# Python Data Structures Repo & Website

# - [Other Repo](https://github.com/bgoonz/python-gitbook)

# - [Website for other repo](https://bgoonz42.gitbook.io/datastructures-in-pytho/stdlib/untitled)

# - [Website for this repo](https://ds-unit-5-lambda.netlify.app/)

---

# Table of contents

- [Home](./_RESOURCES/bgoonz42.gitbook.io/README.md)
- [Navigation](./_RESOURCES/bgoonz42.gitbook.io/navigation.md)

## Cirriculumn

- [Outline](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled.md)
- [wk17](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/README.md)
- [Outline-w17](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/outline.md)
- [homework](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/week-overview.md)
- [D1-Module 01 - Python I](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/untitled-2/README.md)
- [Install Python](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/untitled-2/install-python.md)
- [D2- Module 02 - Python II](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/untitled-1-1.md)
- [D3- Module 03 - Python III](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/untitled-1.md)
- [D4-Module 04 - Python IV](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-3/untitled.md)
- [wk18](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-2/README.md)
- [Outline-W-18](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-2/untitled-4.md)
- [D1- Module 01 - Number Bases and Character Encoding](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-2/untitled-3.md)
- [D2- Module 02 - Hash Tables I](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-2/untitled-2.md)
- [D3-Module 03 - Hash Tables II](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-2/untitled-1.md)
- [D4- Module 04 - Searching and Recursion](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-2/untitled.md)
- [wk19](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-4/README.md)
- [Outline-W-19](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-4/overview.md)
- [D1- Module 01 - Linked Lists](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-4/untitled-3.md)
- [D2- Module 02 - Queues and Stacks](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-4/untitled-7.md)
- [D3- Module 03 - Binary Search Trees](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-4/untitled-8.md)
- [D4- Module 04 - Tree Traversal](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-4/untitled-5.md)
- [wk20](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-1/README.md)
- [Outline-W-20](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-1/overview.md)
- [D1-Graphs I](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-1/untitled-5.md)
- [D2-Graphs 2](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-1/untitled-4.md)
- [DFS](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-1/untitled-1.md)
- [D4](./_RESOURCES/bgoonz42.gitbook.io/cirriculumn/untitled-1/untitled-2.md)

## Utilities

- [Code lab Notebooks](./_RESOURCES/bgoonz42.gitbook.io/utilities/code-lab-notebooks.md)
- [Repl.IT](./_RESOURCES/bgoonz42.gitbook.io/utilities/repl.it/README.md)
- [Trinket](./_RESOURCES/bgoonz42.gitbook.io/utilities/repl.it/trinket.md)
- [Utilites](./_RESOURCES/bgoonz42.gitbook.io/utilities/untitled/README.md)
- [YouTube](./_RESOURCES/bgoonz42.gitbook.io/utilities/untitled/untitled.md)
- [Code Lab Notebook Embeds From Lecture](./_RESOURCES/bgoonz42.gitbook.io/utilities/untitled/code-lab-notebook-embeds-from-lecture.md)

## practice

- [Supplemental Practice:](./_RESOURCES/bgoonz42.gitbook.io/practice/supplemental-practice/README.md)
- [Random Examples](./_RESOURCES/bgoonz42.gitbook.io/practice/supplemental-practice/random-examples.md)
- [Prompts](./_RESOURCES/bgoonz42.gitbook.io/practice/supplemental-practice/prompts.md)
- [Abstract Data Structures:](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/README.md)
- [Untitled](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-7.md)
- [Industry Standard Algorithms](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/industry-standard-algorithms.md)
- [Interview Practice Resources](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/interview-practice-resources.md)
- [Overflow Practice Problems](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/overflow-practice-problems.md)
- [Array](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/array/README.md)
- [Extra-Array](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/array/extra-array.md)
- [Stack](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/stack.md)
- [Queue](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/queue.md)
- [Queue Sandbox](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/queue-sandbox.md)
- [Binary Search](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/binary-search.md)
- [BST Explained](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/bst-explained.md)
- [Binary Tree](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/binary-tree.md)
- [Binary Search Tree](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/binary-search-tree/README.md)
- [BST Insert](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/binary-search-tree/bst-insert.md)
- [Recursion](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-6/README.md)
- [Recursion Explained](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-6/recursion-explained/README.md)
- [Recursion Examples](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-6/recursion-explained/recursion-examples.md)
- [Hash Table](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-5.md)
- [Linked List](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-4/README.md)
- [Double Linked List](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-4/double-linked-list.md)
- [List Operations](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-4/list-operations.md)
- [Sorting](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-3/README.md)
- [SelectionSort](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-3/selectionsort.md)
- [Quick Sort](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-3/untitled-7.md)
- [Merge Sort](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-3/merge-sort.md)
- [Insertion Sort](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-3/insertion-sort.md)
- [Searching](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-2.md)
- [Graphs](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled-1.md)
- [Exotic](./_RESOURCES/bgoonz42.gitbook.io/practice/untitled/untitled.md)

## Resources

- [Python VS JavaScript](./_RESOURCES/bgoonz42.gitbook.io/resources/python-vs-javascript.md)
- [Misc. Resources](./_RESOURCES/bgoonz42.gitbook.io/resources/untitled-1.md)
- [Things To Internalize:](./_RESOURCES/bgoonz42.gitbook.io/resources/things-to-internalize/README.md)
- [Functions](./_RESOURCES/bgoonz42.gitbook.io/resources/things-to-internalize/functions.md)
- [Intro To Python w Jupyter Notebooks](./_RESOURCES/bgoonz42.gitbook.io/resources/intro-to-python-w-jupyter-notebooks.md)
- [Calculating Big O](./_RESOURCES/bgoonz42.gitbook.io/resources/calculating-big-o.md)
- [Python Cheat Sheet](./_RESOURCES/bgoonz42.gitbook.io/resources/python-cheat-sheet.md)
- [Code Signal CGA Sprint Resources](./_RESOURCES/bgoonz42.gitbook.io/resources/code-signal-cga-sprint-resources.md)
- [Youtube](./_RESOURCES/bgoonz42.gitbook.io/resources/youtube.md)
- [Useful Links](./_RESOURCES/bgoonz42.gitbook.io/resources/untitled/README.md)
- [My-Links](./_RESOURCES/bgoonz42.gitbook.io/resources/untitled/my-links.md)
- [Beginners Guide To Python](./_RESOURCES/bgoonz42.gitbook.io/resources/untitled/beginners-guide-to-python.md)

## quick-reference

- [Python Snippets](./_RESOURCES/bgoonz42.gitbook.io/quick-reference/python-snippets.md)
- [Python Module Index:](./_RESOURCES/bgoonz42.gitbook.io/quick-reference/python-module-index.md)
- [Useful Info](./_RESOURCES/bgoonz42.gitbook.io/quick-reference/untitled.md)
- [Python Glossary](./_RESOURCES/bgoonz42.gitbook.io/quick-reference/python-glossary.md)
- [index](./_RESOURCES/bgoonz42.gitbook.io/quick-reference/untitled-1.md)
- [List Of Python Cheat Sheets](./_RESOURCES/bgoonz42.gitbook.io/quick-reference/bash-commands.md)

## Python-Docs <a id="stdlib"></a>

- [Basic Syntax](./_RESOURCES/bgoonz42.gitbook.io/stdlib/basic-syntax.md)
- [Values Expressions & Statments](./_RESOURCES/bgoonz42.gitbook.io/stdlib/values-expressions-and-statments.md)
- [Python Standard Library \(STDLIB\)](./_RESOURCES/bgoonz42.gitbook.io/stdlib/python-standard-library-stdlib.md)
- [Docs](./_RESOURCES/bgoonz42.gitbook.io/stdlib/untitled/README.md)
- [String-Methods](./_RESOURCES/bgoonz42.gitbook.io/stdlib/untitled/string-methods.md)
- [Built In Functions](./_RESOURCES/bgoonz42.gitbook.io/stdlib/built-in-functions.md)
- [Lists](./_RESOURCES/bgoonz42.gitbook.io/stdlib/lists/README.md)
- [Examples](./_RESOURCES/bgoonz42.gitbook.io/stdlib/lists/examples.md)
- [Dictionaries](./_RESOURCES/bgoonz42.gitbook.io/stdlib/dictionaries.md)
- [Classes](./_RESOURCES/bgoonz42.gitbook.io/stdlib/classes/README.md)
- [Python Objects & Classes](./_RESOURCES/bgoonz42.gitbook.io/stdlib/classes/python-objects-and-classes.md)
- [index](./_RESOURCES/bgoonz42.gitbook.io/stdlib/classes/untitled.md)
- [Queue & Stacks](./_RESOURCES/bgoonz42.gitbook.io/stdlib/queue-and-stacks.md)

## MISC

- [Unsorted Examples](./_RESOURCES/bgoonz42.gitbook.io/misc/unsorted-examples.md)
- [Outline](./_RESOURCES/bgoonz42.gitbook.io/misc/outline.md)
- [About Python](./_RESOURCES/bgoonz42.gitbook.io/misc/untitled/README.md)
- [Python VS JavaScript](./_RESOURCES/bgoonz42.gitbook.io/misc/untitled/python-vs-javascript.md)
- [Python Modules & Python Packages](./_RESOURCES/bgoonz42.gitbook.io/misc/untitled/untitled-1.md)
- [Misc](./_RESOURCES/bgoonz42.gitbook.io/misc/untitled/misc.md)
- [Python's Default Argument Values and Lists](./_RESOURCES/bgoonz42.gitbook.io/misc/untitled/pythons-default-argument-values-and-lists.md)
- [SCRAP](./_RESOURCES/bgoonz42.gitbook.io/misc/untitled/untitled.md)

## Interview Prep

- [Interview Resources](./_RESOURCES/bgoonz42.gitbook.io/interview-prep/interview-resources/README.md)
- [How to Write an Effective Resume of Python Developer](./_RESOURCES/bgoonz42.gitbook.io/interview-prep/interview-resources/how-to-write-an-effective-resume-of-python-developer.md)
- [Interview Checklist](./_RESOURCES/bgoonz42.gitbook.io/interview-prep/interview-resources/interview-checklist.md)
- [150 Practice Problems & Solutions](./_RESOURCES/bgoonz42.gitbook.io/interview-prep/interview-resources/150-practice-problems-and-solutions.md)

## Installations Setup & Env

- [python-setup](./_RESOURCES/bgoonz42.gitbook.io/installations-setup-and-env/untitled.md)

## Aux-Exploration

- [Subject](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/README.md)
- [List Directory Contents](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/list-directory-contents.md)
- [Employee Manager](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/untitled-5.md)
- [OS Module](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/untitled-4.md)
- [Untitled](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/untitled-3.md)
- [Untitled](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/untitled-2.md)
- [Untitled](./_RESOURCES/bgoonz42.gitbook.io/aux-exploration/subject/untitled-1.md)

---

# Python syntax was made for readability, and easy editing. For example, the python language uses a `:` and indented code, while javascript and others generally use `{}` and indented code.

Lets create a [python 3](https://repl.it/languages/python3) repl, and call it _Hello World_. Now you have a blank file called _main.py_. Now let us write our first line of code:

_helloworld.py_

print('Hello world!')

> Brian Kernighan actually wrote the first “Hello, World!” program as part of the documentation for the BCPL programming language developed by Martin Richards.

Now, press the run button, which obviously runs the code. If you are not using replit, this will not work. You should research how to run a file with your text editor.

If you look to your left at the console where hello world was just printed, you can see a `>`, `>>>`, or `$` depending on what you are using. After the prompt, try typing a line of code.

Python 3.6.1 (default, Jun 21 2017, 18:48:35)
[GCC 4.9.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
> print('Testing command line')
Testing command line
> print('Are you sure this works?')
Are you sure this works?
>

The command line allows you to execute single lines of code at a time. It is often used when trying out a new function or method in the language.

Another cool thing that you can generally do with all languages, are comments. In python, a comment starts with a `#`. The computer ignores all text starting after the `#`.

_shortcom.py_

# Write some comments!

If you have a huge comment, do **not** comment all the 350 lines, just put `'''` before it, and `'''` at the end. Technically, this is not a comment but a string, but the computer still ignores it, so we will use it.

_longcom.py_

'''
Dear PYer,
I am confused about how you said you could use triple quotes to make
SUPER
LONG
COMMENTS
!

I am wondering if this is true,
and if so,
I am wondering if this is correct.

Could you help me with this?

Thanks,
Random guy who used your tutorial.
'''
print('Testing')

Unlike many other languages, there is no `var`, `let`, or `const` to declare a variable in python. You simply go `name = 'value'`.

_vars1.py_

x = 5
y = 7
z = x*y # 35
print(z) # => 35

Remember, there is a difference between integers and strings. _Remember: String = `""`._ To convert between these two, you can put an int in a `str()` function, and a string in a `int()` function. There is also a less used one, called a float. Mainly, these are integers with decimals. Change them using the `float()` command.

_vars2.py_

x = 5
x = str(x)
b = '5'
b = int(b)
print('x = ', x, '; b = ', str(b), ';') # => x = 5; b = 5;

Instead of using the `,` in the print function, you can put a `+` to combine the variables and string.

There are many operators in python:

- `+`
- `-`
- `/`
- `*` These operators are the same in most languages, and allow for addition, subtraction, division, and multiplicaiton. Now, we can look at a few more complicated ones:
- `%`
- `//`
- `**`
- `+=`
- `-=`
- `/=`
- `*=` Research these if you want to find out more…

_simpleops.py_

x = 4
a = x + 1
a = x - 1
a = x * 2
a = x / 2

You should already know everything shown above, as it is similar to other languages. If you continue down, you will see more complicated ones.

_complexop.py_

a += 1
a -= 1
a *= 2
a /= 2

The ones above are to edit the current value of the variable.
Sorry to JS users, as there is no `i++;` or anything.

> Fun Fact:
> The python language was named after Monty Python.

If you really want to know about the others, view [Py Operators](https://www.tutorialspoint.com/python/python_basic_operators.htm)

Like the title?
Anyways, a `'` and a `"` both indicate a string, but **do not combine them!**

_quotes.py_

x = 'hello' # Good
x = "hello" # Good
x = "hello' # ERRORRR!!!

_slicing.py_

### String Slicing

You can look at only certain parts of the string by slicing it, using `[num:num]`.
The first number stands for how far in you go from the front, and the second stands for how far in you go from the back.

x = 'Hello everybody!'
x[1] # 'e'
x[-1] # '!'
x[5] # ' '
x[1:] # 'ello everybody!'
x[:-1] # 'Hello everybod'
x[2:-3] # 'llo everyb'

### Methods and Functions

Here is a list of functions/methods we will go over:

- `.strip()`
- `len()`
- `.lower()`
- `.upper()`
- `.replace()`
- `.split()`

I will make you try these out yourself. See if you can figure out how they work.

_strings.py_

x = " Testing, testing, testing, testing       "
print(x.strip())
print(len(x))
print(x.lower())
print(x.upper())
print(x.replace('test', 'runn'))
print(x.split(','))

Good luck, see you when you come back!

Input is a function that gathers input entered from the user in the command line. It takes one optional parameter, which is the users prompt.

_inp.py_

print('Type something: ')
x = input()
print('Here is what you said: ', x)

If you wanted to make it smaller, and look neater to the user, you could do…

_inp2.py_

print('Here is what you said: ', input('Type something: '))

Running:
_inp.py_

Type something:
Hello World
Here is what you said: Hello World

_inp2.py_

Type something: Hello World
Here is what you said: Hello World

Python has created a lot of functions that are located in other .py files. You need to import these **modules** to gain access to the,, You may wonder why python did this. The purpose of separate modules is to make python faster. Instead of storing millions and millions of functions, , it only needs a few basic ones. To import a module, you must write `input <modulename>`. Do not add the .py extension to the file name. In this example , we will be using a python created module named random.

_module.py_

import random

Now, I have access to all functions in the random.py file. To access a specific function in the module, you would do `<module>.<function>`. For example:

_module2.py_

import random
print(random.randint(3,5)) # Prints a random number between 3 and 5

> Pro Tip:
> Do `from random import randint` to not have to do `random.randint()`, just `randint()`
> To import all functions from a module, you could do `from random import *`

Loops allow you to repeat code over and over again. This is useful if you want to print Hi with a delay of one second 100 times.

**for Loop**

The for loop goes through a list of variables, making a seperate variable equal one of the list every time.
Let’s say we wanted to create the example above.

_loop.py_

from time import sleep
for i in range(100):
     print('Hello')
     sleep(.3)

This will print Hello with a .3 second delay 100 times. This is just one way to use it, but it is usually used like this:

_loop2.py_

import time
for number in range(100):
     print(number)
     time.sleep(.1)

**while Loop**

The while loop runs the code while something stays true. You would put `while <expression>`. Every time the loop runs, it evaluates if the expression is True. It it is, it runs the code, if not it continues outside of the loop. For example:

_while.py_

while True: # Runs forever
     print('Hello World!')

Or you could do:

_while2.py_

import random
position = '<placeholder>'
while position != 1: # will run at least once
    position = random.randint(1, 10)
    print(position)

The if statement allows you to check if something is True. If so, it runs the code, if not, it continues on. It is kind of like a while loop, but it executes **only once**. An if statement is written:

_if.py_

import random
num = random.randint(1, 10)
if num == 3:
     print('num is 3. Hooray!!!')
if num > 5:
     print('Num is greater than 5')
if num == 12:
     print('Num is 12, which means that there is a problem with the python language, see if you can figure it out. Extra credit if you can figure it out!')

Now, you may think that it would be better if you could make it print only one message. Not as many that are True. You can do that with an `elif` statement:

_elif.py_

import random
num = random.randint(1, 10)
if num == 3:
    print('Num is three, this is the only msg you will see.')
elif num > 2:
    print('Num is not three, but is greater than 1')

Now, you may wonder how to run code if none work. Well, there is a simple statement called `else:`

_else.py_

import random
num = random.randint(1, 10)
if num == 3:
    print('Num is three, this is the only msg you will see.')
elif num > 2:
    print('Num is not three, but is greater than 1')
else:
    print('No category')

So far, you have only seen how to use functions other people have made. Let use the example that you want to print the a random number between 1 and 9, and print different text every time.
It is quite tiring to type:

Characters: 389

_nofunc.py_

import random
print(random.randint(1, 9))
print('Wow that was interesting.')
print(random.randint(1, 9))
print('Look at the number above ^')
print(random.randint(1, 9))
print('All of these have been interesting numbers.')
print(random.randint(1, 9))
print("these random.randint's are getting annoying to type")
print(random.randint(1, 9))
print('Hi')
print(random.randint(1, 9))
print('j')

Now with functions, you can seriously lower the amount of characters:

Characters: 254

_functions.py_

import random
def r(t):
     print(random.randint(1, 9))
     print(t)
r('Wow that was interesting.')
r('Look at the number above ^')
r('All of these have been interesting numbers.')
r("these random.randint's are getting annoying to type")
r('Hi')
r('j')

---

## Devision 01 - Getting Ready with Python

### Installing Python 3, And Launching Python Shell

This video should help you get up and running with Python 3

- [Installing Python 3 and Launch Python Shell](https://www.youtube.com/watch?v=Ji1WW4Suaww)

Installing Python is really a cakewalk. Search for “Python download” on [www.google.com](http://www.google.com/). Download the installable and install it.

A quick word of caution on Windows

- Make sure that you have the check-box “Add Python 3.6 to PATH”, checked.

Once you have installed Python, you can launch the Python Shell.

- Windows - Launch cmd prompt by typing in ‘cmd’ command.
- Mac or Linux - Launch up terminal.

Command to launch Python 3 is different in Mac.

- In Mac, type in `python3`
- In other operating systems, including windows, type `python`

You can type code in python shell and code as well!

You can use `print(5*4)`, and it shows `20`.

You can execute the code, and the shell would immediately give you output.

## Using the the Python Shell is an awesome way to learn Python.

## Devision 02 - Introduction To Python Programming

Most programmers find programming a lot of fun, and besides, it also gets their work done.

Programming mainly involves _problem solving_, where one makes use of a computer to solve a real world problem.

During our journey here, we will approach programming in a very different way. We will not only introduce you to the Python language, but also help you pick up essential problem solving skills.

As a programmer, you need to be able to look at a problem, and identify the important programming concepts relevant to solving it. Finally, you need to be able to use the language features and syntax, to express your solution on the computer. While all this looks complex, we want to make it easy for you. Together, we will tackle a variety of programming challenges, using these same steps. We will start with simple challenges \(such as a Multiplication Table\), and gradually increase the difficulty level over the duration of this book.

Learning to program is a lot like learning to ride a bicycle. The first few steps are the most challenging ones.

Once you get over these initial steps, your experience will become more and more enjoyable.

Are you ready for your first programming challenge? Let’s get going now! We wish you all the best.

**Summary**

In this step, we:

- Were introduced to the concept of problem solving
- Understood how good programmers approach problem solving

### Step 01: Our First Programming Challenge

Our first _programming challenge_ aims to do, what every kid does in math class: read out a multiplication table. We now want to give this task to the computer. Here is the statement of our problem:

**The Print Multiplication Table Challenge \(PMT-Challenge\)**

1. Compute the multiplication table for `5`, with entries from `1` to `10`.
2. Display this table.

The display needs to be:

_**5 \* 1 = 5**_

_**5 \* 2 = 10**_

_**5 \* 3 = 15**_

_**5 \* 4 = 20**_

_**5 \* 5 = 25**_

_**5 \* 6 = 30**_

_**5 \* 7 = 35**_

_**5 \* 8 = 40**_

_**5 \* 9 = 45**_

_**5 \* 10 = 50**_

This is the challenge. For convenience, let’s give it a label, say _PMT-Challenge_. What would be the important concepts we need to learn, to solve this challenge? The following list of concepts would be a good starting point:

- **Statements**
- **Expressions**
- **Variables**
- **Literals**
- **Conditionals**
- **Loops**
- **Methods**

In the rest of this Devision , we will introduce these concepts to you, one-by-one. We will also show you how learning each concept, takes us closer to a solution to _PMT-Challenge_.

**Summary**

In this step, we:

- Stated our first programming challenge
- Identified what programming concepts we need to learn, to solve this challenge

### Step 02: Breaking Down _PMT-Challenge_

Typically when we do programming, we have problems. Solving the problem typically need a step-by -step approach. Common sense tells us that to solve a complex problem, we break it into smaller parts, and solve each part one by one. Here is how any good programmer worth her salt, would solve a problem:

- Simplify the problem, by breaking it into sub-problems
- Solve the sub-problems in stages \(in some order\), using the language
- Combine these solutions to get a final solution

The _PMT-Challenge_ is no different! Now how do we break it down, and where do we really start? Once again, your common sense will reveal a solution. As a first step, we could get the computer to calculate say, `5 * 3`. The second thing we can do, is to try and print the calculated value, in a manner similar to `5 * 3 = 15`. Then, we could repeat what we just did, to print out all the entries of the `5` multiplication table. Let’s put it down a little more formally:

Here is how our draft steps look like

- Calculate `5 * 3` and print result as `15`
- Print `5 * 3 = 15` \(`15` is result of previous calculation\)
- Do this ten times, once for each table entry \(going from `1` to `10`\)

Let’s start with that kind of a game plan, and see where it takes us.

**Summary**

In this step, we:

- Learned that breaking down a problem into sub-problems is a great help
- Found a way to break down the _PMT-Challenge_ problem

### Step 03: Introducing Operators And Expressions

Let’s focus on solving the first sub-problem of _PMT-Challenge_, the numeric computation. We want the computer to calculate `5 * 5` for example, and print `25` for us. How do we get it to do that? That’s what we would be looking at in this step.

**Snippet-01: Introducing Operators**

Launch up Python shell. We want to calculate `5 * 5`. How do we do that?

Using our knowledge of school math, let’s try `5 X 5`.

    >>> 5 X 5
    File "< stdin >", line 1
    5 X 5
      ^
    SyntaxError: invalid syntax

The Python Shell hits back at us, saying “_invalid syntax_”. This is how Python complains, when it doesn’t fully understand the code you type in. Here, it says our code has a “**SyntaxError**”.

The reason why it complains, is because ‘`X`’ is not a valid **operator** in Python.

The way you can do multiplication is by using the ‘`*`’ _operator_ .

“_5 into 5_” is achieved by the code `5 * 5`, and you can see the result `25` being printed. Similarly, `5 * 6` gives us `30`.

    >>> 5 * 6
    30

There are a wide range of other operators in Python:

- `5 + 6` gives a result of `11`.
- `5 - 6` leads to `-1`.

  >>> 5 + 6
  11
  >>> 5 - 6
  -1

`10 / 2`, gives an output of `5.0` . There is one interesting operator, `**`. Let’s try `10 ** 3`. We ran this code, and the result we get is `1000`. Yes you guessed right, the operator performs “to the power of”. “`10` to the power of `3`” is `10 * 10 * 10`, or `1000`.

    >>> 10 / 2
    5.0
    >>> 10 ** 3
    1000

Another interesting operator is `%`, called “_modulo_”, which computes the remainder on integer division. If we do `10 % 3`, what is the remainder when `10` is divided by `3`? `3 * 3` is `9`, and `10 - 9` is `1`, which is what `%` returns in this case.

Let’s look at some terminology:

- Whatever pieces of code we gave Python shell to run, are called **expressions**. So, `5 * 5`, `5 * 6` and `5 - 6` are all _expressions_. An expression is composed of _operators_ and **operands**.
- In the expression `5 * 6`, the two values `5` and `6` are called operands, and the `*` operator _operates_ on them.
- The values `5` and `6` are **literals**, because those are constants which cannot be changed.

The cool thing about Python, is that you can even have expressions with multiple operators. Therefore, you can form an expression with `5 + 5 + 5`, which evaluates to `15`. This is an expression which has three operands, and two `+` operators. You can even have expressions with different types of operators, such as in `5 + 5 * 5`.

    >>> 5 + 5 + 5
    15
    >>> 5 + 5 * 5
    30

Try and play around with the expressions, and understand the output which results.

**Summary**

In this step, we:

- Learned how to give code input to the Python Shell
- Understood that Python has a predefined set of operators
- Used a few types of basic operators and their operands, to form expressions

### Step 04: Programming Exercise IN-PE-01

At this stage, your smile tells us that you enjoy evaluating Python expressions. What if we tickle your mind a bit, to make sure it hasn’t fallen asleep? Here is your first programming exercise.

**Exercises**

1. Write an expression to calculate the number of minutes in a day.
2. Write an expression to calculate the number of seconds in a day.

**Note**

You need to solve these problems by yourself. If you are able to work them out, that’s fantastic! But if not, that’s part of the learning process.

**Solutions**

**Solution 1**

    >>> 24 * 60

    1440

We wanted to calculate the number of minutes in a day. How do we do that? Think about this…

- How many number of hours are there in a day? `24`.
- And how many minutes does each hour have? It’s `60`.
- So if you want to find out the number of minutes in a day, it’s `24 * 60`, which is `1440`.

**Solution 2**

    >>> 24 * 60 * 60

    86400

How many seconds are there in a day?

- Let’s start with the number of hours, `24`.
- The number of minutes in an hour is `60`, and
- The number of seconds in a minute is `60` as well.
- So it’s `24 * 60 * 60`, or `86400`.

**Summary**

In this step, we:

- Solved a Programming Exercise involving common scenarios, using Python code involving:
- Expressions
- Operators
- Literals

### Step 05: Puzzles On Expressions

Let’s look at a few puzzles related to expressions, in this step. Before that, let’s revise some of the terminology we had learned earlier.

`5 + 6 + 10` is an example of an expression. In this expression, `5`, `6` and `10` are operands. The `+` here is the operator. You can have multiple operators in an expression. We also did mention that the operands, namely `10`, `6` and `5`, are literals. Their values will not change.

Here are a few puzzles coming up, to explore aspects of expressions.

**Snippet-01: Puzzles On Expressions**

Think about what would happen when you do something of this kind: `5 $ 2`. You’re right, it would throw a `SyntaxError`. When Python does not understand the code you type in, it reports an error. Here, the expression we’re typing is `5 $ 2`, which does not make sense to Python, hence the `SyntaxError`.

    >>> 5 $ 2
    File "< stdin >", line 1
    5 $ 2
    ^
    SyntaxError: invalid syntax
    >>> 5$2
    File "< stdin >", line 1
    5 $ 2
    ^
    SyntaxError: invalid syntax

Let’s say we type in `5+6+10`, without any spaces between the operands, and the operators. What do you think will happen? Surprisingly, the Python Shell does calculate the value!

    >>> 5+6+10
    21

In an expression, using spaces makes it easier for you to read it, but it’s not mandatory. `5 + 6 + 10` is easier to read than `5+6+10`, but does not make any difference to the Python compiler.

The next puzzle tries to evaluate `5 / 2`, which is “`5` divided by `2`”. What would be the output? `2.5`.

    >>> 5/2
    2.5

If you’re coming from other programming languages like Java or C, this might be a surprising result. If you try this in Java for instance, you would get `2` as the output. Note that even though both operands are integers, the result of the `/` operation is a floating point value, `2.5` . Python does what is expected by a programmer!

The puzzle after that tries to play with `5 + 5 * 6`. What would be the result of this expression? Will it be `5 + 5` or `10`, then `10 * 6`, which is `60`? Or, will it be `5` plus `5 * 6`, which is `5` + `30`, that’s `35`?

    >>> 5 + 5 * 6
    35

The correct result is `35`.

Python decides this is based on the **precedence** of operators.

Operators in Python are divided into two sets as follows:

- `**`, `*`, `/` and `%` have higher precedence, or priority.
- `+` and `-` have a lower precedence.

Sub-expressions involving operators from {`*`, `/`, `%`, `**`} are evaluated before those involving operators from {`+`, `-`}

Let’s try another small puzzle on precedence, with `5 - 2 * 2`. What would be the result of this? Will it be `6`, or `1`? It’s `1`, because `*` has a higher precedence than `-`. Thus `2 * 2` is `4`, and `5 - 4` gives us `1`.

    >>> 5 - 2 * 2
    1

Let’s say we want to execute `5 - 2`, to give an output of `2`. How do we change the operator precedence?

You cannot really change the precedence, but you can add parentheses to group sub-expressions differently.

    >>> (5 - 2) * 2
    6
    >>> 5 - ( 2 * 2 )
    1

Parentheses have the highest precedence in Python, and can be used to override operator precedence. `(5 - 2)` gets calculated first, and the final result of the expression is `6`.

A positive thing about using parentheses is, that it makes expressions more readable. So even in situations such as `5 - 2 * 2`, where we know the result according to precedence, adding parentheses is good.

**Summary**

In this step, we went about solving a few puzzles about expressions, touching concepts such as:

- `SyntaxError` for incorrect operators
- White-space in expressions
- Floating Point division by default
- Operator Precedence
- Using parentheses

### Step 06: Printing Text

In the previous step, we learned how to use expressions to compute values. In this step, let’s see how we can actually print multiplication table entries, that are readable by the user.

**Snippet-01: Printing Text**

How do we go about printing a complete multiplication table entry? We want to print text such as `5 * 6 = 30` . But trying to do so, as we know it, gives us a `SyntaxError`. Clearly, there is a different way to print text, as compared to an expression.

    >>> 5 * 6 = 30
      File "<stdin>", line 1
    SyntaxError: can't assign to operator

Let’s first try to print a simple piece of text, `Hello`. Typing in this piece of code directly on Python Shell also gives us an error.

    >>> Hello
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Hello' is not defined

Only expressions work that way, and `Hello` is not really an expression.

`"Hello"` is typically called a **string**, and represents the text of letters `'H'`, `'e'`, `'l'`, `'l'`, `'o'`. `"Hello"` is hence different from the number `5`.

There are a number of in-built functions in Python to help print strings. One of these is the `print()` function. Can you just say `print Hello`?

    >>> print Hello
      File "<stdin>", line 1
        print Hello
                  ^
    SyntaxError: Missing parentheses in call to 'print'. Did you mean print(Hello)?

The Python compiler gives you an error, that says “missing parentheses”.

Will `print(Hello)` work?

    >>> print (Hello)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Hello' is not defined

Nope! Again, this one failed because you need to indicate that `"Hello"` is a string.

How do I indicate that `"Hello"` is a string? By putting it within double quotes.

Let’s try `print ("Hello")`

    >>> print ("Hello")
    Hello
    >>> print("Hello")
    Hello

`print("Hello")` finally results in `"Hello"` being printed out. To be able to print `"Hello"`, the things we need to do are:

- Typing the method name print ,
- open parentheses \( ,
- Followed by a double quote " ,
- The text Hello,
- and another double quote " ,
- finished off with a closed parentheses \).

What we have written here is called a **statement**, a simple piece of code to execute. As part of this statement, we are **calling** a **function**, named `print()`.

What exactly are we trying to print?

The text `"Hello"`, which is called a **parameter** or **argument**, to `print()`.

Now let’s get back to what we wanted to do, which is to print `5 * 6 = 30`. The most basic version would be something of this kind, `print("5 * 6 = 30")`. Here, we are passing the entire value in the form of a string.

    >>> print("5 * 6 = 30")
    5 * 6 = 30

This prints the text on the console, as-is. The thing you need to understand here is, we aren’t really calculating `30` using the formula `5 * 6`, but directly putting text `30` in here. That’s called **hard-coding**.

In a later step, we will look at how to actually calculate the value and pass it in.

**Summary**

In this step, we:

- Understood that displaying text on the console is not the same as printing an expression value
- Learned about the `print()` function, that is used to print text in Python.
- Found a way to print the text `"5 * 6 = 30"` on the console, by hard-coding values in a string

### Step 07: Puzzles On Utility Methods, And Strings

In the previous step, we learned how to print `5 * 6 = 30`. It was not a perfect solution, because we hard-coded everything. we used an in-built function named `print()`, passed a string to it, and invoked the method.

In this step, let’s look at a number of puzzles related to in-built methods, their parameters, and strings in general.

For example, let’s do `print("5 * 6")`, as in the previous step. What does this code result in?

    >>> print("5*6")
    5*6
    >>> print('5*6')
    5*6

It just prints the string `"5 * 6"`.

Let’s say we try the code `print(5 * 6)`,

    >>> print(5*6)
    30

Without the double quotes, `5 * 6` is an expression. What will be the output? `30`.

If you call `print()` with an expression argument, it prints the value of the expression. However, when we pass something within double quotes, it becomes a piece of text, printed as-is.

An interesting thing to note is, that in Python you can use either double-quotes \(`"` and `"`\), or single-quotes \(`'` and `'`\) with text values.

Let’s look at a few other in-built methods within Python.

Consider `abs()` \(which stands for absolute value\), a method that accepts a numeric value. You can use `abs(10.5)`, passing `10.5` as a value to it, and it prints the absolute value of `10`.

    >>> abs 10.5
      File "<stdin>", line 1
        abs 10.5
               ^
    SyntaxError: invalid syntax
    >>> abs(10.5)
    10.5

If you pass in a string value, will it work? It complains, “`abs()` function will not work with a string, it only works with numeric values”.

    >>> abs("10.5")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: bad operand type for abs(): 'str'

Let’s say you want to use a function that computes “to the power of”, for instance “`2` to the power of `5`”. In Python, there’s an in-built function named `pow()`, which does just what we need. To `pow()`, you can pass two parameters and calculate the result. How do you do that?

Will this work: `pow 2 5`? No, not at all. This code does not work as well: `pow(2 5)`. `pow(2, 5)` is the correct syntax.

    >>> pow 2 5
      File "<stdin>", line 1
        pow 2 5
            ^
    SyntaxError: invalid syntax
    >>> pow(2 5)
      File "<stdin>", line 1
        pow(2 5)
              ^
    SyntaxError: invalid syntax
    >>> pow(2, 5)
    32

You’ll see that `32` is printed.

Let’s see another example, “`10` to the power of `3`”. `pow(10,3)` is the alternative to saying `10 ** 3`. This gives us `1000`, similar to how `pow()` would.

    >>> pow(10, 3)
    1000
    >>> 10 ** 3
    1000

`max()` returns maximum in a set of numbers.`min()` function returns the minimum value.

    >>> max(34, 45, 67)
    67
    >>> min(34, 45, 67)
    34

These are some of the in-built functions in Python, and we saw how to call the in-built functions by passing in a varied number of parameters.

Python is case sensitive. So let’s say I want of calculate `pow(2,5)`. So this would give me `32`. Now, what if I say capital `'P'` instead of small `'p'` here? `Pow(2,5)` would lead to an error.

    >>> pow(2,5)
    32
    >>> Pow(2,5)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Pow' is not defined

The only things not case-sensitive in Python, are string values. Earlier we saw that the code `print("Hello")` displays the text `"Hello"`. Inside a string, the text can be in any case. Hence, `print("hello")` displays `"hello"` ,with a small `'h'`.

    >>> print("Hello")
    Hello
    >>> print("hello")
    hello
    >>> print("hellO")
    hellO
    >>> print ( "hellO" )
    hellO

However inside your code, you need to be very particular about the case of function names, class names, variable names, and the like.

In your code, whitespace does not really matter. You can add space here and here, and you would still get the same output. However, in case of strings, whitespace does matter.

If we say `print("hellO World")`, it would print `"hellO World"`, with a space in between. And if you do `print("hellO World")` with three spaces, it would print the same. In expressions, white-space does not affect the output.

    >>> print ( "hellO World" )
    hellO World
    >>> print ( "hellO     World" )
    hellO     World

The last thing we want to look at, is an **escape sequence**. Let’s say you want to print a double quote, `"`, in the code. If we were to do this: `print("Hello"")`, what would happen? The compiler says error!

    >>> print("Hello"")
      File "<stdin>", line 1
        print("Hello"")
                      ^
    SyntaxError: EOL while scanning string literal

If you want to print a `"` inside a string, use an escape sequence. In Python, the symbol `'\'` is used as an **escape character**. On using `'\'` adjacent to the `"`, it prints `Hello"` \(notice the trailing `"`\). We have used the `'\'` to **escape** the `"`, by forming an _escape sequence_ `\"`.

>>> print("Hello\"")
Hello"
>>>

The other reason why you would want to use a `'\'` is to print a `<NEWLINE>`. If you want to print `"Hello World"`, but with `"Hello"` on one line and `"World"` on the next, `'\n'` is the escape sequence to use.

    >>> print("Hello\nWorld")
    Hello
    World

The other important escape sequence is `'\t'`, which prints a `<TAB>` in the output. When you do `print("Hello\tWorld")`, you can see the tab-space between `"Hello"` and `"World"`.

    >>> print("Hello\tWorld")
    Hello   World

Another useful escape sequence is `\\` . If you want to print a `\` , then use the sequence `\\` . You would see that it prints `Hello\World` . Think about what would happen if we put six `\` . Yes you’re right! It would print this string: `"\\\"` .

    >>> print("Hello\\World")
    Hello\World
    >>> print("Hello\\\\\\World")
    Hello\\\World

One of the things with Python is, it does not matter whether you use double quotes or single quotes to enclose strings. There are some interesting, and useful ways of using a combination of both, within the same string. Have a look at this call: `print("Hello'World")`, and notice the output we get. In a similar way, the following code will be accepted and run by the Python system: `print('Hello"World')`.

    >>> print('Hello"')
    Hello"
    >>> print("Hello'World")
    Hello'World
    >>> print("Hello\"World")
    Hello"World
    >>> print("Hello\"World")
    Hello"World

The above two examples can be used as a tip by newbie programmers when they form string literals, and want to use them in their code:

- If the string literal contains one or more single quotes, then you can use double quotes to enclose it.
- However if the string contains one or more double quotes, then prefer to use single quotes to enclose it.

**Summary**

In this step, we:

- Explored a number of puzzles related to code involving:
- Built-in functions for numeric calculations
- The `print()` function to display expressions and strings
- Covered the following aspects of the above utilities:
- Case-sensitive aspects of names and strings
- The role played by whitespace
- The escape character, and common escape sequences

### Step 08: Formatted Output With print\(\)

In the previous step, we learned how to print a hard-coded string, such as `"5 * 6 = 30"`.

In this step, let’s try to replace the hard-coded `30` with a computed value.

Let’s start with a simple scenario. Let’s say we want to place that calculated value within a string, and display it. How do we do that?

**Snippet-01: print\(\) Formatted Output**

`format()` method can be used to print formatted text.

Let’s see an example:

    >>> print("VALUE".format(5*2))
    VALUE

We were expecting `10` to be printed, but it’s actually printing `VALUE`.

How do we get `10` to be printed then?

    >>> print("VALUE {0}".format(5*2))
    VALUE 10

By having an open brace `{`, closed brace `}`, and and by putting the index of the value between them. Here, the value is the first parameter, and it’s index will be `0`.

`"VALUE {0}"` is what we need.

Let’s take another example. Suppose to the `format()` function, we pass three values: `10`, `20` and `30`.

Typically when we count positions or indexes, we start from `0`.

To print the first value, you need to pass in an index of `0`. To print the second value, pass an index of `1`.

    >>> print("VALUE {0}".format(10,20,30))
    VALUE 10
    >>> print("VALUE {1}".format(10,20,30))
    VALUE 20
    >>> print("VALUE {2}".format(10,20,30))
    VALUE 30

Now going back to our problem, we wanted to display `"5 * 6 = 30"`, but without hard-coding. Instead of `30`, we want the calculated value of `5 * 6`.

    >>> print("5 * 6 = 30".format(5,6,5*6))
    5 * 6 = 30

Let replace `"5 * 6 = 30"` with `"5 * 6 = {2}"`. `2` is the index of parameter value `5*6`.

    >>> print("5 * 6 = {2}".format(5,6,5*6))
    5 * 6 = 30

Cool! Progress made.

Let’s replace `5 * 6` with the right indices - `{0} * {1}`.

    >>> print("{0} * {1} = {2}".format(5,6,5*6))
    5 * 6 = 30

The great thing about this, is now we can replace the values we passed to `print()` in the first place, without changing the indexes! So, we can display results for `5 * 7 = 35` and `5 * 8 = 40`. We are now able to print `5 * 6 = 30`, `5 * 7 = 35`, `5 * 8 = 40`, and can do similar things for other table entries as well.

    >>> print("{0} * {1} = {2}".format(5,7,5*7))
    5 * 7 = 35
    >>> print("{0} * {1} = {2}".format(5,8,5*8))
    5 * 8 = 40
    >>> print("{0} * {1} = {2}".format(5,8,5*8))
    5 * 8 = 40

**Summary**

In this step, we:

- Discovered that Python provides a way to do formatted printing of string values
- Looked at the `format()` function, and saw how to call it within `print()`
- Observed how we could work only with the indexes of parameters to `format()`, and change the parameters we pass without changing the code

### Step 09: Puzzles On format\(\) and print\(\)

In this step, let’s look at a few puzzles related to the format, and the print methods.

**Snippet-01: format\(\) And print\(\) Puzzles**

Let’s say we pass in additional values, such as: `5 * 8`, `5 * 9` and `5 * 10`. However, within the call to `format()`, we are only referring to the values at index `0`, index `1` and index `2`. The values at indexes `3` and `4` are not used at all. What would happen when we run the code?

    >>> print("{0} * {1} = {2}".format(5,8,5*8,5*9,5*10))
    5 * 8 = 40

Would this throw an error? No, it does not. You can see that the additional values which are passed in, are conveniently ignored.

Let’s say instead of passing in a value of `2`, we pass `4`. What would happen?

    >>> print("{0} * {1} = {4}".format(5,8,5*8,5*9,5*10))
    5 * 8 = 50

`5 * 10` is the value at index `4`

Now let’s take a different scenario. We remove all the parameters passed to `format()`. However, inside the call to `print()`, we continue to say `{0} * {1} = {4}`. So we are trying to print the value at index `4`, but are only passing two values to the function `format()`. What do you think will happen?

    >>> print("{0} * {1} = {4}".format(5,8))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    IndexError: tuple index out of range

It says `IndexError`, which means :“you are asking me to fetch the value at index `4`, but only passing in two values. How can I do what you want?”

Let’s look at a few more things related to other data types. We try to format the following inside `print()`: `{0} * {1} = {2}`, and would pass in `2.5`, `2`, and `2.5 * 2` . Here, `2` is an integer value, but `2.5` is a floating point value. You can see that it prints `2.5 * 2 = 5.0`. So this approach of formatting values with `print()`, works also with floating point data as well.

    >>> print("{0} * {1} = {2}".format(2.5,2,2.5*2))
    2.5 * 2 = 5.0

Now, are there are other types of data that `format()` works with? Yes, strings can join the party.

Let’s say over here, we do: `print("My name is {0}".format("Ranga"))`. What would happen?

    >>> print("My name is {0}".format("Ranga"))
    My name is Ranga

Index `0` will be replaced with the first parameter to `format()`.

**Summary**

In this step, we:

- Understood the behavior when the parameters passed to `format()`:
- Exceed the indexes accessed by `print()`
- Are less than the indexes accessed by `print()`
- Are of type integer, floating-point or string

### Step 10: Introducing Variables

We are slowly making progress toward our main goal, which is to print the `5` multiplication table.

In the first statement, we are printing `5 * 1 = 5`, and then changing the literals. To make it print `5 * 2 = 10`, we are changing `1` to `2`. Next, we are changing `2` to `3`. How do we make it a little simpler, so that our effort is reduced?

    >>> print("{0} * {1} = {2}".format(5,1,5*1))
    5 * 1 = 5
    >>> print("{0} * {1} = {2}".format(5,2,5*2))
    5 * 2 = 10
    >>> print("{0} * {1} = {2}".format(5,3,5*3))
    5 * 3 = 15

Let’s try a different approach.

What would happen if you replace `1` with `index`, and `5 * 1` with `5 * index`, and try to run it?

It gives an error! It says: “index is not defined”.

Let’s try and fix this, and execute `index = 2`. What would happen?

    >>> index = 2

Aha! This compiles.

    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 2 = 10

And this statement is printing `5 * 2 = 10`.

Let’s try something else. Let’s make `index = 3`. What would happen?

    >>> index = 3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15

The same statement on being run, prints `5 * 3 = 15`.

How can you check the value that `index` has? Just type in `index`.

    >>> index
    3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15

The `index` symbol we have used here, is what is called a **variable**.

In Python, it’s also called a **name**.

You can see that the value `index` referring to, can change over the duration of a program.

Initially, `index` was referring to a value of `1`. later, `index` was referring to a value of `3`.

Now, think about how you would print the entire table. All that you need to do, is start from `1`, execute the same statement with `print()` and `format()`, to get output `5 * 1 = 5`. Next, Change the value of index to `2`, and then print the same statement. Next, `index = 3`, and print the same statement again.

    >>> index = 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 1 = 5
    >>> index = 2
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 2 = 10
    >>> index = 3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15

With the same statement `print("{0} * {1} = {2}".format(5,index,5*index))`, we are able to print different values. The value of `index` varies, but the code remains the same!

Variables make the program much more easier to read, as well as more generic.

**Snippet-02: Classroom Exercise On Variables**

Let’s do a simple exercise with variables.

We want to create three variables `a`, `b` and `c`. Let’s initially give them some values, say a value of `5` to `a`, `6` to `b` and `7` to `c`.

We want to get output of this kind: `5 + 6 + 7 = 18`, without using the literal values.

You would want to use the values stored in the variables in `a`, `b` and `c`.

If you’re hard-coding, the way to do it is with `print("5 + 6 + 7 = 18")`.

    >>> a = 5
    >>> b = 6
    >>> c = 7
    >>> print("5 + 6 + 7 = 18")
    5 + 6 + 7 = 18
    >>> print("5 + 6 + 7 = 18".format(a,b,c,a+b+c))
    5 + 6 + 7 = 18

The way you can do that is with code like this: `print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))`.

    >>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
    5 + 6 + 7 = 18

How do you confirm we are accessing values stored in the variables?

Let’s change the values of `a`, `b` and `c`. Let’s make `a = 6` , `b = 7` , and `c = 8` . Execute same statement.

    >>> a = 6
    >>> b = 7
    >>> c = 8
    >>> print("{0} + {1} + {2} = {3}".format(a,b,c,a+b+c))
    6 + 7 + 8 = 21

You can see the magic of variables at play here! Based on what values these variables are referring to, you can see that the output of the print statement changes.

**Summary**

In this step, we:

- Were introduced to variables, or names, in Python
- Observed how we could pass in values of variables to the `format()` function

### Step 11: Puzzles On Variables

In the previous step, we were introduced to the concept of variables in Python.

We will start with looking at a few puzzles.

**Snippet-01: Puzzles On Variables**

What if I try to refer to a variable which is not yet created?

    >>> count
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'count' is not defined
    >>> print(count)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'count' is not defined

Before using a variable, you need to have it assigned a value. If you have not defined a variable before, then you cannot use it. Consider `print(count)`, it does not know what count is. So it would throw an error, saying: “`count` is not defined, I have no idea what count is.”

Once you assign a value to a variable, you can use it.

    >>> count = 4
    >>> print(count)
    4

The statement `count = 4` where we are creating a variable named `count` for the first time, is called a **variable definition**.

This is the first time you’re referring to a variable, and assigning a value to it.

Python will create a variable in its memory.

Variable names are case sensitive. `count` and `Count` are not the same thing.

    >>> Count
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Count' is not defined
    >>> count
    4

There are rules to follow while naming variables.

All variable names should either start with an alphabet , or an underscore `_` . `count`, `_count` are valid. `1count` is invalid.

    >>> 1count = 5
      File "<stdin>", line 1
        1count = 5
             ^
    SyntaxError: invalid syntax
    >>> count = 5
    >>> _count = 5
    >>> 1count
      File "<stdin>", line 1
        1count
             ^
    SyntaxError: invalid syntax
    >>> 2count
      File "<stdin>", line 1
        2count
             ^
    SyntaxError: invalid syntax

After the first symbol, you can also use a numeral in variable names.

    >>> c12345 = 5

To summarize the rules for naming variables.

- This should start with an alphabet \(a capital or a small alphabet\) or underscore.
- Starting the second character, it can be alphabet, or underscore, or a numeric value.

**Summary**

In this step, we:

- Understood that a variable needs to be defined before it is used
- Learned that there are certain rules to be followed while giving names to variables

### Step 12: Introducing Assignment

In this step, we will look at an important concept in Python, called **assignment**. In previous steps, we created variables, like `i = 5`.

**Snippet-01: Introducing Assignment**

You can create other variables using whatever value `i` is referring to. If we say `j = i`, what would happen?

    >>> i = 5
    >>> j = i
    >>> j
    5

`j` would start referring to the same value that `i` is referring to. This statement is called an **assignment**.

Let’s try `j = 2 * i`.

    >>> j = 2 * i
    >>> j
    10

`j` refers to a value of `10`

`=` has a different meaning in programming compared to mathematics.

In mathematics, When we execute `j = i`, it means `j` and `i` are equal.

In prgramming, the value of the expression on right hand side is assigned to the variable on the right hand side. Can you use a constant on the left hand side of an assignment? The answer is “No”!

    >>> 5 = j
      File "<stdin>", line 1
    SyntaxError: can't assign to literal

The Python Shell throws an error, saying “Can’t assign to literal”, as `5` is a literal.

Let’s create a couple of variables. `num1 = 5` and `num2 = 3`. We would want to add these and create a fresh variable. Let’s say the name of the variable is `sum`.

    >>> num1 = 5
    >>> num2 = 3
    >>> sum = num1 + num2
    >>> sum
    8

Create 3 variables `a`, `b` and `c` with different values and calculate their sum.

    >>> a = 5
    >>> b = 6
    >>> c = 7
    >>> sum = a + b + c
    >>> sum
    18

We have just seen the mechanics of how assignment works in Python.

**Summary**

In this step, we:

- Learned what happens when you assign a value to a variable, which may or may not exist
- Discovered that literal constants cannot be placed on the left hand side of the assignment\(`=`\) operator

### Step 13: Introducing Formatted Printing

Until now, we have been using the `format()` method to format and print values. Let’s see a better approach to printing values.

This is the approach we used until now.

    >>> a = 1
    >>> b = 2
    >>> c = 3
    >>> sum = a + b + c
    >>> print("{0} + {1} + {2} = {3}".format(a, b, c ,sum))
    1 + 2 + 3 = 6

Python has the concept of formatted strings. The syntax to use a formatted string is very simple - `f""`.

If we want to print the value of a variable `a`, we can use `{a}` in the text.

    >>> print(f"")
    >>> print(f"value of a is {a}")
    value of a is 1
    >>> print(f"value of b is {b}")
    value of b is 2

The variable within braces is replaced by its value.

You can use expressions in a formatted string. Example below uses `{a+b}`.

    >>> print(f"sum of a and b is {a + b}")
    sum of a and b is 3

This feature was introduced in a Python 3 release.

Let’s get back to the original problem we wanted to solve: printing `5 + 6 + 7 = 18`, using formatted strings.

    >>> print(f"{a} + {b} + {c} = {sum}")
    1 + 2 + 3 = 6

You can see how easy it turns out to be!

### Step 14: The PMT-Challenge Revisited

We want to print the `5`-table from `5 * 1 = 5` onward, until we reach to `5 * 10 = 50`. The best solution we have right now, is shown below:

**Snippet-01:**

    >>> index = 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 1 = 5
    >>> index = 2
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 2 = 10
    >>> index = 3
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 3 = 15
    >>> index = 4
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 4 = 20

Can we do something, to make sure that the code remains the same all the time, but the `index` value gets updated?

    >>> index = index + 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 5 = 25
    >>> index = index + 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 6 = 30
    >>> index = index + 1
    >>> print("{0} * {1} = {2}".format(5,index,5*index))
    5 * 7 = 35

We used `index = index + 1` to increment `index` value.

If we execute these same two statements again and again, we can print the entire table! This is exactly what loops help us do: execute the same statements repeatedly.

The simplest loop available in Python is the **for loop**.

When we run a `for` loop, we need to specify the range of values - `1` to `10` or `1` to `20`, and so on. `range()` function helps us to specify a range of values.

    >>> range(1,10)
    range(1, 10)

The syntax of the `for` loop is: `for i in range(1, 10): ...`. Here, `i` is the name of the **control variable**. In Python, you need to put a colon, ‘`:`’, and in the next line give indentation.

    >>> for i in range(1,10):
    ...   print(i)
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9

You would see that it prints from `1` to `9`.

When we run a loop in `range(1, 10)`, `1` is _inclusive_ and `10` is **exclusive**.The loop runs from `1` to the value before `10`, which is `9`.

The leading whitespace before `print(i)` is called **indentation**. We’ll talk about indentation later, when we talk about puzzles related to the `for` loop.

How can you extend this concept to solving our _PMT-Challenge_ problem?

    >>> print(f"{5} * {index} = {5*index}")
    5 * 7 = 35

What we were doing earlier, was calling `print()` with a formatted string. Now we want to print this statement for different values of `i`.

How can you do that?

Let’s start with a simple example.

    >>> for i in range(1,11):
    ...   print(f"{i}")
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10

`print(f"{i}")` prints the value of i.

Now, how do we get it to print `5 * 1 = 5` to `5 * 10 = 50`?

    >>> for i in range(1,11):
    ...   print(f"5 * {i} = {5 * i}")
    ...
    5 * 1 = 5
    5 * 2 = 10
    5 * 3 = 15
    5 * 4 = 20
    5 * 5 = 25
    5 * 6 = 30
    5 * 7 = 35
    5 * 8 = 40
    5 * 9 = 45
    5 * 10 = 50
    >>> 5 * 4 * 50
    1000

`print(f"5 * {i} = {5 * i}")` prints a specific multiple of 5.

### Step 15: Loops

In a previous step, we took a major step in programming. We wrote our first for loop with Python. In this step, let’s try a few puzzles to understand the for loop even further.

The syntax of the for loop we looked at earlier was:

  for i in range(1, 10):
    print(i)

**Snippet-01:**

Let’s say we write a `for` loop, but don’t give a `:` after the `range()` method, to close the first line. What would happen?

    >>> for i in range(1,10)
      File "<stdin>", line 1
        for i in range(1,10)
                           ^
        SyntaxError: invalid syntax

Invalid syntax. A `:` is mandatory within the `for` loop syntax.

Let’s provide a `:` and in the next line, use `print(i)` without space before it \(without indentation\).

    >>> for i in range(1,10):
    ... print(i)
      File "<stdin>", line 2
        print(i)
            ^
    IndentationError: expected an indented block

Most other programming languages use open brace `{` and closed brace `}` as delimiters in a `for` loop. However, Python uses indentation to identify which code is part of a `for` loop, and which is not. So if we are writing the body of a `for` loop, we must use indentation, and leave atleast a single `<SPACE>`.

    >>> for i in range(1,10):
    ...   print(i)
    ...
    1
    2
    3
    4
    5
    6
    7
    8
    9

How do we execute two lines of code as part of the `for` loop?

    >>> for i in range(1,10):
    ...  print(i)
    ...  print(2*i)
    ...
    1
    2
    2
    4
    3
    6
    4
    8
    5
    10
    6
    12
    7
    14
    8
    16
    9
    18

We are indenting both statements with a space - `print(i)` and `print(2*i)`.

When for loop has only one line of code, you can specify it right after the `:`

    >>> for i in range(2,5): print(i)
    ...
    2
    3
    4

However, this is not considered to be a good programming practice. Even though you may want to execute just one statement in a `for` loop, indentation on a new line is recommended.

Another best practice is to use four `<SPACE>`s for indentation, instead of just two. This would give clear indentation of the code.

    >>> for i in range(2,5):
    ...     print(i)
    ...
    2
    3
    4

Anybody who looks at the code immediately understands that this `print()` is part of the `for` loop.

Let’s say you only want to print the odd numbers till `10`, which are `1`, `3`, `5`, `7` and `9`. The `range()` function offers an interesting option.

    >>> for i in range (1,11,2):
    ...   print(i)
    ...
    1
    3
    5
    7
    9

In `for i in range(1, 11, 2)`, we pass in a third argument, called a _step_. After each iteration, the value of `i` is increment by `step`.

**Summary**

In this step, we:

- Looked at a few puzzles about the `for` loop, which lay emphasis on the following aspects of for:
- The importance of syntax elements such as the colon
- Indentation
- Variations of the `range()` function

### Step 16: Programming Exercise PE-BA-02

In the previous step, after initially exploring the Python `for` loop, we looked at a number of puzzles.

In this step, let’s look at a few exercises.

**Exercises**

1. Print the even numbers up to 10. We would want to print 2 4 6 8 10, using a for loop.
2. Print the first 10 numbers in reverse
3. Print the first 10 even numbers in reverse
4. Print the squares of the first 10 numbers
5. Print the squares of the first 10 numbers, in reverse
6. Print the squares of the even numbers

**Solution 1**

Instead of starting with `1`, we need to start with `2`. Each time, `i` it would be incremented by `2`, and `2 4 6 8 and 10` would be printed.

    >>> for i in range (2,11,2):
    ...   print(i)
    ...
    2
    4
    6
    8
    10

**Solution 2**

We would want to print the numbers in reverse. Think about how you would do that using the `range()` function. We’d want go from `10`, `9`, `8`, and so on up to `1`.

    >>> for i in range (10,0,-1):
    ...   print(i)
    ...
    10
    9
    8
    7
    6
    5
    4
    3
    2
    1

The value to start with is `10`. As we discussed earlier, the end value is exclusive. So to print from `10` to `1`, we want to end one value which is `0`. `range(10, 0)` seems to be what we need.

Usually these step value is positive, but we need to go backwards from `10`. Hence, we would give a step value of `-1`.

**Solution 3**

Now, let’s print the first `10` even numbers in reverse.

    >>> for i in range (20,0,-2):
    ...   print(i)
    ...
    20
    18
    16
    14
    12
    10
    8
    6
    4
    2

**Solution 4**

Next, we would want to print the squares of the first 10 numbers.

    >>> for i in range (1,11):
    ...     print(i * i)
    ...
    1
    4
    9
    16
    25
    36
    49
    64
    81
    100

**Solution 5**

Let’s print the squares in the reverse order.

    >>> for i in range (10,0,-1):
    ...     print(i*i)
    ...
    100
    81
    64
    49
    36
    25
    16
    9
    4
    1

**Solution 6**

Print the squares of the even numbers. How to do that?

    >>> for i in range (10,0,-2):
    ...     print(i*i)
    ...
    100
    64
    36
    16
    4

The key part is using a step of `-2`

We leave it as an exercise for you, to print squares of odd numbers.

**Summary**

In this video, we: \* Tried out a few exercises involving the for loop, by playing around with printing sequences of numbers.

- Used the for loop to simplify the solution to the _PMT-Challenge_ problem.

### Step 17: Review: The Basics Of Python

It must have been a roller-coaster ride to solve the multiplication table challenge so far. If you’re new to programming, there are a wide range of topics and concepts, that you would have learned during this small journey.

Let’s quickly revise the important concepts we have learned during this small journey.

- `1`, `11`, `5`, … are all called literals because these are constant values. Their values don’t really change. \_Consider `` 5 _ 4 _ 50`. This is an expression. `_`is an operator, and`5`, `4`and`50 `` are operands.
- The name `i` in `i = 1`, is called a variable. It can refer to different values, at different points in time.
- `range()` and `print()` are in-built Python functions.
- Every complete line of code is called statement. The specific statement `print()`, is invoking a method. The other statement which we looked at earlier, was an assignment statement. `index = index + 1` would evaluate `index + 1`, and have the `index` variable refer to that value.
- The syntax of the `for` loop was very simple. `for var in range(1, 10) : ...`, followed by statements you would want to execute in a loop, with indentation. For the sake of indentation we left four `<SPACE>`s in front of each statement inside the `for` loop.

## So that, in a nutshell, is what we have learned over the course of our first section.

## Devision 03 - Introducing Methods

In the last section, we introduced you to the basics of python. We learned those concepts by applying them to solve the _PMT-Challenge_ problem. The code below is what we ended up with as we solved that chellenge.

**Snippet-01: Current Solution To PMT-Challenge**

    >>> for i in range (1,11):
    ...   print(f"8 * {i} = {8 * i}")

If we wanted to change the code to print the `7` table, we need to change the value `7` used in the for loop, to `8`. It’s simple, but still not as friendly as you would like.

    >>> for i in range (1,11):
    ...   print(f"7 * {i} = {7 * i}")

To print a `7` table, it would be awesome if could say `print_multiplication_table`, and give a value of 7 beside it, and it would do the rest:

    >>> print_multiplication_table(7)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'print_multiplication_table' is not defined
    >>> print_multiplication_table(8)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'print_multiplication_table' is not defined

Similarly, `print_multiplication_table(8)`, could print the multiplication table for `8`!

To be able to do this, we need to create a **method**, or a **function**. Creating a method makes the code _reusable_, and we can invoke that method very easily by passing _arguments_.

In this section, we take an in-depth look at methods.

### Step 01: Defining Your First Method

Methods are very important building blocks in Python programming. In this step, we will create a simple method that prints `"Hello World"`, twice.

**Snippet-01:**

When we talk about a method, we need to give it a name. We are already using an in-built Python method here, which is `print()`.

    >>> print("Hello World")
    Hello World
    >>> print("Hello World")
    Hello World

Similar to that, we need to give a name to our body of code. Let’s say the name is `print_hello_world_twice`.

The syntax to create a method in Python is straightforward:

- At the start, use the keyword `def` followed by a space.
- Followed by name of the method - `print_hello_world_twice`.
- Add a pair of parenthesis: `()`.
- This is followed by a colon `:` \(similar to what we used in a `for` loop\).

  >>> def print_hello_world_twice():
  ...     print("Hello World")
  ...     print("Hello World")
  ...

All statements in a method should be indented. The two `print("Hello World")` are indented. So, they are part of the method body.

`print_hello_world_twice()` defines a method, and it has certain code inside its body.

How do we call this method? Is it sufficient to say `print_hello_world_twice`?

    >>> print_hello_world_twice
    <function print_hello_world_twice at 0x10a71ef28>

Python Shell says, there’s a function defined with that specific name.

How do we execute a method? Very simple! Add a pair of parentheses to the name, `()`!

    >>> print_hello_world_twice()
    Hello World
    Hello World
    >>> print_hello_world_twice()
    Hello World
    Hello World

Now, we are able to run the method.

**Summary**

In this step, we:

- Learned we can define our own methods in the code we write
- Understood how to define a method, and all its syntax elements
- Saw how we can invoke a method we write

### Step 02: Programming Exercise PE-MD-01

We will now leave you with two exercises, based on what we have learned about methods so far.

**Exercises**

1. Write a method called `print_hello_world_thrice()`. It should print `"Hello World"` thrice to the output. Define this method, and also invoke it.
2. Write and execute a method, that prints four statements:
1. “I have created my first variable.”
2. “I’ve created in my first loop.”
3. “I’ve created my first method.”
4. “I am excited to learn Python.” You need to print these four statements on four consecutive lines.

**Solutions**

**Solution 1**

    >>> def print_hello_world_thrice():
    ...     print("Hello World")
    ...     print("Hello World")
    ...     print("Hello World")
    ...
    >>> print_hello_world_thrice()
    Hello World
    Hello World
    Hello World

**Solution 2**

    >>> def print_your_progress():
    ...     print("Statement 1")
    ...     print("Statement 2")
    ...     print("Statement 3")
    ...     print("Statement 4")
    ...
    >>> print_your_progress()
    Statement 1
    Statement 2
    Statement 3
    Statement 4

    def print_your_progress():
        print("Statement 1")
        print("Statement 2")
        print("Statement 3")
        print("Statement 4")

For convenience, we have changed the exact text we need to print. Call this method with the syntax `print_your_progress()`, and you’re able to execute its code.

Now try another exercise. We want to print `"Statement 1"`, `"Statement 2"`, `"Statement 3"` and `"Statement 4"` on different lines, using just one print statement. How can you do that?

    >>> def print_your_progress():
    ...     print("Statement 1\nStatement 2\nStatement 3\nStatement 4")
    ...
    >>> print_your_progress()
    Statement 1
    Statement 2
    Statement 3
    Statement 4

We are using the newline character `\n`.

Let’s look at the difference between defining and executing a method.

When we are writing a method definition, we are writing the code as part of its body. It has a specific syntax, and starts with the `def` keyword.

A definition by itself cannot cause the code in its body to be executed.

`print_your_progress()` represents a method call. The code inside the method is executed.

**Summary**

In this step, we:

- Implemented solutions to a few exercises that test our understanding of Python methods. We touched concepts such as:
- Defining a method body
- The way to invoke a method, to run its code
- The difference between the two

### Step 03: Passing Parameters To Methods

In the previous step,we created methods. We defined `print_hello_world_twice()`, and this printed `"Hello World"` twice. In this step, let’s talk about _method arguments_, or _parameters_.

**Snippet-01:**

    >>> print_hello_world_twice()
    Hello World
    Hello World
    >>> print_hello_world_thrice()
    Hello World
    Hello World
    Hello World

Earlier, we wrote code for `print_hello_world_thrice()`, which prints the message three times.

Let’s say you want to print it five times. You would need to write another method that does what you need. Doesn’t that seem monotonous?

Instead of that, Won’t it be great if I can call the method by the same name, say `print_hello_world(5)`, and it would print “Hello World” five times?

The `5` which we are passing here is called an **argument**.

How do we define our method to accept this argument?

Let’s call our argument `no_of_times`. If you have any experience with other programming languages, they generally need you to specify the parameter type. Something like `This parameter is an integer/float/string, or other types`. But Python does not require parameter type.

    >>> def print_hello_world(no_of_times):
    ...    print("Hello World")
    ...    print(no_of_times)
    ...

Although we are not doing exactly what we set out to, let’s see what would happen. What would happen if we say `print_hello_world()` ?

    >>> print_hello_world()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: print_hello_world() missing 1 required positional argument: 'no_of_times'

Error! Something like “Hey, you have created `print_hello_world` with a parameter, but not passing anything in here! Go ahead and pass a value”. Let’s pass in a value, such as `5`.

    >>> print_hello_world(5)
    Hello World
    5
    >>> print_hello_world(10)
    Hello World
    10
    >>> print_hello_world(100)
    Hello World
    100

With `print_hello_world(5)`, you can see `"Hello World"` and `5` being printed. We are now able to define this method to accept a value, and print that value by invoking it. You can pass in any value, such as`10`, `100`, or others.

Now think of a different solution for this method, where you don’t repeat the same piece of code to print `"Hello World"`. Consider `print_hello_world(5)`, it should still print `"Hello World"` `5` times. How do you do that?

Think about using something along the lines of a loop.

**Snippet-02:**

For now, what we are doing is we are printing `"Hello World"` `10` times.

    >>> def print_hello_world(no_of_times):
    ...    for i in range(1,10):
    ...       print("Hello World")
    ...

    >>> print_hello_world(5)
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World

Our method call `print_hello_world(5)` now prints `"Hello World"` `10` times.

However just print the message `5` times. We need to make use of the parameter `no_of_times` inside the `for` loop as well.

    >>> def print_hello_world(no_of_times):
    ...    for i in range(1,no_of_times):
    ...       print("Hello World")
    ...

    >>> print_hello_world(5)
    Hello World
    Hello World
    Hello World
    Hello World

Now let’s execute the method again. You can see that it’s printing `4` times only.

Why is it not printing `5` times?

That’s because `no_of_times` as a second parameter to `range()` is exclusive.

    >>> def print_hello_world(no_of_times):
    ...    for i in range(1,no_of_times+1):
    ...       print("Hello World")
    ...
    >>> print_hello_world(5)
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World

Great, it’s now printing the message `5` times!

    >>> print_hello_world(7)
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World

If you pass a different argument like `7`, the message is displayed `7` times.

Something you need to always be cautious about in Python, is the indentation. Over here, the `for` loop is part of the method body. So we have extra indentation for it. The print is part of the `for` loop body. So guess what, even more indentation for that code.

**Summary**

In this step, we:

- Learned how to pass arguments to a method
- Understood that the method definition needs to have parameters coded in
- Observed that arguments passed during a method call can be accessed inside a methods body

### Step 04: Classroom Exercise CE-MD-01

In this step, Let’s look at a few exercises related to the method parameter.

**Exercises**

1. Write a method called `print_numbers()`, that would print all successive integers from `1` to `n`.
2. The second one is to write a method called `print_squares_of_numbers()`, that prints squares of all successive integers from `1` to `n`.

**Solutions**

**Solution 1**

    >>> def print_numbers(n):
    ...    for i in range(1, n+1):
    ...       print(i)
    ...
    >>> print_numbers(5)
    1
    2
    3
    4
    5
    >>>

If you are programming in other languages such as Java, you are used to naming methods in this way: `printNumbers()`. This convention is popularly known as “Camel Case”.

That’s NOT how Python programmers name their methods. Pythonic way is to use underscore `_` to separate words in the method name, as in `print_numbers()`.

**Solution 2**

Let’s define `print_squares_of_numbers()`. This would be very similar to `print_numbers()`, working with the same range. Only, we need to say `print(i*i)` .

    >>> def print_squares_of_numbers(n):
    ...    for i in range(1, n+1):
    ...       print(i*i)
    ...
    >>> print_squares_of_numbers(5)
    1
    4
    9
    16
    25

How is a parameter different from an argument?

- Inside the definition of the method, the name within parentheses is referred to as a **parameter**. In our recent exercise, `n` is a parameter, because it’s used in the definition of `print_squares_of_numbers`.
- When you are passing a value to a method during a method call, say `5`, that value is called an **argument**.
- Don’t worry too much about it. Just follow this convention for now:
- In the method call, call it an _argument_.
- In a method definition, call it a _parameter_.

**Summary**

In this step, we looked at a few simple exercises related to passing method arguments

### Step 05: Methods With Multiple Parameters

In this step, let’s look at creating a method with multiple parameters.

**Snippet-01:**

`print_hello_world` accepts one parameter and prints “Hello World” the specified number of times.

    >>> def print_hello_world(no_of_times):
    ...    for i in range(1,no_of_times+1):
    ...       print("Hello World")
    ...

Let’s say we want to print another piece of text `Welcome To Python`, a specified number of times. How do you do that?

You can always create another method similar to the first one, such as `print_welcome_to_python(no_of_times)` and print the necessary text inside.

However, is that what a good programmer does?

A good programmer tries to create a more generic solution.

    >>> def print_string(str, no_of_times):
    ...    for i in range(1,no_of_times+1):
    ...       print(str)
    ...
    >>> print_string("Hello World", 3)
    Hello World
    Hello World
    Hello World

The good programmer that you are, you created a new method called `print_string(str, no_of_times)` accepting a text parameter, in addition to `no_of_times`.

Syntax rules for method parameters are quite strict. If we say `print_string("Welcome to Python")` and run it, we get an error! Python Shell says: “I need `no_of_times` to be present in here”.

    >>> print_string("Welcome to Python")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: print_string() missing 1 required positional argument: 'no_of_times'

Let’s say you want to assign default values for `str` and `no_of_times` in `print_string()`. By default, we want to always print `"Hello World"`, and that too `5` times.

The Python language makes this very easy. `def print_string(str = "Hello World", no_of_times=5)`. The rest of the method remains the same.

    >>> def print_string(str="Hello World", no_of_times=5):
    ...    for i in range(1,no_of_times+1):
    ...       print(str)
    ...

Now you can call `print_string()`, and `"Hello World"` is displayed `5` times.

    >>> print_string()
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World

If it’s `print_string("Welcome To Python")`, what does it do? It prints `"Welcome To Python"`, `5` times.

    >>> print_string("Welcome to Python")
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python

Consider `print_string("Welcome to Python", 8)`, it would print that string `8` times.

    >>> print_string("Welcome to Python", 8)
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python
    Welcome to Python

Isn’t that cool!

**Summary**

In this step, we:

- Looked at how to pass multiple parameters to a method, starting with two arguments
- Learned how you can define default values for those parameters
- Observed we could pass default arguments for none, some or all of those parameters

### Step 06: Back To Multiplication Table - Using Methods

Let’s get back to our original goal, of why we needed methods. We wanted to create a multiplication table for a number, and observed that each time we needed to we needed change that number, we were forced to make a change in the code. This is not something we liked, and that’s why we started investigating how methods can be used.

In this step, Let’s try our hand at creating a multiplication table method.

**Snippet-01:**

    >>> for i in range (1,11):
    ...    print(f"7 * {i} = {7 * i}")

Let’s define a method called `print_multiplication_table()`, and pass in a parameter to it.

    >>> def print_multiplication_table(table):
    ...    for i in range(1,11):
    ...       print(f"{table} * {i} = {table * i}")
    ...
    >>> print_multiplication_table(7)
    7 * 1 = 7
    7 * 2 = 14
    7 * 3 = 21
    7 * 4 = 28
    7 * 5 = 35
    7 * 6 = 42
    7 * 7 = 49
    7 * 8 = 56
    7 * 9 = 63
    7 * 10 = 70

Now you have the entire multiplication table for `7`.

You can then call `print_multiplication_table()` with arguments `8`, `9`,and so on, by simply changing the `table` arguemnt value.

We now want to create even better `print_multiplication_table()` method.

We want to control the start point, as well as the end point, in the call to `range()`. We want to say `print_multiplication_table(7, 1, 6)`, to print the `7` table with entries from `1` to `6`. How can you do that?

    >>> def print_multiplication_table(table, start, end):
    ...    for i in range(start, end+1):
    ...       print(f"{table} * {i} = {table * i}")
    ...
    >>> print_multiplication_table(7, 1 , 6)
    7 * 1 = 7
    7 * 2 = 14
    7 * 3 = 21
    7 * 4 = 28
    7 * 5 = 35
    7 * 6 = 42

Simple! Define those range limits as additional parameters!

The other thing we can obviously do, is have default values for the `start`, and the `end`.

    >>> def print_multiplication_table(table, start=1, end=10):
    ...    for i in range(start, end+1):
    ...       print(f"{table} * {i} = {table * i}")
    ...

    >>> print_multiplication_table(7)
    7 * 1 = 7
    7 * 2 = 14
    7 * 3 = 21
    7 * 4 = 28
    7 * 5 = 35
    7 * 6 = 42
    7 * 7 = 49
    7 * 8 = 56
    7 * 9 = 63
    7 * 10 = 70

Calling `print_multiplication_table(7)` would give us entries from `7 * 1 = 7` to `7 * 10 = 70`.

Now you can actually send out this method, to your friends, who would find it easy to use, and cool!

**Summary**

In this step, we:

- Learned how to define a method to print the multiplication table for a number
- Looked at how to enhance this method to make table printing more flexible
- Further enhanced that method to accept default arguments while printing a table

### Step 07: Indentation Is King

In Python, indentation denote blocks of code. So if you want to put something in a `for` loop, or outside it, proper indentation would be sufficient. In this step, let’s explore indentation in depth. Let’s start by creating a simple method.

**Snippet-01:**

    >>> def method_to_understand_indentation():
    ...     for i in range(1,11) :
    ...        print(i)
    ...
    >>> method_to_understand_indentation()
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10

Consider the code below: `print(5)` is indented at the same level as `for loop`.

    >>> def method_to_understand_indentation():
    ...     for i in range(1,11) :
    ...        print(i)
    ...     print(5)
    ...

You can see that `print(5)` is called only once. It is not part of the `for loop`.

    >>> method_to_understand_indentation()
    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    5

Let’s change the code in this method a bit. `print(5)` is indented the same way as `print(i)`

    >>> def method_to_understand_indentation():
    ...     for i in range(1,11) :
    ...        print(i)
    ...        print(5)
    ...

`print(5)` is part of the for loop. It is executed 10 times.

    >>> method_to_understand_indentation()
    1
    5
    2
    5
    3
    5
    4
    5
    5
    5
    6
    5
    7
    5
    8
    5
    9
    5
    10
    5

Whether we’re talking about loops, methods or conditionals, proper indentation is very important in Python.

We indicate a block of code, by having all lines of that block at the same indentation level. There are no specific delimiters like for instance a pair of braces `{...}`, as in other programming languages.

**Summary**

In this step, we:

- Ran through a few examples to see how indentation works in Python

### Step 08: Puzzles on Methods - Named Parameters

In this step, let’s look at a variety of puzzles related to methods.

**Snippet-01:**

Consider the following method: I would want to print the default string 6 times. How do we do it?

    >>> def print_string(str="Hello World", no_of_times=5):
    ...     for i in range(1,no_of_times+1):
    ...        print(str)
    ...
    >>> print_string()
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World

Will it work if we call the method as in: `print_string(6)`?

    >>> print_string(6)
    6
    6
    6
    6
    6

`6` is passed as the first parameter. `6` is matched to `str`, and the method prints `6` the default number of times, which is `5`.

to default to `"Hello World"`, and print it `6` times.

You can do this in Python by using **named parameters**. During the method call, you can specify `no_of_times = 6`. **`no_of_times`** is a named parameter.

> There is no provision of doing something like this, in other languages like Java.

Call it as `print_string(no_of_times=6)`:

    >>> print_string(no_of_times=6)
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World
    Hello World

`str` gets a default value, and `"Hello World"` is printed `6` times.

Named parameters are very useful, when a method has a number of parameters, and you would want to make it very clear which parameter you’re passing a value for.

Let’s call `print_string(7, 8)`. what happens?

    >>> print_string(7, 8)
    7
    7
    7
    7
    7
    7
    7
    7

You would see that `7` is printed `8` times.

Since `print()` method is quite flexible, you can pass a number as the first argument. You can even pass a `float`.

    >>> print_string(7.5, 8)
    7.5
    7.5
    7.5
    7.5
    7.5
    7.5
    7.5
    7.5

What would be the result of this - `print_string(7.5, "eight")`?

    >>> print_string(7.5, "eight")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 2, in print_string
    TypeError: must be str, not int

Note how `no_of_times` is used inside the method… as an argument to `range()`. `range()` only accepts integers, nothing else. When you run the code with `print_string(7.5, "eight")`, we get an error.

It says: ` TypeError: ```no_of_times``` must be ```int```, not string `.

A simple rule of thumb is, if you have a parameter, you can pass any type of data to it. That could be an integer, a floating point value a string, or a boolean value. The Python language does not check for the type of a parameter. However, Python will throw an error if the function which is using that parameter, expects it to be of a specific type. The `range()` function expects that the `no_of_times` is an integer value.

**Snippet-02:**

The last thing which we would be looking at, is method naming conventions. We named our methods in a consistent way: `print_string`, `print_multiplication_table`, and the like.

This is exactly the format which most Python developers use, to name their methods.

Convention is to use underscore to separate words in a name.

However, there are a few rules for naming a method: One of the important rules is also related to variable names. We observed that a variable name cannot start with a number.

    >>> def 1_print():
      File "<stdin>", line 1
        def 1_print():
             ^
    SyntaxError: invalid token

Similarly, `1_print` will not be accepted as a method name.

- You can start a name with an alphabet, or with an underscore.
- From the second character onward, you are allowed to use numeric symbols.

Methods and variables cannot be named using Python keywords.

Now, what is a keyword? For example, when we talked about `for` loop, as in:

- **`for`** is a keyword
- **`in`** is a keyword
- **`def`** is a keyword.

Later we will look at a few other keywords, such as **`while`**, **`return`**, **`if`**, **`else`**, **`elif`**, and many more.

>>> def def():
File "<stdin>", line 1
def def():
^
SyntaxError: invalid syntax
>>> def in():
File "<stdin>", line 1
def in():
^
SyntaxError: invalid syntax
>>> def for():
File "<stdin>", line 1
def for():
^
SyntaxError: invalid syntax

**Summary**

In this step, we:

- Were introduced to the concept of named parameters
- Explored the typical naming rules and conventions for methods in Python
- Observed that reserved keywords cannot be used to name variables or methods

### Step 09: Methods - Return Values

Let’s try and understand the importance of return values from a method. We will learn how to return a value from a method.

**Snippet-01:**

Let’s name our method as `product_of_two_numbers()`, and let’s have parameters `a` and `b` that it accepts:

>>> def product_of_two_numbers(a,b):
... print(a * b)
...
>>> product_of_two_numbers(1,2)
2

Can we take the product of these two numbers into a variable, and use it in other code, in the same program?

Suppose we say a `product = product_of_two_numbers(1,2)`, is this allowed?

Let’s run this code, and see what’s stored in `product`.

>>> product = product_of_two_numbers(1,2)
2
>>> product

It’s empty.

The `product_of_two_numbers()` method is not really returning anything back, to be used elsewhere.

Have a look at some of the built-in Python functions, such as `max()` for example.

>>> max(1,2,3)
3
>>> max(1,2,3,4)
4
>>> maximum = max(1,2,3,4)
>>> maximum
4
>>> maximum * 5
20

If I call `max()` with four parameters, as in `maximum = max(1,2,3,4)`, the value `4` gets stored in maximum.

Later on in the code that follows, we can say `maximum * 5`, or we can print the value of `maximum`, or a similar calculation. This gives our programs a lot more flexibility.

So instead of just printing `a*b`, if this function could return a value, that would be quite useful.

>>> def product_of_two_numbers(a,b):
... product = a * b;
... return product
...
>>> product_of_two_numbers(2,3)
6

We are creating a variable `product` and doing a `return product`.

Lets run `product_result = product_of_two_numbers(2, 3)`

>>> product_result = product_of_two_numbers(2,3)
>>> product_result
6
>>> product_result * 10
60

You can see how simple it is to return values from a method!

**Summary**

In this step, we:

- Learned how to return values from inside a method
- Observed how we can store the values returned by a method call

### Step 10: Programming Exercise PE-MD-02

In this step let’s look at a couple of exercises about returning values from methods.

**Exercises**

1. Write a method to return the sum of three integers.
2. Write a method which takes as input two integers, representing two angles of a triangle, and computes the third angle.

Hint: The sum of the angles in a triangle is `180` degrees. So if I am passing `50` and `50`, `50` plus `50` is `100`. So some of three angles should be `180`, so the third angle will be `180 - 100`, which is `80`.

**Solution 1**

>>>def sum_of_three_numbers(a, b, c):
... sum = a + b + c
... return sum
...

>>> sum_of_three_numbers(1,2,3)
6
>>> something = sum_of_three_numbers(1,2,3)
>>> something * 5
30

The shorter way of doing that would have been to have a temporary variable called instead of `sum`. We could directly `return a + b + c`.

>>> def sum_of_three_numbers(a, b, c):
... return a + b + c
...
>>> something = sum_of_three_numbers(1,2,3)
>>> something * 5
30

In methods, you can use `return expression` as well. That `expression` gets evaluated, and the value gets returned back. You’d see that the result remains the same.

**Solution 2**

The second is to write a method to take two integers, representing two angles of a triangle, and compute the third one.

>>> def calculate_third_angle(first, second) :
... return 180 - ( first + second )
...
>>> calculate_third_angle(50, 20)
110

In your programming career, you would be writing a number of methods. It’s very important that you are comfortab