

For declaration of variable in python no need to declare data type of it.
Syntax : variable_name = assigned_value
example :
name="abc"
x=23
roll=1 etc
You may assign values to more than one variable at a time.
example : a=b=c=2 means a=2,b=2,c=2.
You may also assign like this_
a,b=2,3 i.e. a=2 and b=3 (this is not possible in language like C or C++)

Rules for declaration of a variable

Variable name not start with a number (show syntax error) but you can use number inside the variable name.

Variable name only starts with any letters ,underscore(_) . this is for the first letter of the variable name
Not use any special symbol ($,^,@,%) in the variable name neither at the start nor in the middle or end.
keywords are not variables. They just have special meaning to the interpreter..
See below list for the python keywords these are not used as a variable :

=>> This is all about variable as we continue we know more about variables as we go deeper in it.

Thank You....