Introduction to Python Programming Part-I

Dr. Harpreet Singh,

Head PG Department of Bioinformatics

Hans Raj Mahila Maha Vidyalaya Jalandhar

https://www.python.org/

Programming basics

• code or source code: The sequence of instructions in a program.
• syntax: The set of legal structures and commands that can be used in a particular programming language.
• output: The messages printed to the user by a program.
• console: The text box onto which output is printed.
• Some source code editors pop up the console as an external window,

and others contain their own console window.

3

Compiling and interpreting

• Many languages require you to compile (translate) your program into a form that the machine understands.

compile execute

• Python is instead directly interpreted into machine instructions.

​

interpret

output

source code

Hello.java

byte code

Hello.class

output

source code

Hello.py

4

The Python Interpreter

• Python is an interpreted

language

• The interpreter provides an interactive environment to play with the language
• Results of expressions are printed on the screen

>>> 3 + 7

10

>>> 3 < 15

True

>>> 'print me'

'print me'

>>> print 'print me' print me

>>>

Expressions

6

• expression: A data value or set of operations to compute a value.

Examples:

1 + 4 * 3

42

• Arithmetic operators we will use:

+ - * /

%

**

addition, subtraction/negation, multiplication, division modulus, a.k.a. remainder

exponentiation

• precedence: Order in which operations are computed.
• * / % ** have a higher precedence than + -

1 + 3 * 4 is 13

• Parentheses can be used to force a certain order of evaluation.

(1 + 3) * 4 is 16

Integer division

7

• When we divide integers with / , the quotient is also an integer.

• More examples:
• 35 / 5 is 7
• 84 / 10 is 8
• 156 / 100 is 1

​

• The % operator computes the remainder from a division of integers.

3 4 ) 14

12

2

43 5 ) 218

20

18

15

3

Real numbers

8

• Python can also manipulate real numbers.
• Examples: 6.022 -15.9997 42.0 2.143e17

​

• The operators + - * / % ** ( ) all work for real numbers.
• The / produces an exact answer: 15.0 / 2.0 is 7.5
• The same rules of precedence also apply to real numbers: Evaluate ( ) before * / % before + -

​

• When integers and reals are mixed, the result is a real number.
• Example: 1 / 2.0 is 0.5
• The conversion occurs on a per-operator basis.

Math commands

8

• Python has useful commands (or called functions) for performing calculations.

• To use many of these commands, you must write the following at the top of your Python program:

from math import *

Numbers: Floating Point

• int(x) converts x to an integer
• float(x) converts x to a floating point
• The interpreter shows

a lot of digits

>>> 1.23232

1.2323200000000001

>>> print 1.23232

1.23232

>>> 1.3E7

13000000.0

>>> int(2.0) 2

>>> float(2)

2.0

Variables

x 5 gpa 3.14

• A variable that has been given a value can be used in expressions.

x + 4 is 9

​

• Exercise: Evaluate the quadratic equation for a given a, b, and c.

10

• variable: A named piece of memory that can store a value.
• Usage:
• Compute an expression's result,
• store that result into a variable,
• and use that variable later in the program.

​

• assignment statement: Stores a value into a variable.
• Syntax:

name = value

• Examples:

x = 5

gpa = 3.14

Example

>>> x = 7

>>> x 7

>>> x+7

14

>>> x = 'hello'

>>> x

'hello'

>>>

print

• print : Produces text output on the console.
• Syntax:

print "Message" print Expression

• Prints the given text message or expression value on the console, and

moves the cursor down to the next line.

print Item1, Item2, ..., ItemN

• Prints several messages and/or expressions on the same line.

​

• Examples:

print "Hello, world!" age = 45

print "You have", 65 - age, "years until retirement"

Output:

Hello, world!

You have 20 years until retirement

12

Example: print Statement

>>> print 'hello' hello

>>> print 'hello', 'there'

hello there

• Elements separated by commas print with a space between them
• A comma at the end of the statement (print ‘hello’,) will not print a newline character

input

• input : Reads a number from user input.
• You can assign (store) the result of input into a variable.
• Example:

age = input("How old are you? ")

print "Your age is", age

print "You have", 65 - age, "years until retirement"

Output:

How old are you? 53

Your age is 53

You have 12 years until retirement

14

• Exercise: Write a Python program that prompts the user for his/her amount of money, then reports how many Nintendo Wiis the person can afford, and how much more money he/she will need to afford an additional Wii.

Input: Example

print "What's your name?" name = raw_input("> ")

​

print "What year were you born?" birthyear = int(raw_input("> "))

​

print "Hi “, name, “!”, “You are “, 2016 – birthyear

% python input.py What's your name?

> Michael

What year were you born?

>1980

Hi Michael! You are 31

Repetition (loops) and Selection (if/else)

The for loop

18

• for loop: Repeats a set of statements over a group of values.
• Syntax:

for variableName in groupOfValues:

statements

• We indent the statements to be repeated with tabs or spaces.
• variableName gives a name to each value, so you can refer to it in the statements.
• groupOfValues can be a range of integers, specified with the range function.

​

• Example:

for x in range(1, 6):

print x, "squared is", x * x

​

Output:

range

19

• The range function specifies a range of integers:

• range(start, stop)

- the integers between start (inclusive) and stop (exclusive)

• It can also accept a third value specifying the change between values.
• range(start, stop, step) - the integers between start (inclusive)

and stop (exclusive) by step

• Example:

for x in range(5, 0, -1): print x

print "Blastoff!"

Output:

5

4

3

2

1

Blastoff!

• Exercise: How would we print the "99 Bottles of Beer" song?

Cumulative loops

20

• Some loops incrementally compute a value that is initialized outside the loop. This is sometimes called a cumulative sum.

sum = 0

for i in range(1, 11):

sum = sum + (i * i)

print "sum of first 10 squares is", sum

​

Output:

sum of first 10 squares is 385

• Exercise: Write a Python program that computes the factorial of an integer.

if

21

• if statement: Executes a group of statements only if a certain condition is true. Otherwise, the statements are skipped.
• Syntax:

if condition:

statements

​

• Example:

gpa = 3.4

if gpa > 2.0:

print "Your application is accepted."

if/else

2

• if/else statement: Executes one block of statements if a certain condition is True, and a second block of statements if it is False.
• Syntax:

if condition:

statements

else:

statements

​

• Example:

gpa = 1.4

if gpa > 2.0:

print "Welcome to Mars University!"

else:

print "Your application is denied."

​

• Multiple conditions can be chained with elif ("else if"):

if condition:

statements elif condition: statements

else:

statements

1

Example of If Statements

import math

x = 30

if x <= 15 : y = x + 15

elif x <= 30 :

y = x + 30

else :

y = x print ‘y = ‘,

print math.sin(y)

In file ifstatement.py

>>> import ifstatement y = 0.999911860107

>>>

In interpreter

while

23

• while loop: Executes a group of statements as long as a condition is True.
• good for indefinite loops (repeat an unknown number of times)
• Syntax:

while condition:

statements

• Example:

number = 1

while number < 200: print number, number = number * 2

​

• Output:

1 2 4 8 16 32 64 128

While Loops

x = 1

while x < 10 : print x

x = x + 1

>>> import whileloop 1

2

3

4

5

6

7

8

9

>>>

• In whileloop.py

• In interpreter

Logic

• Exercise: Write code to display and count the factors of a number.

25

• Many logical expressions use relational operators:

• Logical expressions can be combined with logical operators:

Loop Control Statements

More Examples For Loops

• Similar to perl for loops, iterating through a list of values

range(N) generates a list of numbers [0,1, …, n-1]

More Data Types: Objects

Everything is an object

• Everything means everything, including functions and classes (more on this later!)
• Data type is a property of the object and not of the variable

>>> x = 7

>>> x 7

>>> x = 'hello'

>>> x

'hello'

>>>

Numbers: Integers

• Integer – the equivalent of a C long
• Long Integer – an unbounded integer value.

>>> 132224

132224

>>> 132323 **

2

17509376329L

>>>

Numbers: Floating Point

• int(x) converts x to an integer
• float(x) converts x to a floating point
• The interpreter shows

a lot of digits

>>> 1.23232

1.2323200000000001

>>> print 1.23232

1.23232

>>> 1.3E7

13000000.0

>>> int(2.0) 2

>>> float(2)

2.0

Numbers: Complex

• Built into Python
• Same operations are supported as integer and float

>>> x = 3 + 2j

>>> y = -1j

>>> x + y

(3+1j)

>>> x * y (2-3j)

String Literals

■ + is overloaded to do concatenation

>>> x = 'hello'

>>> x = x + ' there'

>>> x

'hello there'

String Literals

• Can use single or double quotes, and three double quotes for a multi-line string

>>> 'I am a string' 'I am a string'

>>> "So am I!"

'So am I!'

Substrings and Methods

>>> s = '012345'

>>> s[3]

'3'

>>> s[1:4] '123'

>>> s[2:]

'2345'

>>> s[:4] '0123'

>>> s[-2]

'4'

• len(String) – returns the number of characters in the String

• str(Object) – returns a String representation of the Object

>>> len(x) 6

>>>

str(10.3)

'10.3'

String Formatting

• Similar to C’s printf
• <formatted string> % <elements to insert>
• Can usually just use %s for everything, it will convert the object to its String representation.

>>> "One, %d, three" % 2 'One, 2, three'

>>> "%d, two, %s" % (1,3)

'1, two, 3'

>>> "%s two %s" % (1, 'three') '1 two three'

>>>

References

• https://www.python.org/
• https://plantae.org/new-teaching-tool-plants-and-python/
• https://sites.cs.ucsb.edu/~tyang/class/240a16w/slides/pythonQuick.pdf
• https://blog.eduonix.com/software-development/python-popular-among-programmers/

​

Thanks