1 Introduction To Python Programming

1.1 What is programming?

Consider following sentences in different languages.

दोन आणि पाच यांचा गुणाकार करा आणि आपल्या नोटबुकमध्ये उत्तर लिहा
రెండు మరియు ఐదు గుణించి, మీ నోట్‌బుక్‌లో ఫలితాన్ని రాయండి
இரண்டு மற்றும் ஐந்தைப் பெருக்கி, உங்கள் நோட்புக்கில் முடிவை எழுதுங்கள்
બે અને પાંચને ગુણાકાર કરો અને તમારી નોટબુકમાં પરિણામ લખો
দুই এবং পাঁচটি গুণান এবং আপনার নোটবুকের ফলাফল লিখুন
दो और पांच गुणा करें और अपनी नोटबुक में परिणाम लिखें
multiply two and five and write result in your notebook

When you read it what do you do? These are instructions for you to think and act. Similarly ‘programs’ are instructions for computer to think and act. Languages that humans speak have wide spectrum right from very primitive to advanced one. Similarly computers also have wide spectrum of languages primitives to advanced. Instructions in humans language can be rigorous and plenty to do simple tasks. And there can be few elegant and poetic sentences which can inspire people to think and act in complicated and challenging scenarios. Here we are going to learn poetic, pythonish way of writing programs which will inspire machines to do seemingly complicated jobs.

1.2 Working with jupyter notebook

Another fancy way of programming python is jupyter notebook. once you launch jupyter from console or from start menu, a browser will be launched. Create a new notebook. To work with notebook you need to know few key strokes.

  ===========   ====================================
  keys          action                              
  ===========   ====================================
  esc+m         convert the cell to markdown        
  esc+y         convert the cell to code            
  shift+enter   execute the cell                   
  !command      execute system command from jupyter 
  ===========   ====================================

1.3 Numeric And Text data

Very primitive way of looking at Programming is manipulating numbers and text. To manipulate numbers python has got basic numeric compabilities. Python interpreter can be directly used as calculator::

  >> 42 + 42 # text after # is a comment
  84
  >> 42 - 42
  0
  >> 42 * 2 # multiplication
  84
  >> 5 / 2 # real number division
  2.5
  >> 5 // 2 # integer division
  2
  >> 2**5 # 2 raised to power 5!
  32
  >> 5 % 2 # remainder when divided by 2
  1

Numeric data in python has two forms , integers and real numbers. Real numbers are represented by something called as float in python. floats have decimal point in it while integers do not have decimal.

  >>> 1.1 + 1.1
  2.2
  >>> 1.0 - 0.1
  0.9
  >>> 1.0 * 2
  2.0
  >>> 5.0 / 2
  2.5
  >>> 5.0 // 2.0
  2.0
  >>> 2.0 ** 5
  32.0

These symbols +, -, *, **, /, // are called as operators. And calculator commands above are called statements. Two or more operators can be used in single statement. When two operators come in single statement , there is predefined hirarchy or precedence. Here is list arranged from highest precedence to lowest precedence operator.

  ============   ========
  operators      priority
  ============   ========
  ``**``         1
  ``% // / *``   2
  ``+ -``        3
  ============   ========

Have a look at following expressions carefully and find out whats the answer. Can you think of steps involved? which operator will be evaluated first and whats the sequence in which all of the operators in expression will be evaluated?

  >>> 2**5%5/2*7
  >>> 32%5/2*7
  >>> 2/2*7
  >>> 1.8*7
  >>> 7.0

Also observe how round braces change the precedence of operations. If one wants to give precedence to some operation which is in low precedence from above table, then we make use of braces to change the precedence. For example

  >>> 7 + 2*3
  13
  >>> (7+2)*3
  27

Now that we know how to calculate, lets do some book keeping with text data. Python supports text data in the from of very useful data type string. Anything enclosed in single quote ' or double quote " is string or text data in python. These are examples::

  >>> "Hello this is text"
  "Hello this is text"Data
  >>> 'And this is also text'

Python supports multiline text too. As we will learn later python allows all those things which makes your code look more human readable. Anything that is enclosed in triple quotes is multiline string.::

  >>>"""This is first line of my poem
  ... This is second line
  ... This is third line"""
  'This is first line of my poem\nThis is second \nand this s third line'
  >>>'''This is first line of my poem
  ... This is second line
  ... This is third line'''
  'This is first line of my poem\nThis is second \nand this s third line'

Make note of this special character \n. This is called as newline character. This character when printed will become newline or carriage return. Special characters are escaped with this special charecter \, for example.

  ===========    =========
  escape char    meaning
  ===========    =========
  ``\n``         new line
  ``\t``         tab
  ``\\``         \
  ===========    =========

String works with some opeartors just like numbers.::

  >>> "*"*5
  '*****'
  >>> "hello" + "world"
  'helloworld'

1.4 Variables and literals

In addition to arithmatic operators that we saw earlier there is a special operator called assignment operator =. It works like this. Suppose you write a statement which involves assignment operator.

  >>> x = 10

This results in creation of object for integer 10. which will be stored in python’s memory space. Next it creates a name called x in something called as active namespace. Then it connects this name x with the location in python’s memory where 10 is stored. Now magical thing happens due to this namespace. whenever we type x we get::

  >>> x
  10

wow! so we stored the value in something which can be recalled with name x. This is called as variable. Till now we were talking about literals.::

  >>> 10
  10

This is a literal. while what we see below is Variable::

  >>> x
  10

Be carefull with string literals.::

  >>> vikrant = 10
  >>> "vikrant" # not a variable
  'vikrant'
  >>> vikrant
  10

here vikrant is different from 'vikrant'. vikrant is variable and 'vikrant' is litteral string.

What can be used as variable name has some rules.

  - The variable name can't start with number
  - It can be single word (meaning no space or hyphen allowed.)
  - It can have alphabets, numbers and underscore
  - the names are case sensitive

The assignment operator also allows us to assign multiple values at a time.::

  >>> a, b = 2, 3
  >>> a
  2
  >>> b
  3

Now lets work slightly more with strings. Now that we can store strings in a variable, let’s store text data in a variable and play with it.

  >>> s = "hello"

We can access elements from this string with integer indices. Index starts at 0 and goes till length minus one.

  >>> s[0] # 0th character in string
  'h'
  >>> s[4]
  'o'
  >>> s[-1] # last character
  'o'

Indices work as shown below.::

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

1.5 Collections

1.5.1 List and tuple

Other than basic data types we feel need of collecting basic data types together to form an array of sequencially arranged items. List is varsatile higher level data type which allows us to keep any number of items, sequencially.::

  >>> [1, 1, 1]
  [1,1,1]

You can save any similar basic datatypes, or data of different types together in a list::

  >>> numbers = [1, 2, 3, 4]
  >>> words = ["hello", "these", "are","words"]
  >>> words
  ['hello', 'these', 'are', 'words']
  >>> mixed = [1, "word", 2]
  >>> mixed
  [1, "word", 2]

You can actually save lists inside list too.::

  >>> [['a','b','c'], 1, 2, [1, 1, 1]]
  [['a','b','c'], 1, 2, [1, 1, 1]]

You can access elements from a list with it’s index. Lists are nothing but arraging objects in a serial manner. Every item will have unique index, first one starting at index zero. If index more than length -1 is given , python will throw error::

  >>> words[0]
  'hello'
  >>> words[2]
  'are'
  >>> words[3]
  'words'
  >>> words[-1]
  'words'
  >>> words[5]
  ---------------------------------------------------------------------------
  IndexError                                Traceback (most recent call last)
  <ipython-input-19-f6a2fb6dbef1> in <module>
  ----> 1 words[5]

  IndexError: list index out of range

Lists also support modification inplace. For example in a list we can go and change element at specific index.::

  >>> words
  ['hello', 'these', 'are', 'words']
  >>> words[3] = "elements"
  >>> words
  ['hello', 'these', 'are', 'elements']

Just like strings , our lists support + and * operators.::

  >>> [1, 1]*3
  [1, 1, 1, 1, 1, 1]
  >>> [1, 1] + [0, 0]
  [1, 1, 0 , 0]

There is a sibling of list, called tuple. It is exactly similar to list except , it can not be modified like lists.::

  >>> color = (0, 0, 256)
  >>> color[0]
  0
  >>> color[-1]
  256
  >>> color + color
  (0, 0, 256, 0, 0, 256)
  >>> color * 2
  (0, 0, 256, 0, 0, 256)
  >>> color[0] = 100
  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  <ipython-input-31-6f0411612089> in <module>
  ----> 1 color[0] = 100

  TypeError: 'tuple' object does not support item assignment

Lists and tuples allow us to save items by location, i.e by index we can access items.

1.5.2 Dictionary

But there is one more interesting hogher level datatype called dictinary. Dictionary allows to save items in a collection with a name. In a small classroom it is more natural to call out students by name than roll number (index!)::

  >>> scorebyname = {"rupali":20, "alice":19, "maya":18, "kavya":20}
  >>> scorebyname['rupali']
  >>> scorebyname['kavya']
  >>> scorebyname['seema']
  ---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  <ipython-input-36-350bc8d22721> in <module>
  ----> 1 scorebyname['seema']

  KeyError: 'seema'
  >>> scorebyname['seema'] = 15
  >>> scorebyname
  {'rupali': 20, 'alice': 19, 'maya': 18, 'kavya': 20, 'seema': 15}
  >>> scorebyname['seema']
  15

Here is another example of dictionary::

  >>> stock = {"name":"IBM", "open":123, "high":126, "low": 120, "close":123.5}
  >>> stock['open']
  123

1.6 Boolean

There are boolean types supported in python which has only two possible literal values

  >>> True
  >>> False

1.7 Key points to remembers

to make a list we make use square brackets []
to make a string we make use of single or double quotes
to make a tuple we make use of round brackets ()
to make a dictionary we make use of curley brackets {}, inside you have to give key:value pairs separated by comma
to access item from a string we use square bracket and integer index inside that string[2]
to access item from a list we use square bracket and integer index inside that list[2]
to access item from a tuple we use square bracket and integer index inside that tuple[2]
to access item from a dictionary we use square bracket and key inside that d[‘key’]to make a dictionary we make use of curley brackets {}, inside you have to give

1.8 Functions

Now that basic and some higher level data types are known to us and statements as well, lets see functions. Function is nothing but collections of statememnts put together to do more complex task. For time being we will see some built in functions in python. len is one function which we will be using a lot. Function call consists of calling a function with some arguments. argumets are some data on which function will operate and try to calculate some value or try to perform some operation. For example len is used to find length of any collection as well as of string. let’s say we have a string stored in a variable name. we want to find length of string stored inside name. To do this we call function len with name as argument to it.::

  >>> name = "Rupali"
  >>> len(name)
  6
  >>> numbers = [1, 1, 2, 2, 1]
  >>> len(numbers)
  5
  >>> point = (0, 0, 2)
  >>> len(point)
  3
  >>> stock = {"name":"IBM", "open":123, "high":126, "low": 120, "close":123.5}
  >>> len(stock)
  6

1.9 Types and Converting

As we know variable is nothing but just a name. So if we want to know what is it that is stored with the given name?::

  >>> name = "rupali"
  >>> type(name)
  str
  >>> numbers = [1, 2, 3]
  >>> type(numbers)
  list
  >>> point = (0, 0, 1)
  >>> type(point)
  tuple
  >>> stock = {"name":"IBM", "open":123, "high":126, "low": 120, "close":123.5}
  >>> type(stock)
  dict
  >>> type(1)
  int
  >>> type(1.2)
  float

str function can be used to convert other datatypes into string.::

  >>> str(23)
  '23'

int can be used to convert string or float to integer::

  >>> int("42")
  42

list can be used to make a list from string (for that matter from any collection)::

  >>> list("1234")
  ["1","2","3","4"]
  >>> r5 = range(5)
  >>> list(r5)
  [0,1,2,3,4]

max function can find maximum value from collection like list or tuple.::

  >>> max([23, 12, 34, 13, 5, 6, 12, 35])
  35

min function can find minimum value from list or tuple::

  >>> min([23, 12, 34, 13, 5, 6, 12, 35])
  5

sum function sums all items from a list or tuple::

  >>> sum([1, 1, 1, 1])
  4

1.10 More built in functions

We have seen max, min, sum, int, str, float, type functions earlier. Now let us see some more built in functions. Usually programming in any language starts with hello world. It is time to make python’s hello world.::

  >>> print("Hello World")
  'Hello World'
  >>> print("Hello", ",", "Let's", "print", 1, 2)
  Hello , Let's print 1 2

Next we will take input from user::

  >>> x = input("Input value for x:")
  Input value for x:3
  >>> print(x)
  3

We have already seen that functions sum, max, min operate on lists. Here is another function which works on a list. sorted takes a list or tuple and returns sorted list.::

  >>> sorted([3,2,4,1,1])
  [1, 1, 2, 3, 4]
  >>> sorted([3,2,4,1,1], reverse=True)# this is optional argument for sorted
  [4, 3, 2, 1, 1]