ecesem7.files.wordpress.com · Chapter 1 Introduction to Python and setting up Python Development...

Chapter 1 Introduction to Python and setting up

Python Development Environment

1.1 Why Python?

Python is considered as one of the most versatile programming languages. If you

have even a little experience in programming, then you will soon notice the

difference. Let's take a look at the features of Python.

1.2 Features of Python

1. It has simple easy-to-use syntax.

2. It can run on almost any platform including Windows, Linux's all distros(All

versions of Linux), Mac OS X, Unix etc.

3. You can even combine the pieces of other programming languages like C, C++,

etc with python to utilize the best of both worlds.

4. It's large standard library support makes real life programming much easier.

5. Other than that, it's open source, and have a huge community support working

actively with Python programming.

6. It's Object Oriented (also called OO Programming or OOP) which makes it

more applicable to be used in real world application programming.

7. Also, it can not only be used to write complex programs but can also be used to

design applications with GUI(Graphical User Interface).

8. Python is an Inerpreted language, as its the Interpreter which executes the

python code line by line, which makes it easier to debug. But it is a topic of never

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ending discussion that is Python compiled? Well, technically it is, from user

written code to bytecode, and the bytecode is then sent to the Interpreter to

execute it.

9. Python has numerous implementations like CPython(The Standard

implementation of python), Jython(Targeted for integration with java

programming language) etc.

As far as its simplicity of syntax is concerned, let's see an example code. Consider a

program where you want the user to enter something from the keyboard and you

want to save that value in a variable. For C++, a popular programming language,

here is how you will be writing it:

#include<iostream>

using namespace std;

int main(){

int x;

cin >> x;

return 0;

}

For Core Java, another popular programming language, it will be:

import java.util.Scanner;

class Test{

Scanner input = new Scanner(System.in);

public static void main(String args[]){

int x;

x = input.nextInt();

}

}

And in Python, it is (*drum roll*):


x = input()

No kidding. That's all it will take. No file importing, no curly braces, no semicolons

and just one line. It's really not necessary to understand that code piece right now. It

was just meant to demonstrate you that how Python can be a programmer's relief.

Another thing that can be noticed in above code is, in the case of C++ and Java user

can only type any integer as the input to variable x, because we have declared

value x as an integer by specifying the code: int x;. In case of Python, programmers

don't have to explicitly specify the data type while declaring a variable, python's

compiler will do that itself, based on the type of value assigned to the variable.

1.3 Few things about the course

To be precise there are currently two releases of Python that are available on their

official website i.e., Python 2.x and Python 3.x. In this course we will be dealing

with Python 2.x, the reason behind this is because it is ideal to learn Python 2.x first.

Python 2.x is legacy, Python 3.x is the present and future of the language

Python 2.x is what has been used in the programming world until recently when

Python 3.x stable version was released. Hence, for beginners Python 2.x is more than

perfect to use, because you will find plenty of resources for it on the Internet.

Moreover, Python 3.x has a poor library support, as it is very new, which means we

will have less options to find and use ready-made functions/libraries. Certainly, this

is not suitable for a beginner, but you can shift to Python 3.x once you get a hold over

Python programming. So, let's get started with Python 2.x.

Also, there is not too much difference between Python 3.x and Python 2.x, the major

change is in the way how you print anything. It was just print in Python 2.x and in

Python 3.x they made it print(), yes added the brackets.


1.4 Applications of Python

One can say, you can do almost anything with Python.

1. Web Application: Python can be used to develop scalable and secure web

applications. Frameworks like Django, Flask, Pyramid etc are amazing to design

and develop web based applications.

2. Computer Software or Desktop Applications: As python can be used to

develop GUI too, hence it is a great choice for developing desktop

applications. Tk is an open source widget toolkit which can be used to develop

desktop applications with python. Kivy is another such platform.

3. Scientific Computing Application: For its amazing computational power and

simple syntax, python is used for scientific computing applications. Python

libraries like SciPy and NumPy are best suited for scientific computations.

4. AI and ML(Artificial Intelligence and Machine Learning): Python is at the

fore front of the paradigm shift towards Artificial Intelligence and Machine

Learning.

5. Image Processing: Python is known for its image processing capabilities, which

includes traversing and analysing any image pixel by pixel. There are numerous

python libraries available for image processing, for example: Pillow, scikit-image

etc.


Starting with something that you might already be using: Websites

like Instagram, Reddit, Mozilla, have been developed using Python. There are

various web frameworks like Django (most popular one) and Pyramid, based on

Python, which can be used to develop modern web applications.

1.5 Installing Python

Steps to set up Python environment in your Windows system:

1. Go to python.org/download

2. Download the latest release for Python 2.x for 32-bit or 64-bit depending upon

your PC.

3. Open Installer and follow these steps:

Click on Next, once you have checked the selected options in the window.


https://www.python.org/downloads/

4. Once these steps had been followed, the installation will begin.

5. You can check whether the installation was successful by typing python -

V in cmd. If it returns the Python version that you have installed, then

congratulations you're all set.

1.6 Set Environment Variable

Windows users will also have to set the Environment Variable path with the

location of python directory, where it is installed. To set the path variable:

1. Right click on My Computer → Properties. Then in the left sidebar, click on the

last option with Advanced system settings


2. A new window will be opened, click on the button Environment Variable

3. Then to add a new User variable, click on New

4. Then write path as the variable name and copy the path of Python folder in the

value.

5. And it's done.

If you have a Macbook or you are using Ubuntu Operating System, then you do not

have to do anything, as Python comes pre installed with these operating systems.

1.7 Introduction to IDLE - Default Python IDE

In this section, you will get familiar with the development environment which has been

used throughout this tutorial series to run python code. It's probably the best

environment to write and execute the python code while learning. Moreover, there is no

extra installation required since it comes bundled with Python's compiler. So, if you

have followed the installation steps as given in the last chapter, you might already be

having IDLE in your system.

It's time to open IDLE now, you can do it by typing IDLE

in Windows/Mac/Linux search bar and it should appear.


This is pretty much how it looks. You can see the cursor blinking right after >>>. This is

where you will be writing your code. Also, the current running version of Python is also

mentioned at the top.


In IDLE we write code line by line. One line will handle one thing. You type whatever

you want in that line and press enter to execute it. IDLE works more like a terminal or

command prompt - You write one line, press enter, it executes.

We can also create python file which will contain the complete multiline program and

can execute that using IDLE as well. A python script has an extension .py.

Python takes some time to compile, its compilation is not fast, thus writing the example

code in a file, then compiling the whole code again and again gets tedious and is not

suited for beginners. When we open the IDLE, a session is created, which saves all the

lines of code that you write and execute in that one window as a single program. This is

the reason why, what you wrote above may affect what you will write later, eg. using a

variable. Here is a preview of how we will be typing in IDLE.

IDLE is pretty neat in its own way. You can choose custom colours for the background

and text, to give it your own style. There is an auto-complete feature, which predicts

what you are typing and suggests it (pretty much like Google search).


Above all, it is available for all major operating systems!

Chapter 2 Basic Syntax & First Program

In this tutorial we will try to understand the syntax of python programming. Although,

syntax is something that you will understand as you will see more and more programs

and examples, but there are a few things that you must know beforehand.

2.1 Python Syntax Rules

1. Python is case sensitive. Hence a variable with name yoyostudytonight is not same

as yoYoStudytonight

2. For path specification, python uses forward slashes. Hence if you are working with

a file, the default path for the file in case of Windows OS will have backward slashes,

which you will have to convert to forward slashes to make them work in your

python script.


For window's path C:\folderA\folderB relative python program path should

be C:/folderA/folderB

3. In python, there is no command terminator, which means no semicolon ; or

anything.

So if you want to print something as output, all you have to do is:

print "Hello, World!"

Oops! we just shared the first python program too with you. Yes, just one single

statement.

4. In one line only a single executable statement should be written and the line

change act as command terminator in python.

To write two separate executable statements in a single line, you should use

a semicolon ;to separate the commands. For example,

print "Hello, World!" ; print "This is second line"

Live Example →

5. In python, you can use single quotes '', double quotes "" and even triple quotes '''

""" to represent string literals.

word = 'word'

sentence = "This is a one line sentence."

para = """This is a paragraph

which has multiple lines"""


https://www.studytonight.com/code/python/use-semicolon-for-multiple-statements.php

6. In python, you can write comments in your program using a # at the start. A

comment is ignored while the python script is executed.

# this is a comment


# this is a

# multiline comment

7. Line Continuation: To write a code in multiline without confusing the python

interpreter, is by using a backslash \ at the end of each line to explicitly denote line

continuation. For example,

sum = 123 + \

456 + \

789

Live Example →

Expressions enclosed in ( ), [ ] or { } brackets don't need a backward slash for line

continuation. For example,

vowels = ['a', 'e', 'i',

'o', 'u']

8. Blank lines in between a program are ignored by python.

9. Code Indentation: This is the most important rule of python programming. In

programming language like Java, C or C++, generally curly brackets { } are used to

define a code block, but python doesn't use brackets, then how does python knows

where a particular code block ends. Well python used indentation for this.


https://www.studytonight.com/code/python/use-forward-slash-multiline.php

It is recommended to use tab for indentation, although you can use spaces for

indentation as well, just keep in mind that the amount of indentation for a single

code block should be same.

if True:

print "Yes, I am in if block"

# the above statement will not be

# considered inside the if block

the correct way would be,

if True:

# this is inside if block


Also, the following code will give error, as the statements are differently indented:

if True:



# this will give error

print "me too"

again, the correct way to do so is to keep all the statements of a particular code

block at same indentation.

if True:



print "me too"

So these are some basic rules that you must know so that it becomes easier for us to

learn various concepts of python programming in the coming tutorials.


2.2 First Python Program

We already shared the first python program with you while learning the syntax. Yes, we

were not joking, it's just one single line, nothing above it, nothing below it. To

print Hello, World! on screen, all you have to do is:


Live Example →

You can write and execute this code in IDLE, or you can save this code in a python code

file, name it test.py(you can name it anything, just keep the extension of the file as .py).

To run the test.py python script, open IDLE, go to the directory where you saved this

file using the cdcommand, and then type the following in command prompt or your

terminal:

python test.py

This will execute the python script and will show you the output in the line below.

From the next tutorial we will start learning the various concepts of python

programming language.

Chapter 3 Python Numbers, Variables and built-

in Math Functions

In this section, we will be learning about Numbers and various Math functions available

in python language. In Numbers, we will see some of the most commonly used math

operators that we can use to perform various operations on the numbers in python.

Under Math functions section, we will learn about some shortcuts (called functions),

which are very helpful in calculating some of the complex mathematical expressions

like power, sine/cosine, factorials etc. So, let's begin. We recommend keeping the IDLE

open, while reading, so that you can practice and learn simultaneously.


https://www.studytonight.com/code/python/python-first-program.php

3.1 Numbers

In Python we have 6 basic mathematical operators, they are:

1. Addition

2. Subtraction

3. Multiplication

4. Division

5. Modulo

6. Power

Most of you must be familiar with all of the above operators except for

the modulo operator. Don't worry we will explain it. Let's start from the beginning.

3.1.1 Addition

As you might have guessed it's just simple addition of numbers. In order to test the

operator, just go to IDLE and type a number, then addition sign +, and then another

number to add to the first number. Press Enter. This must look like this.

Example: Taking 8 and 19 as example,

>>> 8+19

27

On pressing return(or enter), the answer will appear just below the code line. And this

is how the output will be displayed, all the time - just below your code line. As you'll hit

the enter key, output will appear in the line below.

Don't stop with this example, try using the addition operator with other numbers. Try

number with decimal places, like 4.5 + 5.5 and so on.


3.1.2 Subtraction

Just like addition, subtraction has the same syntax. Just change the operator to -. Again,

pick some random numbers and try.

Example: We took 89.33 and 23.67, which gave the output 65.55.

3.1.3 Multiplication

Same again! Just change the operator to *, also known as an asterisk. You do know that

it's used for multiplication, right? Go ahead and try it in you IDLE.

Example: Take any two numbers and multiply them using the * operator, just like we

did below.

3.1.4 Division

Use / sign this time. And try with random numbers. Caution: If you're a beginner, you

might find some difficulty in this one. How? Let's see. Let's take some integer numbers

(numbers without decimal) like 16 and 2, and divide them.

>>> 16/2

8


Very well. Next, try with 15 and 2. What do you expect the answer would be? Well,

according to proper mathematics the answer should obviously be 7.5, but if you actually

try this in IDLE, the answer will turn out to be 7. This happened, because if we perform

any mathematical operation on an integers then the answer would be an integer. In our

case, 15 and 2 both are integers, hence, our answer is 7, as the answer has to be an

integer.

You might be wondering if it had to be an integer, why it turned out to be 7 and why not

any other integer number. Well, that is because the answer is determined as the closest,

smaller integer to the original answer. In our case, the original answer is 7.5, thus the

nearest integer to it is 7 and 8, and since we have to pick the smaller one; 7 is picked as

the answer. In mathematics, it is also known as floor function (it's there in Python too).

Now talking about the solution to the above problem, all you have to do is, convert any

one of the integers(that you want to divide) into decimal, i.e. write 15.0 instead of 15

and/or 2.0 instead of 2.

3.1.5 Power

This mathematical operator is not usually found in common programming languages. In

fact, Python is the only language we know which does have an operator for this. In rest

of the languages, they use some pre-defined functions (shortcut as we mentioned

before) to calculate this. Getting to the point, just put two asterisks like ** between any

two numbers. Example, to calculate 2 to the power 10, you have to write:

>>> 2**10

1024


With that, we now know about all the commonly used mathematical operators of

python. Now you can try to combine multiple operators and use them to form one

expression. We will recommend using brackets so that python can understand what

you want as the answer, i.e. instead of writing 2-9.0/2, write 2-(9.0/2).

Remember BODMAS, how a mathematical expression with multiple operators is solved

in mathematics.

3.1.6 Modulo

Modulo operator is denoted by % percentage sign. If you are familiar with the computer

programming world, chances are you already know this function. In case you don't, no

need to worry. You know division, right? Then you know what remainder is, correct?

This Modulo operator, when used with two operands, returns the remainder as the

answer. Here are some quick examples.

12%2 = 0, since 2 perfectly divides 12.

13%2 = 1, since dividing 13 with 2 leaves 1 as remainder.

19%5 = 4, because, again, 19/5 leaves 4 as the remainder.

It is used in pretty much the same way as it has been explained here.


To see all the Math operators covered above, live in action, click on the Live Example

button,

3.2 Math Functions in Python

As you learn more about python, you might decide to create a scientific calculator for a

project or anything. For that, along with simple mathematical operations, you will have

to evaluate some complex mathematical operations as well, like trigonometric

operations, logarithmic operations etc. Forget about the calculator, there can be various

situations where you might need these functions. Like a software for civil engineers to

calculate various parameters of any structure that they are building, or any aerospace

software - where they need various kinds of calculations about satellite trajectory,

shuttle trajectory and what not. In a nutshell, the complex mathematical operations are

used in various real life programs and softwares, hence you must know about them.

Now in Python, some nice guys have already created code pieces (libraries) for almost

every mathematical function. We can use these codes without any hesitation and the

plus point is, we won't have to re-write it again. Forget about rewriting, we don't even

have to know what the complete code is. We only need a few key information to be able

to use these readymade code pieces.

Alright, so unofficially function part had already begun. We'll learn about functions in

detail in later chapter, thus we'll keep this one short.

Function can be described as a piece of code that may or may not take some value(s) as input,

process it, and then finally may or may not return any value as output.


As you can see in the figure above, here input x is given to a function f and it is giving

some value f(x) as the output. Although in general programming world, depending upon

the purpose of the function, input and output are completely optional. But for a

mathematical function, it's very important to have both.

For example, in the trignometric function sin(x), there must be some value of x in order

to calculate and return the answer, and that pretty much establish why mathematical

functions have both inputand output.

In python, there are two types of pre-defined functions.

Inbuilt functions: These are the functions which doesn't require any other(external) code

file (known as, Modules or Library Files). These are a part of the python core and are just

built within the Python compiler hence there is no hassle of importing these

modules/libraries in our code.

The second type of functions require some external files(modules) in order to be used. The

process of using these external files in our code is called importing. So all we have to do is

import the file into our code and use the functions which are already written in that file.


It's time to try some of the functions. Let's begin with power functions.

3.2.1 Power - pow(x,y)

I know what you might be thinking. We just tried that, didn't we? Well, we did saw

something that can calculate power, but it was an operator and this one is an inbuilt

function (yes, the first type). So, with that just consider this one as an alternative way to

calculate power.

Since this one is an inbuilt function, you don't need to import any other library files

(or modules), hence it's pretty easy to implement.

Since power function will be needing two numbers(inputs) to perform the operation,

i.e. base and exponent, hence we will have to provide two numbers to the function. Go

ahead, open the IDLE and write:

>>> pow(3,2)

Now let's analyse what we did and what will happen. First, we wrote pow, which is

simply the name of the function that we are trying to call. This will tell the python

compiler to look out for an inbuilt function named pow and discover what it can do.

Next, within the brackets we wrote two numbers separated with a comma, i.e. 3 and 2.

Here the first number 3 is base and the second number 2 is an exponent, and we are

trying to calculate 32.

Once the python compiler has ensured that all the syntax (the grammar of

programming) is correct, it will look for the implementation of the function pow and use

it to find 32. So as you might have expected, the output would be:


With that, we now know, how a function is called. Especially for math functions, we can

generalise it as following:

>>> functionName(input1, optionalInput2, optionalInput3, ...)

The values inside brackets that had been separated by commas, which we mentioned to

you as input to functions, are called Arguments. As in pow(x, y) example given above, 3

and 2 were the arguments. There can be any number of arguments in a function. And as

we discussed earlier, for a mathematical function there usually is at least one argument

present. Let's see some another inbuilt mathematical functions.

3.2.2 Absolute - abs(x)

Absolute function, also known as Modulus (not to be confused with Modulo), returns

the non-negative value of the argument value. Therefore, absolute value of any non-

negative number is the same, while for negative numbers, their positive value is

returned.

Example: absolute value of -3 will be 3, absolute value of -8.74 will be 8.74 and so on.

Syntax:

>>> abs(-99.99)

Since -99.99 is a negative number, it's positive counterpart will be the output, i.e. 99.99.

Now let's try some functions where we have to import some modules(or library files).


3.2.3 Sine - sin(x)

Since we know sine is a trigonometric function, hence it accepts only one value as an

argument, i.e. x. Here x should be in radians, so you better not confuse it with a degree.

As we mentioned before we won't be able to use this function directly. If you do, you

might get an error, something like this, which will say name sin is not defined.

This is because the compiler doesn't know what it is supposed to do when it

encounters sin()function, as we have not defined this function but we are trying to use

it. So, in order to use it, we will have to import python's math module which consists

the implementation of the sin() function, which will guide the python compiler to

understand what to do when sin() is called.

What we are about to do is called importing a module and it's most oftenly done to use

already available ready-made functions. Importing a module takes just one extra line:

>>> import math

Hit enter, and you're done. Now in order to use the sin() function, go to a new line and

type:

>>> math.sin(3.14159)

Since 3.14159 is approximately the value of π hence the answer would be near to zero.

As you can see after math.sin(3.14159) statement, the answer returned was something

like 2.653589335273e-6, it might seem a little messy but it is an equivalent

representation of 2.653589335273 × 10^-6, or 0.000002653589335273.


Because of approximation in the value of π, the answer too just got deviated a little bit

from 0, but you can see the value is almost zero. And at the same time, you can see how

accurate the results are.

Now of course there are several other functions available inside math module,

like floor() (floor function; We mentioned this one in division

operator), exp() (Exponential function), log()(Logarithmic function), sqrt() (Square root)

and a lot more.

3.3 Variables in Python

This lesson deals with variables. Those who already know some programming must be

familiar with the concept of variable and its importance. Variable is an important

element of programming world, visually, you can consider variable as a box, that is

capable of storing some value within it.

In more formal terms, variables are used to store information to

be referenced and manipulated in a computer program.

They also provide a way of labelling data with a descriptive name, so our programs can

be understood more clearly by the reader and ourselves. It is helpful to think of

variables as containers that hold information. Their sole purpose is to label and store

data in the memory. This data can then be used throughout your program.

Let's see some examples. A variable is supposed to have a name. There are some rules to

assign a name to the variable.

The variable name can consist of alphabet(s), number(s) and underscore(s) only.

The first character in variable's name cannot be a number. Hence i-am-

variable, variable!, 1variable, #variable, are all invalid variable names.

While i_am_variable, variable, variable1, _variable, are all valid names.


3.3.1 Storing value in a variable

Time to see how we can store a value in our variable. Consider a variable (or a box, if

you like it casually) named x. We want this variable (or box) to store a number, say 11.

Then in order to do that just go to IDLE and type:

>>> x = 11

And press Enter key. With that our variable gets created, named x, and with a default

value 11stored in it, using the equal to = operator. Remember, Equal to operator is

always used to assign a specific value to any variable. Variable's name will always be on

the left side and it's value will be on the right. Let's create another variable, say y and

assign it value 25.

>>> y = 25

Now in the current IDLE session, python is dealing with two variables, x and y, which

have values 11 and 25 respectively. If you want to check the value of any variable in

IDLE, simply type the name of that variable in a new line and press the Enter key. The

value stored in it will be printed on the IDLE screen in a new line.

>>> x

11

>>> y

25

Now try to look at the below code, what do you think this will do?

>>> x = y

As you can see on the LHS(Left Hand Side) we have x and y on the RHS(Right Hand

Side), hence as we explained before, the value on the right will get assigned to the

variable on the left. Since y has a value 25 stored in it, this statement will modify the

value inside x from 11 to 25. And hence if you ask again for the value of x, it'll be 25 now.

In this case, we just over-wrote the value inside xvariable.

>>> x

25


Once you are done with the above example, let's be more creative this time while

naming our variables. Let's create a variable and assign your name as its value. So here

we are having a box named name, which is capable of storing any word. The process is

quite similar to what we did above but with just a tiny change. Watch carefully,

>>> name = "Sudytonight"

As you can see, we quoted our website's name within the double quotation marks. This

is because we don't want python compiler to get confused. Since Studytonight is a

word (or more precisely, stringin the programming world), we will have to surround it

with quotation marks. By doing so we tell python that it is a word. But, what is supposed

to happen if we write Studytonight without the quotation marks? Like this,

>>> name = Studytonight

Since there is no quotation marks, python will consider Studytonight as another variable

and will try to find the value stored within it so that it can further assign it to the

variable name. But since we never declared any variable with the name Studytonight,

python won't be able to find any value for it and in the end, it will throw an error saying

that the variable with name Studytonight is not defined.

Also, you can use both single quotation as well as double quotation in order to represent

a word (or string).

>>> name = "Studytonight.com"

>>> name = 'Studytonight'

Live Example →

Both are fine.


https://www.studytonight.com/code/python/using-variables.php

Try to use the variables as the arguments for the function. For example,

>>> x = 3

>>> y = 2

>>> pow(x, y)

9

>>> z = -7

>>> abs(z)

7

Next, you can try to save the answer of any mathematical function in a variable. Like,

>>> p = pow(5, 2)

>>> import math

>>> q = math.log(x)

Try using mathematical operators with these variables. Like,

>>> x+y

5

>>> x-y

1

Try using these variables in a mathematical expression including some mathematical

function and operators, all together, like,

>>> x = pow(x**y, p+2)

In the above code, python will first calculate the expression on the RHS(Right Hand

Side) first, and will use the old value for variable x, which is 3 and once the expression is

solved the answer will be stored in the variable x, which will become its new value.


3.4 Modules and Functions in Python

In one of the previous section, we learnt about the math module. We also checked some

of its functions like the sin, log, etc., and also learned how they are used. Now just like

the math module, there are several other modules available in python library which can

be used directly inside any python program. Some of them are so important that

programmers literally can't make any real-life app without these modules.

In this lesson, we will revisit Modules and the Functions within them and will try to

have a better understanding of them.

3.4.1 Functions lie inside the Module

Consider modules as, just another piece of python code, saved somewhere within the

python package and you can reuse the code from the module files in any other (or your)

python code file. Now try to recall what we did after importing the math module. We


tested some of the functions defined inside the math module,

like math.floor(8.4), math.sin(3.14159) etc.

NOTE: pow(5,2), math.floor(8.4), sin(3.14159) are all examples of calling a function.

To see how we can use the import statement to import external modules

like math module, checkout the live example,

Now if, what we have written above is just a way to call and use these functions. Where

is the actual logic(code) to perform these complex mathematical operations stored? i.e.

calculating power, floor, or sine for the provided value(s). All this (code or logic), reside

inside the module file and is called Function definition. Function definition sets the

rules that how a function will interpret the input provided, perform some operation on

it and further what is to be returned as the output or result.

In fact, it is the function definition itself that decides the type and number of input(s)

and output (output is always one, but inputs can be multiple) for the functions. These

inputs have an official term for them, they are known as Parameters.

Parameter: Structure that defines the type of the input accepted, while defining the

function.

Argument: The actual value that is inserted as an input while calling/using the function.

Suppose we have created a module file StudyTonight.py (.py implies that it's a python

code file; Yes! you can create a module file yourself too, it's just python code after all).

Inside the module file, we have defined a function that can solve a quadratic

equation ax2 + bx + c = 0. Let the name of the function be qSolver(a, b, c).

All you have to do to use this function is, provide the value of a, b and c, i.e., coefficient

values of the equation, and the function will print whether the two solutions are real or

not and what are they. So, lets see how can we import the module and use the function.

Step 1: First import the module.

>>> import StudyTonight


Step 2: Next call the function with appropriate arguments, i.e., for 2x2-6x+3=0, a=2, b=-

6 and c=3. Hence we will call,

>>> qSolver(2, -6, 3)

You don't have to try this function on IDLE since this won't work. Why? Well because

we hadn't actually created any module named StudyTonight.py in your local

system(laptop/computer). This was just to explain to you how module and functions

actually work and how we can create custom modules with functions defined by us. You

have to follow the same steps in order to call every function from the modules.

There are already many modules available for python. Some might have already been

installed in your system when you installed python, while others can be easily

downloaded from the internet. It all depends on what you need.

3.4.2 How to use a function defined inside a

module

Following are the things you should know while using a function defined inside a

module:

1. You must know which module contains that function.

2. Check if it's already in your system. If it is, then all good, otherwise go ahead and download

it from the internet.

(NOTE: You can check by importing that module in IDLE. Error means it's not

there.)

3. Next, identify the name of the function that you want to use.

4. Next, call the function. <module-name> is module's name, <function-name> is function's

name and <agrument-1>, <argument-2>... are the arguments of functions.

>>> <module-name>.<function-name>(<argument-1>, <argument-2>, ...)


Note that the number of arguments depend on the function we are using. In some cases,

function might not accept any argument. It entirely depends on which function you are

calling, hence you must check the documentation to see the function definition before

using any function. Above called function may or may not return any value (function's

output). In some cases, we are often require to store the output for future use. We do so

by storing it in a variable, which we have already learnt in the last tutorial.

>>> variable-name = <module-name>.<function-name>(<argument-1>, <argument-2>, ...)

This only make sense when the function returns any value (output). Also, functions can

only return one output value or none at all.

Let's use what we have understood till now, to use a function lower() which is defined in

the stringmodule.

3.5 Additional Topic (What is String?)

String is just a term used to refer to a word like "Studytonight" or sentences

like "Studytonight is a good website".

They must be enclosed in single or double quotes ('...'/ "..."). In the previous chapter, we

have already explained how they can be stored in a variable. Variable which stores a

string inside them are called string variables.

Now the string module is different from the above explained String. This one is just a

module with name string with functions defined to perform various operations on

string variables.

So, the string module consists of some functions which deal with operations on strings.

In the example below, we will use lower(), which is used to convert every character of a

string to lowercase.

1. We already know that string is the library/module which consists of the function lower().

2. Import the string module to check if it's available. Since it's a basic module, it's very likely

that it will be there.

3. Check the documentation and find the function lower() in the module's page. It will give

you the information required to use it.


https://www.studytonight.com/python/variables-in-python

4. It only has one argument which is a string. So create a string variable with value as "Let's

StudyTonight".

5. Next call the function,

>>> string_variable = "Let's StudyTonight"

>>> string.lower(string_variable)

Or, if you want to be direct and don't waste memory on saving a variable, just do it

like,

>>> string.lower("Let's StudyTonight")

Calling a function like we did above will only print the result when we hit enter. Just

like this,

As you can see, the returned value is having all uppercase letters (Capital letters)

converted to lowercase, i.e. L to l, S to s and T to t. In case you used a variable, keep in

mind that using a function like this doesn't modify the value of the original variable.

Thus even after using the function, the value of string_variable will remain as it was

before. So if you want, you can save the modified value in some other variable, or even

in the same variable itself, like:

>>> string_variable = string.lower(string_variable)

This statement is perfectly valid since python will compile everything from right to left,

i.e., it will first use string.lower() to get the modified string and then later assign the

output value to the string_variable variable using the = (equal to) operator.


Chapter 4 Input and Output in Python

If you think about it, you will realise that most of the real world programs or softwares

that you use, requires some input from the user.

Be it any smartphone application, like Facebook or Instagram where you have to first

input your email/username and password to login, then some posts in form of text or

photos in order to make it available on your profile. Or, in any ticket booking software

where you first have to specify the train/bus/movie name; etc.

Software is a bundle of programs, working together to perform a task or solve a

problem, that makes our life easier. And in order to perform any task or solve the

problem, the software must know the parameters of the problem(or, task to perform).

This is where programs take input from the user. Also, after taking these inputs the

program must process them and return some result. This is where outputcomes into the

picture.

In this chapter we will learn how to create a program in python that can take some

input from the user and print it back.

4.1 Taking Input from user

So lets say, you want to make a simple calculator in python. For that, you would have to

take one or maybe two input values from the user, then perform the user requested

mathematical operation on the input numbers and return the output. For simplicity,

let's just fix the user requested mathematical operation is addition for now.

We can add more operations to our simple calculator later, but let's stick to addition for

now. In order to perform addition, you will have to take two numbers as input from the

user (much like an ordinary calculator). The inputs must be assigned to or stored

in variables (say x and y). Now, below is the code, to accept user input, let’s see:

>>> x = input()


Write the above code line in the IDLE and just press the Enter key. You will notice that

the cursor will not move to the new >>> line. Rather, it will start waiting for the user

input.

This is the time when you enter any number from the keyboard (lets say, 3). Just enter

any number and press the Enter key again and now you will see the newline with >>>.

And just like that, using the input() function (yes, it is a built-in function) you just stored

a value in the variable x. In order to make it more fancy, try this:

>>> x = input("Enter the first number:")

Live Example →

Now you will notice that the next line will print the message: "Enter the first

number:" and will wait for the user input. As you might have already guessed, printing a

message like this will inform the user that they should enter a value/number/input for

the program to proceed ahead. A good program should always have such intuitive

messages.

Next, ask for the value of y variable.

>>> y = input("Enter the second number:")

Now, its time to play around a little. Try taking input for variable x or y again and this

time instead of typing a number try typing any alphabet or any string. As you press the

Enter key, you will see python will throw an error.


https://www.studytonight.com/code/python/input-output-example.php

This clearly shows that input() method is not capable of handling anything other than

numbers. Then, of course there must be some other way to take words and other non-

number characters as input.

raw_input() is the function which helps us to do that. This function can take any non-

numeric data as input, be it a string or a character. Every input,

with raw_input() function, is stored in the form of a string during the input.

NOTE: Remember, there is huge difference between a number 1 and a string "1". The

number 1 is just a number to python which can be added, subtracted, multiplied etc., while

a string "1" isn't a number, instead it's a symbol (anything between double/single quote is

a symbol). Python can't do any mathematical operations on a string. We will discuss this in

later lessons.

Thus, if you use raw_input() and input any number, python won't be able to do any

mathematical operation on it. Now try to create a code snippet that can accept your

name. Remember, name is a string, hence we will have to use raw_input().

4.2 The Output

Now how can we display the input values on screen? You might think that all we have to

do is just type the variable and press the Enter key. Well, it is true that we have been

doing this the whole time, but this only works when you are working on IDLE.


While creating real world python programs you have to write statements that outputs

the strings, or numbers explicitly.

We use the print statement to do so. Consider, printing a popular statement "Hello,

World". To do so, just type,

>>> print "Hello, World"

Don't forget the quotes, double or single, both will do just fine. Press the Enter key and

you will see the statement being printed at the bottom. If you try to just write "Hello,

World" and remove print statement, like,

>>> "Hello, World"

You will see that this will also print the line, however this time with quotes around it.

This is how you can differentiate among the two methods. Next, lets try printing a

number:

>>> print 9

Now try, printing two strings together. Create two variables, a and b. Assign them two

string values,

>>> a = "Hello"

>>> b = "World"

Now print them together.

>>> print a + b

HelloWorld

This is how it must appear. This is the example, showcasing how python can interpret

one single thing in two ways. Here the + operator, is used to join two strings instead of

performing mathematical addition (which is actually impossible to perform on two


strings anyways). Since, the mathematical addition of two strings doesn't make any

sense, hence whenever python encounter two strings with a + operator in between

them, it will just join them both.

The above operation can be performed in an another way as well:

>>> print "Hello" + "World"

HelloWorld

Now try printing a number and string together.

>>> print 5 + " is a number"

You will get an Error. Can you guess why?

This was because we tried to join an integer with a string. So lets establish a rule here

that-

"While printing we can't mix up two literals of different types."

Solution? We will have to convert the number 5 into a string. One way can be,

>>> print "5" + " is a number"

Or,

>>> print str(5)+ " is a number"

str() function, is just another built-in function that can be used to convert an integer into

a string. It is quite useful when it comes to converting a numeral variable (variable

consisting of numbers) into string. Like,

>>> x = 1

>>> print str(x)+ "st rule of Fight Club we don't talk about Fight Club"

1st rule of Fight Club we don't talk about Fight Club

To conclude, let's finish our calculator with addition function. So, far this is what we

have done:

>>> x = input("Enter the first number")

>>> y = input("Enter the second number")

Now, its time for the output.


>>> print x+y

Or,

>>> print str(x+y)

Both will give the same output. Though, result of x+y will be a number, while the result

of str(x+y)will be a string.

Chapter 5 Data Types in Python

Let's do a quick overview of the data types that are available in Python. Of course,

this will become more and more clear as you use practice python more.

First, let's understand what Data Types are? As the name itself suggests, these are

some different types of data that the python compiler understands. Just like the

Computer can understand only binary numbers (i.e., 1 and 0), so that is the one and

only data type that a Computer can understand but python has more. As far as

Python, programmers are provided with various ways of representing data - like

your name, roll number, marks, to the height of Mt. Everest, world population etc.

Basically, anything that holds some raw information.

5.1 Types of data in Python

Data in Python can be one of the following 5 types:


1. Numbers

2. None

3. Sequences

4. Sets

5. Mappings

5.1.1 Numbers

Number is a data type that we have already dealt with in the previous lessons. We

used them with mathematical operators, in functions, with variables, and in input-

output tutorial as well, we guess everywhere till now, and that's what make them

crucial. As you might have noticed at a few places in the previous tutorials, we have

mentioned about Integers and Floating point numbers, well this is where we'll learn

the difference between them. Numbers are further divided into three more types:

5.1.1.1 Integers and Boolean

This set comprises of all the Integers like 1, 2, 3,..., 0, -1, -2, -3,... and Boolean values

(the one that computer understands) which are true or false (1 or 0 respectively).

5.1.1.2 Floating Point Numbers

This set deals with the numbers having decimal point values (or any real number).

For example, 1.2, 4.76832, 8.0 are all floating point numbers.

5.1.1.3 Complex Numbers

Yes, unlike any other programming language, there is a data type for complex

numbers in Python. Type either 1+2j or 1+2J in IDLE and it won't return any error

because it's a perfectly valid complex number in python language. We know,


traditionally we have been using iota (or i) for representing complex numbers but

python opted for j instead i. Which is totally fine. Who are we to judge?

5.1.2 None

Think of a scenario where you don't want the variable to have any value. What will

do? You can't use 0 because it's a number after all. That is where None comes into

picture. Just put:

>>> x = None

And x will have no value.

5.1.3 Sequence

A Sequence is an ordered collection of items, indexed by positive integers. It is a

combination of mutable (a mutable variable is one, whose value can be changed)

and immutable (an immutable variable is one, whose value can not be changed)

data types. There are three types of sequence data type available in Python, they are:

Strings

Lists

Tuples

5.1.3.1 String

It is an ordered sequence of letters/characters. They are enclosed in single quotes

(' ') or double quotes (" "). The quotes surrounding the string are not part of the

string. They only inform the compiler about where the string begins and ends. They


can have any character or sign, including space in them. These are immutable. A

string with length 1 represents a character in python. You know we have used

strings on several occasions in earlier tutorials. Basically, they help us to store and

implement literature words in Python. For

example, "Saharsh", 'Saharsh', "123", '123' are all strings.

5.1.3.2 Lists

It is also a sequence of values of any type. Values in the list are

called elements/items. Lists are mutable and indexed/ordered. The list is

enclosed in square brackets [ ]. For example, [2, 6, 8, 3, 1] or ["Python", "Java",

"C++"] are both lists. You can notice that it's no different from the list we use in our

daily life. A List is just a bunch of numbers or strings that are kept together inside

the square bracket, sequentially. Each element of the list can be accessed by using

it's index number.

5.1.3.3 Tuples

They are a sequence of values of any type and are indexed by integers. Tuples are

enclosed in curved brackets i.e., (). Tuples are also pretty much like Lists, except that

they are immutable, hence once we assign it some value, we cannot change it later.

At the same time, it is faster in implementationthan List.

5.1.4 Sets

Set is an unordered collection of values of any type with no duplicate entry. It

is immutable (their values can't be changed later; try to keep that definition in

mind).


5.1.5 Mapping

This data type is unordered and mutable. Dictionaries fall under Mappings.

Dictionaries

Dictionaries pretty much like our dictionary can store any number of python objects.

What they store is a key-value pair, which is accessed using the key. Dictionary is

enclosed in curly brackets { }.

5.2 String in Python

This isn't the first time that we are encountering Strings since we have started learning

python. In many of the previous tutorials we have used strings in examples or discussed

about it, so it shouldn't be an ambush for you. Nonetheless, this chapter will give you

more insight about how they can be used, manipulated and implemented in python

world. We will also checkout some handy string functions to manipulate string. So,

without wasting time let's jump right into it.

5.2.1 What is a String?

String can be defined as a sequence of characters, and that's the most basic

explanation of string that you can provide. In this definition, we can see two important

terms, first being sequence and other is characters. What is Sequence data type and

how Strings are a type of sequence. Just for revision, in python, Sequence is a data type

which is made up of several elements of same type, i.e., integers, float, characters,

strings etc.


Note: There is a unique code provided to all existing characters. The coding convention

had been labelled as Unicode format. It consists of characters of almost every possible

languages and in fact emoticons too (yes, emoticons had been declared as characters too).

Hence, strings can be considered as a special type of sequence, where all its elements

are characters. For example, string "Hello, World" is basically a sequence ['H', 'e', 'l', 'l',

'o', ',', ' ', 'W', 'o', 'r', 'l', 'd'] and its length can be calculated by counting number of

characters inside the sequence, which is 12.

Note: Yes, space, comma everything inside those quotes will be a character if the length is

1.

Generally in programming languages there is a different data type dedicated to

characters only, while in Python, there is no character data type. Instead characters are

just treated as a string of length 1.

5.2.2 Declaration of Strings

>>> mystring = "This is not my first String"

>>> print mystring

This is not my first String

Live Example →

You can access each individual character of a string too. Just like accessing each element

of a Sequence, we can use index numbers for this purpose. To access first character

of mystring, we can do following:

>>> print mystring[0]

T

Since T is the first character of our string This is not my first String, hence it will have

index number as 0 (zero). Similarly, for further characters we can use index 1, 2, 3 and

so on, i.e., in order to access ith element we will have to use (i-1)th index.


https://www.studytonight.com/code/python/introduction-to-strings.php

There is another trick to access elements of the sequence from its end. For example, if

you want to access the last element of the sequence just do the following:

>>> print mystring[-1]

Writing -1 in the index will imply that you are asking for the 1st element from the last.

Similarly, in order to access 2nd last element use -2 as index, for 3rd last use -3 and so

on, i.e., for ith element from the last use -ith as the index. So that settles the

generalization for accessing each character from both forward and backward side in a

string. Note that positive index number implies you are accessing character from the

forward side, while negative index number means you're accessing it from the rear end.

We can conclude the what we have covered till now in a simple table. Consider a

string PYTHON. For this each character can be accessed in two ways - from the front, or

from the rear end.

Characters P Y T H O N

Forward Index 0 1 2 3 4 5

Backward Index -6 -5 -4 -3 -2 -1

5.2.3 Escape Sequence

Suppose you want a string to store a quote by Mahatma Gandhi.


"You must be the change you wish to see in the world" - Gandhi

This is the exact line you want to display in the console. And you also wish to have the

quotes surrounding this sentence. As you go ahead and print the statement, you will see

that it isn't that simple.

Python will instantly return a syntax error. This is because of those extra double quotes

that we added. In above image you can notice that Gandhi's quoted text is in black

colour, while "- Gandhi" is in green. Also, if you have used IDLE enough you might know

that all the characters inside the string are highlighted in green in the IDLE (it can be

other colours too depending upon text editor, python version, OS etc). This clearly

means that Python isn't treating You must be the change you wish to see in the world part

of the sentence as a string. Therefore, this concludes that whenever we open a quote

and close it, to declare a string, whatever we write after the closing quote, is just

considered as some python keyword.

Like for the above quotation text, we started the string with two double quotes and

wrote You must be the change you wish to see in the world just next to it, since

double quote was already closed before this phrase, hence Python considered the entire

sentence as some non-understandable python keywords. After the phrase, another

double quote started, then came - Gandhi after that and finally the closing double quote,

since - Gandhi part is within a pair of double quotes hence its totally legitimate.

Now you understand the problem that we can face if we use uneven number of double

quotes. Now let's see how we can actually have a quote in a string. Well, there are two

ways to do so:


1. First one is a bit compromising. You can use single quotes inside of double quotes,

like:

>>> print "'You must be the change you wish to see in the world' - Gandhi"

‘You must be the change you wish to see in the world' - Gandhi

Hence, it's legitimate to use single quote inside double quotes, however, reverse is

not true, i.e.,

>>> '"You must be the change you wish to see in the world" - Gandhi'

Will give an error.

2. Second one is for those who hate to compromise, or just want to use the double

quotes. For you people, there is something called escape sequence or literally

speaking, a back-slash\. You can use it like:

>>> print "\"You must be the change you wish to see in the world\" – Gandhi"

Can you guess what happened? We used backslash or escape sequence at two

places, just before the quotes which we directly want to print. If you want to inform

the compiler to simply print whatever you type and not try to compile it, just add an

escape sequence before it. Also remember, you must use one escape sequence for

one character. For example, in order to print 5 double quotes, we will have to use 5

backslashes, one before each quote, like this:

>>> print "\"\"\"\"\""

5.2.4 Operations on String

String handling in python probably requires least efforts. Since in python, string

operations have very low complexity compared to other languages. Let's see how we

can play around with strings.


1. Concatenation: No, wait! what? This word may sound a bit complex for absolute

beginners but all it means is - to join two strings. Like to join "Hello" with "World",

to make it "HelloWorld". Yes, that's it.

>>> print "Hello" + "World"

HelloWorld

Yes. A plus sign + is enought to do the trick. When used with strings, the + sign joins

the two strings. Let's have one more example:

>>> s1 = "Name Python "

>>> s2 = "had been adapted "

>>> s3 = "from Monty Python"

>>> print s1 + s2 + s3

Name Python had been adapted from Monty Python

Live Example →

2. Repetition: Suppose we want to write same text multiple times on console. Like

repeat "Hi!" a 100 times. Now one option is to write it all manually,

like "Hi!Hi!Hi!..." hundred times or just do the following:

>>> print "Hi!"*100

Suppose, you want the user to input some number n and based on that you want a

text to be printed on console n times, how can you do it? It's simple. Just create a


https://www.studytonight.com/code/python/string-operation-examples.php

variable n and use input() function to get a number from the user and then just

multiply the text with n.

>>> n = input("Number of times you want the text to repeat: ")

Number of times you want the text to repeat: 5

>>> print "Text"*n

TextTextTextTextText

3. Check existence of a character or a sub-string in a string: The keyword in is

used for this. For example: If there is a text India won the match and you want to

check if won exist in it or not. Go to IDLE and try the following:

>>> "won" in "India won the match"

True

Amongst other datatypes in python, there is Boolean datatype which can have one

of the possible two values, i.e., either true or false. Since we are checking if

something exists in a string or not, hence, the possible outcomes to this will either

be Yes, it exists or No, it doesn't, therefore either True or False is returned. This

should also give you an idea about where to use Boolean datatype while writing

programs.

Go ahead, try out some more examples of the in keyword, where you can take any

string and substring or character as input one by one from the user and then

print true or false using the in keyword.


4. not in keyword: This is just the opposite of the in keyword. You're pretty smart if

you guessed that right. Its implementation is also pretty similar to the in keyword.

>>> "won" not in "India won the match"

False

You can see all the above String operations live in action, by clicking on the below Live

example button. Also, we suggest you to practice using the live compiler and try

changing the code and run it.

Live Example →

5.2.4 Converting String to Int or Float datatype

and vice versa

This is a very common doubt amongst beginners as a number when enclosed in quotes

becomes a string in python and then if you will try to perform mathematical operations

on it, you will get error.

numStr = '123'

In the statement above 123 is not a number, but a string.


https://www.studytonight.com/code/python/string-operation-examples.php

Hence, in such situation, to convert a numeric string into float or int datatype, we can

use float() and int() functions.

numStr = '123'

numFloat = float(numStr)

numInt = int(numFloat)

Live Example →

And then you can easily perform mathematical functions on the numeric value.

Similarly, to convert an int or float variable to string, we can use the str() function.

num = 123

# so simple

numStr = str(num)

5.2.5 Slicing

Slicing is yet another string operation. Slicing lets you extract a part of any string based

on a start index and an end index. For example, if we have a string This is Python

tutorial and we want to extract a part of this string or just a character, then we can use

slicing. First lets get familiar with its usage syntax:

string_name[starting_index : finishing_index : character_iterate]

String_name is the name of the variable holding the string.

starting_index is the index of the beginning character which you want in your sub-

string.

finishing_index is one more than the index of the last character that you want in

your substring.

character_iterate: To understand this, let us consider that we have a string Hello

Brother!, and we want to use the slicing operation on this string to extract a sub-

string. This is our code:


https://www.studytonight.com/code/python/python-string-to-int-float-coversion.php

>>> str = "Hello Brother!"

>>> str[0:10:2]

Live Example →

Now str[0:10:2] means, we want to extract a substring starting from the

index 0 (beginning of the string), to the index value 10, and the last parameter

means, that we want every second character, starting from the starting index. Hence

in the output we will get, HloBo.

H is at index 0, then leaving e, the second character from H will be printed, which

is l, then skipping the second l, the second character from the first l is printed, which

is o and so on.

It will be more clear with a few more examples:

Let's take a string with 10 characters, ABCDEFGHIJ. The index number will begin

from 0 and end at 9.

A B C D E F G H I J

0 1 2 3 4 5 6 7 8 9

Now try the following command:

>>> print s[0:5:1]

Here slicing will be done from 0th character to the 4th character (5-1) by

iterating 1 character in each jump.

Now, remove the last number and the colon and just write this.


https://www.studytonight.com/code/python/python-string-slicing.php

>>> print s[0:5]

You'll see that output are both same.

You can practice by changing the values. Also try changing the value of the character

iterate variable to some value n, then it will print every nth character from starting

index to the final index.

5.2.6 Built-in String Functions in Python

For the final portion, we will see some really useful string functions to work with strings

in python. Below we have mentioned a few useful string functions.

5.2.6.1 len(string)

len or length function is used to find the character length of any string. len returns a

number and it takes a string as an argument. For Example,

>>> s = "Hello"

>>> len(s)


5

5.2.6.2 find(subString)

In case you want to find the position of any character or of a subString within any given

string, you can use the find function. It's implementation is a little different than a

normal function but it's not tough to understand. Obviously to find a subString in a

string, we will have to provide both the main string and the subString to be found, to the

funtion. For Example,

>>> s = "Hello"

>>> ss = "He"

>>> print s.find(ss)

0

Since, He is present at the beginning of the string Hello, hence index 0 is returned as the

result. It can be directly implemented/used as follows(in case you hate useless typing;

which every programmer do):

>>> print "Hello".find("He")

0

5.2.6.3 string_name.lower()

lower() function is used to convert all the uppercase characters present in a string into

lowercase. It takes a string as the function input, however the string is not passed as

argument. This function returns a string as well.

>>> print "Hello, World".lower()

hello, world


5.2.6.4 string_name.upper()

upper() is used to turn all the characters in a string to uppercase.

>>> print "Hello, World".upper()

HELLO, WORLD

5.2.6.5 string_name.islower()

islower() is used to check if string_name string is in lowercase or not. This functions

returns a boolean value as result, either True or False.

>>> print "hello, world".islower()

True

>>> print "Hello, World".islower()

False

5.2.6.6 string_name.isupper()

isupper() is used to check if the given string is in uppercase or not. This function also

returns a boolean value as result, either True or False.

>>> print "HELLO, WORLD".isupper()

True

>>> print "Hello, World".isupper()

False


5.2.6.7 string_name.replace(old_string, new_string)

replace() function will first of all take a string as input, and ask for some subString

within it as the first argument and ask for another string to replace that subString as the

second argument. For Example,

>>> print "Hello, World".replace("World", "India")

Hello, India

5.2.6.8 string_name.split(character, integer)

Suppose you're having a string say,

>>> mystring = "Hello World! Welcome to the Python tutorial"

Now we can use the split() function to split the above declared string.

If we choose to split the string into two substring from the exclamation mark !. We can

do that by putting an exclamation mark ! in the character argument. It will basically split

the string into different parts depending upon the number of exclamation marks ! in the

string. All the sub-pieces of the string will be stored in a list. Like,

>>> print mystring.split("!")

['Hello World', ' Welcome to the Python tutorial']

You can store these values to another variable and access each element of it like this:

>>> myNEWstring = mystring.split("!")

>>> print myNEWstring[0]

>>> print myNEWstring[1]

Hello World

Welcome to the Python tutorial


In the example below we have broken the string into 2 subStrings from a full stop !

Chapter 6 Lists in Python

In the last section, we discussed about strings and the various properties and

functions of strings, like how it can be thought of as a collection of various characters

just like a list. In other words, the list is an ordered set of values enclosed in square

brackets [ ].

An important difference to be noted is that list is mutable, i.e. it's values can be

modified.

As it is nothing but a set of values, we can use the index in square brackets [ ] to

identify an value belonging to the list.

The values that make up a list are called its elements, and they can be of any type.

We can also say that list data type is a container that holds a number of elements in a

given order. For accessing an element in the list, indexing is used, which can be

called from both forward and backward direction (similar to strings).

6.1 Constructing a List


As already explained, the list is a collection of similar type of data. This data can be

an integer, a float, a complex number, a string or any legitimate datatype in

python. The list is always provided with some name, similar to a variable, and each

element is accessed by their index number. Python has both forward and backwards

index number, so for each element there exists one positive and one negative

number to access that element (discussed in the next topic). While defining a list, we

have to enclose each element of the list within square brackets, and each element is

separated by commas.

Suppose, if you want to create an empty list (which is possible in case you want to

add the elements later in the program or when user gives the input), then you can

initialize it by declaring empty square brackets,

>>> myEmptyList = []

For example, list of some integers will be,

>>> myIntegerList = [9, 4, 3, 2, 8]

A list of float values,

>>> myFloatList = [2.0, 9.1, 5.9, 8.123432]

A list of characters,

>>> myCharList = ['p', 'y', 't', 'h', 'o', 'n']

A list of strings,

>>> myStringList = ["Hello", "Python", "Ok done!"]

Live Example →

These were some basic methods to create a list. Now let's try some other methods.

6.2 Deriving from another List

Suppose there is an existing list,

>>> myList1 = ['first', 'second', 'third', 'fourth', 'fifth']


https://www.studytonight.com/code/python/python-list.php

And now you want to create a new list which consists some or all the elements

of myList1, then you can you do a direct assignment for complete copying of

elements or use slicing, and take only some elements.

For complete copying,

>>> myList2 = myList1

Use slicing, for copying only the first three elements,

>>> myList2 = myList1[0:3]

Live Example →

For copying last three elements,

>>> myList2 = myList1[-4:-1]

6.3 Adding Serial Numbers in a List

Suppose you want to add serial whole numbers (i.e., 0, 1, 2, 3, ...) into your list and

just don't want to write all of them one by one, do not worry, there is a shortcut for

doing this.

For storing 0 to (n-1) numbers in a list use range(n) function.

>>> myList1 = range(9)

This will create a list with numbers 0 to 8 inside it.

>>> myList1 = range(5, 9)

Live Example →

This will create a list having numbers 5, 6, 7, 8 i.e., argument's first number to

the (argument's last number - 1).

This range() function can be used for various usecases.




Suppose you want to create a list with squares of all whole numbers. Although there

exists various programming approaches for this problem but here is a one line

method in python. We will be introducing something new, so it can get a little tricky.

>>> myQuickList = [x**2 for x in range(5)]

The final list will contain elements like [0, 1, 4, 9, 16], i.e. x**2 where x is varying

from 0 to (5-1).

for is the keyword here that you may not be familiar with. for is one of the keywords

that makes programming do a lot of mundane and redundant task for a programmer,

for example, it can create loops of execution. Although there will be a whole chapter

dedicated to this, the important thing to note here is that for is the keyword which is

responsible for iterating (or repeating) x in range 0 to 4 and finally storing the

iterated value's square i.e. (x2) in the list.

6.4 Appending to a List

Appending basically means adding to the existing. If you remember we told you

how you can create an empty list in python by just declaring an empty square

bracket. Now, what to do when you actually have to insert some elements inside the

list? This is where append function comes in use. You can add any number of

elements to the end of the list in one line without any hassle.

>>> emptyList = []

>>> emptyList.append('The Big Bang Theory')

>>> emptyList.append('F.R.I.E.N.D.S')

>>> emptyList.append('How I Met Your Mother')

>>> emptyList.append('Seinfeld')

Live Example →

So, that will basically create a list of some of the best sitcoms (and isn't empty

anymore). If you print the list.

>>>print emptyList



['The Big Bang Theory', 'F.R.I.E.N.D.S', 'How I Met Your Mother', 'Seinfeld']

6.5 Indexing of elements

We have already covered about using index to access the sequence of characters in a

string. Similarly, in python, index numbers can be used to access the elements of a

list too. Let's create a list of strings and try to access individual strings using index

numbers. Here is an example,

>>> fruitsList = ["Orange", "Mango", "Banana", "Cherry", "Blackberry", "Avocado", "Apple"]

As you can see in the code above, we have a list with 7 elements. To find the number

of elements in a list, us the function len just like for strings.

>>> len(fruitsList)

7

Live Example →

For every element in the list, following are the index numbers that we should use:

(try to understand how we assigned the index numbers.)

Element Forward index Backward index

Orange 0 -7

Mango 1 -6

Banana 2 -5

Cherry 3 -4

Blackberry 4 -3



Avocado 5 -2

Apple 6 -1

6.6 Utilising list elements in Python

Till now, we have created a list and accessed individual elements of that list, now its time

to learn how to use all of the elements in a list by iterating over them, using loops.

When you have a list with hundred or a thousand elements, and you want to access a

few of them or all of them to perform some operation or calculation on them, it would

be very hectic to use index numbers to access all the elements. In such cases, we use an

iterative method, to iterate over all the list items. To understand how to iterate over list

elements, we will be using loops. Don't worry loops will be covered in details in the next

chapter.

Chapter 7 Introduction to Loops

Loops are used for doing redundant and repetitive tasks.

7.1 Using for loop

Let's start with for loop. For iterating over a list, a for loop will need two things - First, a

reference variable to store the element being iterated and second, a source (list or

a range(), as in the previous case). When a for loop executes, elements of the source are

one by one copied to the reference variable for utilizing(performing operations) them

inside the loop. For example,

for x in range(0,5):

print x


0

1

2

3

4

In the above case, x is our reference variable in which, each element in the range(0, 5) is

getting stored, iteratively(one by one), simple enough.

Also, you must know that range(0, 5) returns a list with elements 0 to 4 in it.

7.2 Using while loop

Another way of iterating is by using the while loop. The while loop requires one

condition to function. The loop keeps on iterating till that condition is true. As soon as it

becomes false, the loops get terminated. For example,

i = 0

while i < 5:

print i

i = i+1

0

1

2

3

4

You can see that both the loops returned the same output, but the while loop, used the

condition i < 5 in order to get the output. In simple english, it implies that "while i is less

than 5, keep printing i and keep incrementing the value of i". Do you know why we

increment the value of i, everytime after printing its value? Well, it's because if we


hadn't incremented its value, the value would have remained the same as it was

declared initially, which is, i = 0, and the condition i < 5, would have always

been true and hence the loop would have never ended. Such condition leads to

something that we call an infinite loop.

7.3 Using Loops with List

Enough about loops, now let's see how we can use them to access the List elements.

Take a list with any type of element. Now, using the for loop we can access each element

of the list very easily. For accessing the elements, we have to use a reference

variable and our list variable as source.

myList = ['Last Of Us', 'Doom', 'Dota', 'Halo', ' ']

for x in myList:

print x

Last Of Us

Doom

Dota

Halo

7.4 Iterate two Lists simultaneously -

zip() method

Suppose there are two lists and you want to ADD each element of the first list serially to

each element of the second list and save it to a third (empty) list.

We can achieve this by executing a while loop like shown in below code, but this is not

the right way.


i = 0

A = [1, 2, 3, 4]

B = [5, 6, 7, 8]

C = []

while i < len(A):

C.append(A[i]+B[i])

i=i+1

In the above code, we will have to add a lot of conditions to make sure that it runs

without error all the time. For example, What if the size of both the list is not same?

We will have to do something different in such case because we have to iterate over the

elements of two different lists, together. This can be done using the function zip().

Suppose the two lists are,

>>> A = [9, 8, 7, 6, 5]

>>> B = [3, 4, 5, 6, 7]

Then, let's create an empty list to save the results.

>>> C = []

Then the iteration process will require two reference variables, one for each list.

for x, y in zip(A, B):

C.append(x+y)

zip() function can accept any number of list arguments. In this case, we have passed two

lists A and B. Since both A and B have 5 elements, hence zip() will make the loop iterate

5 times. But, what if both the lists have different number of elements? Suppose A

have n elements and B have melements, if m < n then zip() will make loop m times,

otherwise n times.

Another approach to this problem could have been using the index number of the list A

and B.

for i in range(0,5):

C.append(A[i]+B[i])


Although while using this technique it is necessary to know the size of both the lists

prior to appending, while the previous approach (zip function) can be used in almost

any situation.

7.5 Deleting an element from a List

It may happen, that you might have to delete some elements from your list. Below we

have listed some methods to do the same:

7.5.1 pop( ) function:

This method requires the index number of the element that you want to delete. For

example, if you want to delete the 5th element of the list, then just do it like:

>>> myList.pop(4)

7.5.2 del keyword:

This also requires the index number.

>>> del myList[4]

This will delete the 5th element of the list. We can combine this with slicing to delete

sequence of elements together, like if we want to delete all elements from index 3 to 6,

then:

>>> del myList[3:7]

This will remove all elements from index 3 to 6.


7.5.3 remove( ) function:

This function, instead of index number, requires the value that has to be deleted. For

example,

>>> myList = ['Apple', 'Orange', 'Apple', 'Guava']

>>> myList.remove('Apple')

This will remove the first 'Apple' element from the list. Other elements with value

'Apple' won't be deleted. i.e., our list will be:

>>> print myList

['Orange', 'Apple', 'Guava']

7.6 Convert a List to String

If you want to convert your list into a string to print it as output, you can do it easily.

7.6.1 Without using any loop

Here is the code, if you do not want to use for or while loop for coverting list to string.

mylist = ['butter', 'jam', 'curd']

myStr = ', '.join(mylist)

print myStr

butter, jam, curd

If you have a list of int values, then

mylist = [1, 11, 111]

myStr = str(mylist).strip('[]')

print myStr


1, 11, 11

7.6.2 Using for loop

If we have a list of int values,

mylist = [1, 11, 111]

myStr = ''.join(str(e) for e in mylist)

print myStr

1, 11, 11

7.7 More functions for Lists

7.7.1 . insert(int, item)

This function is used to insert values, anywhere in the list. The first argument of the list

takes the index where the items will be inserted and second is the value that has to be

inserted. For example,

>>> myList = ['Python', 'C++', 'Java', 'Ruby', 'Perl']

>>> myList.insert(1, 'JavaScript')

>>> print myList

['Python', 'JavaScript', 'C++', 'Java', 'Ruby', 'Perl']

Notice how the value 'JavaScript' got inserted at index the 1.


7.7.2. reverse()

As obvious by the name, it is used to reverse the order of the elements in the list.

>>> myList.reverse()

7.7.3. sort()

One of the most important functions in real life applications is sort(). Sorting arranges

all the elements of the List in ascending or descending order.

>>> myList.sort()

If the list consists of numbers then this will make the sequence in ascending order, if the

list consists of strings then the elements will be sorted in lexicographic ascending order.

What if you want to sort in descending order?

>>> myList.sort().reverse()

This will first sort the elements in ascending order and then we have used

the reverse() function to reverse the list.

Chapter 8 Dictionaries in Python

In this chapter, we will learn what a Dictionary is? How it can be used? and some

useful functions used for manipulating Dictionary.

Dictionaries are much like lists with an extra parameter called keys. Recall, how in

lists and strings, we used the index as the parameter to access each element of the

string/list. The main differentiating factor between a list and a dictionary would be,

that instead of the index we use keys to access the elements of a dictionary (or

values to access keys, works both ways).

Also, unlike an index, keys can be of any data type varying from integer to string.

This makes them more flexible to use.


8.1 Creating a Dictionary

Since we have flexibility in providing the key for each element in the dictionary, we

will have to define each key explicitly. Below we have a dictionary in tabular format.

For each element in the dictionary we have a key linked to it.

Key Value

Key-1 Element-1

Key-2 Element-2

Key-3 Element-3

Key-4 Element-4

Key-5 Element-5

A dictionary in python can be created as:

>>> myDictionary = {'Key-1': 'Element-1', 'Key-2': 'Element-2', 'Key-3': 'Element-3', 'Key-4': 'Element-4'}

Notice the curly braces that are used here, unlike square braces in the list. Here Key-

1, Key-2... are the keys for Element-1, Element-2... respectively. Therefore, if you

want to access any element of a dictionary, you should know the key for that

element. For example, to access element of Key-3, then just use,

>>> myDictionary['Key-3']

'Element-3'


Also, in a dictionary, each element must have a unique key, since a key is used to

uniquely identity each element of the dictionary, however, the reverse is not true,

which means that elements can be repeated, but key must be unique.

8.1.1 Dictionary with integer keys:

>>> integerDictionary = {10: "C++", 20: "Java", 30: "Python", 40: "Ruby", 50: "C#", 60: "Perl"}

>>> integerDictionary[30]

"Python"

8.1.2 Dictionary with string as keys:

>>> identity = {"name": "StudyTonight", "type": "Educational", "link": "https://studytonight.com", "tag":

"Best place to learn"}

>>> print identity['name'] + ": " + identity['tag']

StudyTonight: Best place to learn

To create an empty dictionary, do the following:

>>> emptyList = {}

The above line of code successfully initialized an empty dictionary. We can easily

add elements to an empty dictionary after its initialization. Suppose you want to

add Delhi with key India to your dictionary, then do it like,

>>> emptyList["India"] = "Delhi"

And this element will get appended to the dictionary.

>>> emptyList


{"India": "Delhi"}

8.2 Accessing elements of a dictionary

Elements stored in a dictionary can be accessed just like lists in python, i.e, using

the for loop. However, while iterating over each element, we will get the key and not

the value of the element, therefore, to access the value of the element, we have to use

the key just like index, For example: myDictionary[key].

for i in myDictionary:

print "Key: " + i + " and Element: " + myDictionary[i]

Key: Key-1 and Element: Element-1





8.3 Deleting element(s) in a dictionary

Elements can be deleted using del keyword, which is similar to how its done in a list.

For example, considering our website details dictionary,

>>> identity = {"name": "StudyTonight", "type": "Educational", "link": "http://studytonight.com", "tag":

"Best place to learn"}

If we want to delete the link key and the value associated to it, then

>>> del identity["link"]

will delete that element.


>>> identity

{"name": "StudyTonight", "type": "Educational", "field": "tag": "Best place to learn"}

8.4 Appending element(s) to a dictionary

Suppose you want to add an extra element to your already initialized list which has

elements, then, all you have to do is:

>>> identity["email": "[email protected]"]

And it will be added to the dictionary.

>>> identity

{"name": "StudyTonight", "type": "Educational",

"tag": "Best place to learn", "email": "[email protected]"}

8.5 Updating existing element(s) in a

dictionary

The update() function is used for merging two dictionaries into one. The common

values of both the lists get overwritten by the latter dictionary. For example, let's

assume that there is another dictionary containing the list of the available courses

on StudyTonight, along with the list used in the example above.

So now we have 2 lists - identity and courseAvail

>>> courseAvail = {"Java": "Full-course", "C/C++": "Full-course", "DBMS": "Full-course"}

Suppose we want to copy all elements of courseAvail to the list identity, then we just

have to do th following:


>>> identity.update(courseAvail)

Note: In this case dictionary identity will get updated, and there would be no effect

on dictionary courseAvail.

8.6 Dictionary Functions in Python

Let's check out some important functions that are quite helpful when we are playing

around with dictionaries in python.

8.6.1 len()

As you might have already guessed, it gives the number of elements stored in the

dictionary or you can say just the number of keys in it.

>>> len(myDictionary)

5

8.6.2 clear()

If we ever need to delete all elements of the dictionary using the del keyword, for

each key value, that would be quite troublesome. Hence clear() function makes

emptying a dictionary, a single line task.

>>> myDictionary.clear()

>>> myDictionary

{}


8.6.3 values()

Suppose you don't want to access the keys while displaying the value stored in the

dictionaries, then values() function can be used. This function will show all the values

stored in the dictionary.

>>> myDictionary.values()

{"Element-1", "Element-2", "Element-3", "Element-4", "Element-5"}

8.6.4 keys()

This function is opposite to the function values(). As the name suggests, in case you

want to view only the keys for a dictionary, then you can use the keys() function.

>>> myDictionary.keys()

{"Key-1", "Key-2", "Key-3", "Key-4", "Key-5"}

8.6.5 items()

In case you want to display both, keys and values with a representation, where both

are well mapped, then use the items() method.

>>> myDictionary.items()

{('Key-1': 'Element-1'), ('Key-2': 'Element-2'), ('Key-3': 'Element-3'),

('Key-4': 'Element-4'), ('Key-5': 'Element-5')}


8.6.6 has_key()

To check if a particular key exists in the dictionary, this function can be used. If

the key that you are looking for exists then it returns True, otherwise False.

>>> myDictionary.has_key("Key-2")

True

>>> myDictionary.has_key("Key-6")

False

8.6.7 cmp():

In case you ever need to compare two dictionaries, cmp() function can be used to do

so. It can return 3 possible values, i.e., 1, 0 or -1. If both dictionaries are equal, then 0.

If second have greater elements than first, then -1 and 1 for the reverse. It can be

used like

>>> x = {1: 1, 2:2, 3:3}

>>> y = {1:1, 2:2, 3:3}

>>> cmp(x, y)

0

Since both are same, hence the output is 0.

Now let's add one an extra element to x.

>>> x[4] = 4

>>> cmp(x, y)

1


Now that x is having more elements, hence the output is 1. Also, if we

compare y with x, then

>>> cmp(y, x)

-1

The output becomes -1, because y has less elements.

Chapter 9 Tuples in Python

Tuples are a lot like lists, and that's why we can define them in a pretty similar way

as we did to define the lists. Simply put, a tuple is a sequence of data.

What makes them different from lists is that tuples are immutable, i.e., the data

inside the tuples can't be modified, which is opposite in the case of lists. Other than

this, tuples are very similar to lists and that would make it much easier for us to

understand as we already know about lists.

9.1 Defining a Tuple

To define a tuple, we just have to assign a single variable with multiple values

separated by commas, and that variable will be known as a Tuple.

>>> myTuple = 1, 2, 3, 4

If you try to print it in IDLE,

>>> print myTuple

(1, 2, 3, 4)

You can see in the above example, that myTuple variable is actually a collection of

integers 1, 2, 3 and 4. Also, note those circular brackets which appears while


printing, around the integers, these will actually help you to distinguish between

lists and tuples. Because in case of lists, we have square brackets around the list

elements.

You can obviously add data of different types in a single tuple,

>>> secondTuple = 1, 2, "python", 4

>>> print secondTuple

(1, 2, "python", 4)

An empty tuple can be created using the tuple() function or by just using an empty

bracket ().

>>> emptyTuple = ()

>>> anotherEmptyTuple = tuple()

The above statements will create tuples with no elements in it. And the compiler

would know that emptyTuple and anotherTuple are tuples, with no elements in them.

9.2 Indexing in Tuples

Indexing in tuples is also pretty similar to that in lists, the first element has

index zero, and it keeps on increasing for the next consecutive elements.

Also, backward indexing is also valid in tuples, i.e., the last element can be accessed

using the index -1 and the consecutive previous numbers by -2, -3and so on. Let's

take an example,

>>> example = "apple", "orange", "banana", "berry", "mango"

>>> example[0]


'apple'

In the table below we have marked the tuple elements for both forward and

backward indexing:

Value Forward Indexing Backward Indexing

apple 0 -5

orange 1 -4

banana 2 -3

berry 3 -2

mango 4 -1

9.3 Adding Elements to a Tuple

As we know, that tuples are immutable, hence the data stored in a tuple cannot be

edited, but it's definitely possible to add more data to a tuple. This can be done using

the addition operator. Suppose there is a tuple,

>>> t = (1, 2, 3, 4, 5)

In case you want to add another element, 7 to the tuple, then you can do it as

follows:

>>> t = t + (7,)


As you can see, we used the addition operator to add(7,) to the tuple t.

>>> print t

(1, 2, 3, 4, 5, 7)

Hence, we can add any kind of element to a tuple, using the + operator.

If we try to think, what else can we do with the + operator, we might realize that it

can be used to combine two tuples as well. For example:

>>> print (1, 2, 5, 8) + (2, 9, 4)

(1, 2, 5, 8, 2, 9, 4)

You can use a tuple(s) to create another tuple.

9.4 Deleting a Tuple

In order to delete a tuple, the del keyword is used. In order to delete a tuple

named myTuple(which we defined earlier), follow the example below:

>>> del myTuple

And myTuple will be deleted from the memory.

9.5 Slicing in Tuples

Slicing in tuples, works exactly the same like in the case of lists. Let's start with an

example:

>>> t = (1, 2, 3, 4)

>>> t[2:4]


(3, 4)

Here, t[2:4] means, slice the tuple starting from index 2 upto the index 4, and take

that slice out.

Slicing can be done backwards as well using the negative indexes for traversing the

tuple from backward direction.

9.6 Basic Operations and Functions

The various operations that we can perform on tuples are very similar to lists. In

fact, you just saw the + operator with tuples, it works with tuples just like it works

with a list. Some other operators for tuples include:

9.6.1. Multiplication

Multiplying a tuple by any integer, x will simply create another tuple with all the

elements from the first tuple being repeated x number of times. For

example, t*3 means, elements of tuple t will be repeated 3 times.

>>> t = (2, 5)

>>> print t*3

(2, 5, 2, 5, 2, 5)

9.6.2 Addition

Using the addition operator, with two or more tuples, adds up all the elements into a

new tuple. For example,


>>> t = (2, 5, 0) + (1, 3) + (4,)

>>> print t

(2, 5, 0, 1, 3, 4)

9.6.3 in keyword

in keyword, can not only be used with tuples, but also with strings and lists too. It is

used to check, if any element is present in the sequence or not. It returns True if the

element is found, otherwise False. For example,

>>> t = (1, 2, 3, 6, 7, 8)

>>> 2 in t

>>> 5 in t

True

False

9.6.4 len() function

As you might have already guessed, this function is used to get the number of

elements inside any tuple.

>>> t = 1, 2, 3

>>> print len(t)

3


9.6.5 cmp() function

This is used to compare two tuples. It will return either 1, 0 or -1, depending upon

whether the two tuples being compared are similar or not.

The cmp() function takes two tuples as arguments, where both of them are

compared. If T1 is the first tuple and T2 is the second tuple, then:

if T1 > T2, then cmp(T1, T2) returns 1

if T1 = T2, then cmp(T1, T2) returns 0

if T1 > T2, then cmp(T1, T2) returns -1

9.6.6 max() and min() function

To find the maximum value in a tuple, we can use the max() function, while for

finding the minimum value, min() function can be used.

>>> t = (1, 4, 2, 7, 3, 9)

>>> print max(t)

>>> print min(t)

9

1


Chapter 10 Python Relational and

Logical Operators

Relation and Logic are the fundamental bricks of a program that defines its

functionality. With these fundamentals, you decide what should be the flow of

execution and what conditions should be kept to make sure the flow stays that way.

In every programming language including python, to manage the flow of any

program, conditions are required, and to define those conditions, relational and

logical operators are required.

Remember those days when your mathematics teacher in school used to ask you if 3

is greater than 2, say yes, otherwise no, that is pretty much what we do in

programming world too.

You provide the compiler with some condition based on an expression, compiler

computes the expression and executes the condition based on the output of the

expression. In the case of relational and logical expressions, the answer will always

be either True or False.

Operators are the conventional symbols, that bring one, two or more operands

together to form an expression. Operators and operands are two key deciding

factors of the output.

Now let's see some operators that are available in python language.

10.1 Python Relational Operator

Relational operators are used to establish some sort of relationship between the two

operands. Some of the relevant examples could be less than, greater than or equal

to operators. Python language is capable of understanding these types of operators

and accordingly return the output, which can be either True or False.


Let's checkout a few relational expressions. Open your IDLE and try this:

>>> 5 < 9

True

Since 5 is less than 9, thus the output returned is True.

The list of operators available includes:

1. Less than → used with <

2. Greater than → used with >

3. Equal to → used with ==

4. Not equal to → used with !=

5. Less than or equal to → used with <=

6. Greater than or equal to → used with >=

You can try each of the operators to practice with some numbers (or even strings).

>>> "abc" > "aaa"

>>> "abc" == "bcd"

True

False

10.2 Python Logical Operators

Logical operators, as the name suggests are used in logical expressions where

the operands are either True or False. The operands in a logical expression, can be

expressions which returns Trueor False upon evaluation. There are three basic types

of logical operators:


1. Logical AND: For AND operation the result is True if and only if both operands

are True. The keyword used for this operator is and.

2. Logical OR: For OR operation the result is True if either of the operands is True.

The keyword used for this operator is or.

3. Logical NOT: The result is True if the operand is False. The keyword used for this

operator is not.

Let's see a few examples:

>>> True and False

False

>>> not True

False

Now, we also know that the relational expressions return a Boolean value as their

output, therfore know we can combine relational and logical expressions to create

something more meaningful. For example,

>>> (2 < 3) and (2 < 5)

True

>>> (2 < 3) and (2 < 1)

False

Taking a bit more realistic programming example, consider you have a variable x as

input and you want to check if the user entered value is between some range, say 0

to 100, then:

>>> x = int(raw_input())

25


>>> (x > 0) or (x < 100)

True

Chapter 11 Conditional Statements

In the last chapter, we studied about the logical and relational expressions and tried

to understand their usage with help of various examples. We also saw examples

about how multiple relational expressions can be put together with the help of

logical operators to create meaningful logical expressions.

We also mentioned how these logical and relational expressions can be used to

control the flow of execution.

In this section, we will learn some more ways to control the flow of execution. In

programming, there are two ways to achieve this, and they are known

as conditional statements and looping.

In this tutorial, we will be discussing about the conditional statements, so let's begin.

11.1 Conditional Statements

There will be various situations while writing a program when you will have to take

care of different possible conditions that might arise while execution of the program.

In such situations, if conditions can be used.


11.1.1 The if condition

Syntax for using the if keyword is as follows:

if [conditional expression]:

[statement(s) to execute]

if keyword and the conditional expression is ended with a colon. In [conditional

expression] some conditional expression is introduced that is supposed to return a

boolean value, i.e., True or False. If the resulting value is True then the [statement to

execute] is executed, which is mentioned below the if condition with

a tabspace(This indentation is very important).

Taking a real-life example, let's say in a savings bank account a user A, has Rs.1000.

The user A, visits the ATM to withdraw money from his savings account. Now the

program which handles the ATM transactions should be able to perform in every

situation, like the program should be aware of the user's bank account balance, and

should not allow the user to withdraw money more than the available balance in his

account. Also, the program must update the account balance once the user has

withdrawn money from the account, to keep the records updated. Let's write a small

piece of code to stop user from withdrawing money more than the available account

balance.

>>> savingAmt = 1000

>>> withdrawAmt = int(raw_input("Amount to Withdraw: "))

>>> if withdrawAmt > savingAmt:

print "Insufficient balance"

Amount to Withdraw: 2000

Insufficient balance


In the above program if the user enters any amount more than Rs. 1000, a message

is displayed on the screen saying "Insufficient Funds".

There are two more keywords that are optional, but can be accompanied

with if statements, the are:

else

elif (also known as else if)

11.1.2 The else condition

Talking about else, let's first see how it is used alongside the if statement.

if[conditional expression]:

[statement to execute]

else:

[alternate statement to execute]

Continuing with the bank account-ATM example, if you noticed in the program

above, we forgot to subtract the withdrawn amount from the savings account

balance. Since now we have an additional else condition to use, we can print a

warning with the message, "Insufficient balance" if the saving amount is less than

the withdraw amount, or else we can subtract the withdrawn amount from the

savings account amount to update the account balance. So the if condition written in

the previous example, will get an additional else block.

if withdrawAmt > savingAmt:

print "Insufficient balance"

else:

savingAmt = savingAmt - withdrawAmt

print "Account Balance:" + str(savingAmt)


And with the else block introduced, in case there is sufficient balance,

the withdrawAmt will be subtracted from the savingAmt and the updated account

balance will be displayed on screen.

With if and else, now we can diverge the flow of execution into two different

directions. In case of a savings account, there will be only two cases, you have

sufficient money or you don't, but what if we come across some situation where

there are more than two possibilities? In such cases, we use yet another statement

that is accompanied with the if and else statements, called elif (or else if in proper

English).

11.1.3 The elif condition

elif statement is added between if and else blocks.

if[condition #1]:

[statement #1]

elif[condition #2]:

[statement #2]

elif[condition #3]:

[statement #3]

else:

[statement when if and elif(s) are False]

With this, you can add as many elif blocks as you want depending upon the

possibilities that may arise.

Let's say you are given a time and you have to tell what phase of the day it is-

(morning, noon, afternoon, evening or night). You will have to check the given time

against multiple ranges of time within which each of the 5 phases lies. Therefore, the

following conditions:


1. Morning: 0600 to 1159

2. Noon: 1200

3. Afternoon: 1201 to 1700

4. Evening: 1701 to 2000

5. Night: 2000 to 0559

Below we have a simple program, using the if, elif and else conditional statements:

if (time >= 600) and (time < 1200):

print "Morning"

elif (time == 1200):

print "Noon"

elif (time > 1200) and (time <= 1700):

print "Afternoon"

elif (time > 1700) and (time <= 2000):

print "Evening"

elif ((time > 2000) and (time < 2400)) or ((time >= 0) and (time < 600)):

print "Night"

else:

print "Invalid time!"

Notice the logical operators that have been used in each condition in the program,

this example demonstrates how you will generally be using them with if-else

statements.

11.1.4 Nesting if-else statements

Simply put, nesting if else means that you will be writing if-else statements inside

another if-else statements. Syntactically,

if[condition #1]:

if[condition #1.1]:


[statement to exec if #1 and #1.1 are true]

else:

[statement to exec if #1 and #1.1 are false]

else:

[alternate statement to execute]

Needless to say you can write the same if-else block inside an else block too. Or in

fact you can add an elif condition, according to your needs. Although if you think

about it, nested if-else is actually an alternative to elif. And just like we created a

program which printed the phase of the day by checking the range of time, that can

be done without using elif as well, by nesting if and elseonly.

if (time >= 600) and (time < 1200):

print "Morning"

else:

if (time == 1200):

print "Noon"

else:

if (time > 1200) and (time <= 1700):

print "Afternoon"

else:

if (time > 1700) and (time <= 2000):

print "Evening"

else:

if ((time > 2000) and (time < 2400)) or ((time >= 0) and (time

< 600)):

print "Night"

else:

print "Invalid time!"

As you can see in the above program, all we did was, we started by adding a

condition to the ifblock, and then used another if-else block in its else block. And we

kept on doing the nesting until all the conditions were taken care of. This, however,


is a little tedious than using elif. However, that's not all it is about. Nesting is very

useful in various other situations, and in some cases, it's the only way forward.

Also, if you remember our first example while explaining the if statement, the

savings bank account example. There is actually a quicker way to do it, a one-line

way because, it's python. The following is the logic:

if (saving > withdraw) then saving = (saving - withdraw), else saving will remain as

it is and there will be no transaction.

>>> savingAmt = savingAmt - withdrawAmt if (savingAmt > withdrawAmt) else savingAmt

Chapter 12 Loops in Python

In this section, we will see how loops work in python. Looping is simply a functionality

that is commonly used in programming for achieving repetitive tasks. It can vary from

iterating each element of an array or strings, to modifying a whole database.

The thumb rule for using loops is:

If you're writing a similar piece of code, again and again, it's time to go for the loops.

Instead of doing,

>>> a = 1

>>> b = 2

>>> c = 3

>>> d = 4

and so on, do it like,

>>> a = []

>>> {run a loop in this line to store 1, 2, 3, 4 in list a}


In the second code snippet, we will be using a loop, where we will only have to write

whatever tedious task we have to achieve, only once, and then put it on a loop. With this,

we will be able to achieve an iterative flow for execution.

In python, there are two ways to achieve iterative flow:

1. Using the for loop

2. Using the while loop

12.1 The for loop

Let us first see what's the syntax,

for [ELEMENT] in [ITERATIVE-LIST]:


else:

[statement to execute when loop is over]

[else statement is completely optional]

for loop is frequently used to iterate elements of lists. Considering the above syntax, let's

say there is a list myList:

>>> myList = [8, 9, 2, 3, 4, 6]

If we want to iterate all the elements of the list myList using the for loop:

for i in myList:

print i

As we can see we are using a variable i, which represents every single element stored in

the list, one by one. Our loop will run as many times, as there are elements in the lists.

For example, in myList there are 6 elements, thus the above loop will run 6 times.

During the first iteration, the value of i will be 8, while in next it will be 9 and so on,

uptill 6, in the last iteration. Doing so, one can easily use these individual list element

values inside the loop, as in our case, we have printed them.


Besides this, there is another way of doing it. If you can recall, our old traditional way of

accessing an element of the list by index - myList[i]. Although, in this case, we will be

using another function range() (or alternatively xrange() can also be

used). range() function has already been mentioned in the previous sections too. If you

can remember range() function can return you a list of numbers according to the

arguments that you provide. For example:

>>> range(5)

[0, 1, 2, 3, 4]

Or,

>>> range(5, 10)

[5, 6, 7, 8, 9]

Or,

>>> range(2, 12, 2)

[2, 4, 6, 8, 10]

xrange() function is also pretty much the same, although it's a little faster.

We can use range() function along with the for loop to iterate index number

from 0 to len(myList). It will be pretty much like:

for i in xrange(0, len(myList)):

print myList[i]

As you might have noticed, in this case, we will be iterating over a temporary list which

contains the index numbers of the actual list. This temporary list is generated

using xrange() function.

The else block in for loop

The else block with the for loop, is executed, once all the elements of the list are iterated

or there are no more elements left to iterate in the list. It'll be safe to say


that else statement is executed at the end of the loop. Although, as already mentioned in

the syntax, it's completely optional to use and in fact, it's not even frequently used.

Looking at some more complex examples, let's say there is a list of lists:

>>> bigList = [[1, 3, 6], [8, 2,], [0, 4, 7, 10], [1, 5, 2], [6]]

How do you think, we will be accessing each element of the list?

The answer is, using a loop within another loop. Since there are lists within a list, thus

the outer loop will help in accessing each list of the bigList, and inside that loop, for

every list element we will be using another loop.

for i in bigList:

for j in i:

print "Element of list within a list - ", j

Element of list within a list - 1














12.2 The while loop

It's time to learn about the while loop.

Syntax:

while [condition]:


else:

[statement to execute if condition is false]

[else statement is completely optional]

Again, in the while loop too, there is an else block, although it's optional and rarely used.

In the first statement, while loop sets a condition, then a statement, and the compiler

following the flow of execution, first checks the condition provided in the condition

block, if the condition holds True, then the statement is executed.

Once the statement is executed, the compiler checks the condition again, if

it's True again then the statement is again executed, otherwise the else block is executed

(if provided) or simply the flow control breaks out of the while loop (if else statement is

not mentioned).

Here is an example:

Caution: Don't try this example in IDE or IDLE or anywhere.

while True:

print "I am printing because the condition is True"

Since here we have set our condition to be True, hence the compiler will execute the

statement the first time. In next iteration, the condition will still be True again and thus

the statement will be executed again. This will keep on happening and as you might

have guessed there will be no end for this loop. Such kind of looping is called infinite

loop and should be avoided in programs.

Time for another example:

i = 0


while i < 5:

i=i+1

print "i =", i

In this case, in the first iteration condition i < 5 is checked, since 0 < 5, hence the

statement is executed. During execution, value of i is incremented by 1.

The for loop, is frequently used for iterating elements of a list, while loop on the other

hand, is capable of serving several purposes, and it can iterate a list too! The logic is

pretty much similar for accessing an element using indices. Run the loop

from 0 to len(myList) and simply access the elements. And, there will be no need of

using range() or xrange() function.

myList = [8, 9, 2, 3, 4, 6]

i = 0

while i < len(myList):

print myList[i]

i = i+1

Quickly going through the example for lists within a list:

bigList = [[1, 3, 6], [8, 2,], [0, 4, 7, 10], [1, 5, 2], [6]]

i = 0

j = 0

while i<len(bigList):

while j<len(bigList[i]):

print “Element of list within a list -”,bigList[i][j]

j=j+1

i=i+1

12.3 The continue keyword


continue keyword is used inside the loops for altering the loop execution.

Using continue inside the loop will skip the current statement execution, and it will

check the condition again. Suppose there is a string,

>>> name = "StudyTonight"

So let's have a quick example demonstrating use of continue keyword:

for i in name:

continue

print i

It will print nothing. Let's see why? In the first iteration, first element of the

list name will be picked, i.e. "S", the next statement inside the for loop is continue, thus as

soon as this keyword is found, the execution of rest of the for loop statements is skipped

and the flow gets back to the for loop starting to get the next element and check the

condition. Same steps are followed by the next element of our string, i.e., "t" and

the continue interrupts the flow again, and this will happen everytime the execution

flow reaches the continue keyqord.

So, now if we want to write a program with a loop which prints only

the lowercase letters of the string stored in the name variable, then, we can use

the continue keyword to skip those letters which are not lowercase.

for i in name:

if not i.islower():

continue

print i

if condition checks whether the element i is in lowercase, using islower() function. If it is

not in lowercase then the rest of the statements are simply skipped and

the print statement is never executed, although, if i is in lowercase then the if statement

returns False and continuestatement is not executed.

The continue keyword works in the exact same way for the while loop as well.


12.4 The break keyword

break keyword is used inside the loop to break out of the loop. Suppose while iterating

over the characters of the string stored in the variable name, you want to break out of it

as soon as the character "T" is encountered. This is how it can be done:

for i in name:

if i == "T":

break

print i

Now, as soon as the if condition gets the character "T", the compiler will execute

the breakstatement and flow of execution will get out of the loop.


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