Introduction to Programming

with Python(202-1-9041)

Lecture 1: Introduction to Python

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Welcome to

”Introduction to programming with python”

course !

• We will learn to program in Python.

• Goal: enable you to use programming as a tool to solve

“real world” problems.

• Work is required!

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Course Staff

Lectures

Zion Sicsic sicsic@cs.bgu.ac.il

Office hours in Zoom: Monday 14:00

(by appointment only)

Recitations:

Alexander Gerovichev gerovich@post.bgu.ac.il

Office hours: Sunday 10:00

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Administration

Website:

https://www.cs.bgu.ac.il/~ipp211

What’s there:

• Lectures

• Recitations

• Homework guidelines

• Assignments

• Code examples

• Announcements

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A Personal Note on HW

It will take you time and

effort to make the code

work.

But

There is no other way to

learn how to program

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Exam

• Final grade is composed out of homework (30%) and

final exam (70%)

• You must pass the exam to pass the course

• Written exam

• Includes all course material:

• Lectures, recitations, and HW

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Course Objectives

Develop basic programming and algorithmic skills

Remember: we learn programming, not how computer hardware works !

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Syllabus

• Python programming basics

• Lists, dictionaries and other data types

• Functions

• Sort & Search algorithms

• Exceptions

• Recursion

• Object-Oriented Programming

• Scientific Calculations using NumPy , SciPy & Matplotlib

• Modules

• Introduction to computational complexity

• Data Analysis

• Image Processing

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Resources

• Course slides and pointers to relevant bibliography.

• Recommended resources:

• Book: Think Python, by Allen B. Downey

http://greenteapress.com/thinkpython/thinkpython.html

• Manual: Python 3.6 documentation

http://docs.python.org/3.6 (the official language

manual)

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Preface

• We assume no prior knowledge.

• However, we advance fast.

• The only way to keep on track is to practice!

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Today

• Brief background

• Python basics:

• Variables

• Numbers

• Strings

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Computers understand only machine language.

Basically looks like a sequence of 1’s and 0’s.

Very inconvenient to work with and non-intuitive.

All other computer languages were created for human

convenience.

The computer does not understand C/Python/Java,

it must be “translated” into machine language

In this course, we do not care how the computer does that

Machine Code (Language)

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Programming Languages Basics

• A computer program is a sequence of text instructions

that can be “understood” by a computer and executed.

• A programming language is a machine-readable

artificial language designed to express computations

that can be performed by a computer.

Over 500 different computer languages

are listed by Wikipedia

Computer Program

• A sequence of instructions designed to achieve a

specific purpose

• The instructions are executed sequentially. No instruction is executed before the previous is completed

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Running Programs

• The transformation from high level to machine level

languages comes in two flavors:

– Interpreters,

– Compilers.

• The interpreter is a machine level program, which

interprets and executes the high level program, line

by line.

• The Compiler translates the complete high level

program to a machine level program

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Compiler

(C, C++)

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Interpreter

(Python, Matlab)

a Word on Writing Code

• Getting a program to work as planned is an

interesting process

• It can be not just interesting, but frustrating as well.

• Planning what your program should do, and how it is

going to do it, is crucial.

• It is very tempting to skip such planning and go

straight to writing lines of code.

• We strongly advise you not to skip the planning stage

• Take care: the simple but worst method is “trial and error”

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Language Selection and Python

Python (since 1991):

• Quick development

• Easy to learn

• Huge community

• Short development-execution rounds

• Fast enough for most applications

• Cross-platform

Guido van Rossum

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Python is Good for Your Future..

Python is widely used in industry (Google, Yahoo!,

YouTube, BitTorrent, Intel, IDF, NASA)

Take a look at Python's community conference

• Short development-execution rounds

• Huge community

• Fast enough for most applications

• Cross-platform

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Programming Environment

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Using Anaconda

• run idle.exe to open the Idle terminal.

• The executable file is located in

INSTALL_DIR\Anaconda\Scripts, INSTALL_DIR stands for

the installation directory, usually C:\ or C:\Program Files

• It is recommended to create a shortcut on your desktop.

This is how idle shell looks like:

• A video on working with IDLE:

http://www.youtube.com/watch?v=lBkcDFRA958

My First Python Program:

Hello World!

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Hello World

• The first line of code taught in all programming

languages is a print

• The text to the right of the prompt is the command to

the Python interpreter.

• The text in the next line is the value returned by the

interpreter

• print is a built-in Python function (colored purple by

the interpreter).

• We will later see that Python has a collection of

reserved words

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Read, eval, print

• An interaction with the interpreter has 3 steps:

• Read: the interpreter reads the sequence of characters

we type following the prompt

• Eval: the interpreter evaluates (computes) the code

that was read, and produces a result (and perhaps

additional effect)

• Print: the interpreter prints the result as a sequence of

characters, then prints the prompt for the next

interaction

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Memory

• The computer memory is composed of a long list of

bits (0 and 1)

• Bits are grouped into bytes (8 bits) and words (4

bytes, 8 on 64-bit systems)

• Every byte is numbered sequentially

• This number is called an address

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What are Variables ?A location in the computer’s memory

A variable:

- has a name

- holds a value

- has type – according to its value

- The variable's name is a sequence of letters and

digits, starting with a letter.

This is how data is handled

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Why do We Need Variables?

• Computer programs manipulate data

• Data is given as input or calculated throughout the program

• To access them later, variables must be remembered

• Thus, variables are stored in the memory

• Variable name memory address

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Why Do We Need Different Types?

• Saving memory

• Execution speed

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Trueb

Variables and Assignments

>>> n = 10

>>> m = (10 + 4) * 5

The left-hand side is a variable.

The right-hand side is an expression.

The interpreter:

1. evaluates the expression

2. assigns its value to the variable.

10 70

n m

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Hands On

Numbers and their Types

>>> 4

4

>>> type(4)

<class 'int'> # integers type

>>> 3.14159

3.14159

>>> type(3.14159)

<class 'float'> # floating point ("reals") type

What type is 8/5 ? , 8/5.0 ? 8//5? Let’s check…

Variables Assignments:

Changing the value of a variable:

>>> n = 11

>>> n

11

Changing the type of a variable:

>>> n = 1.3141

>>> n

1.3141

Variables can be used in expressions:

>>>2*n+1

Variables and Assignments – Cont.

Referring to undefined variables leads to runtime error

>>> check_this

Traceback (most recent call last):

File "<pyshell#16>", line 1, in <module>

check_this

NameError: name 'check_this' is not defined

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Arithmetic Operators

Operator Use Description

+ x + y Adds x to y

- x - y Subtracts x from y

* x * y Multiplies x by y

** x ** y X to the power y

/ x / y Divides x by y

// x / / y Floor divides x by y (rounded to the next smallest whole

number)

% x % y Computes the remainder of dividing x by y

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Playing with Variables

>>> a = 3

>>> a

3

>>> b = 5

>>> b

5

>>> c = a + b

>>> c

8

>>> c = c * 2

>>> c

16

>>> b**a

125

>>> first = (a + b) * 2

>>> second = a + b * 2

>>> first, second

(16, 13)

>>> b / a

1.66666666666667

>>> b % a

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Strings

• Strings are text sequences.

• An ordered list of characters

“Hello world”

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String Type

Immutable

Strings Access

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>>> a=‘Hello’

>>> a[1:3]

'el'

>>> a[1:]

'ello'

>>> a[-4:-2]

'el'

>>> a[:-3]

'He'

>>> a[-3:]

'llo’

H e l l o

0 1 2 3 4 5

-5 -4 -3 -2 -1

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Strings concatenation

>>> s1 = "He"

>>> s2 = "llo"

>>> s3 = s1 + s2

>>> s3

'Hello'

>>> s4 = s3 + " World"

>>> c = "!"

>>> print(s4, 2020, c)

Hello World 2020 !

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Strings Structure

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Strings are Immutable>>> a = "abc"

>>> a[0]='a'

Traceback (most recent call last):

File "<pyshell#21>", line 1, in <module>

a[0]='a'

TypeError: 'str' object does not support item assignment

However, pointing to another string is valid:

>>> a = "abc"

>>> a = “ggg"

Immutable variables cannot be changed after created.

Applying operations on immutable variables usually return

a new variable rather changing the original variable

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Strings Built In Methodshttps://docs.python.org/release/3.5.4/library/string.html

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Strings Built In Methodshttp://www.tutorialspoint.com/python/python_strings.htm

• Escape characters: \n (new line) \t (tab)

• Special string operators:Operator Description Example

+ Concatenation - Adds values on either side of the

operator

a + b will give

HelloPython

* Repetition - Creates new strings, concatenating

multiple copies of the same string

a*2 will give HelloHello

][ Slice - Gives the character from the given index a[1] will give e

] : [ Range Slice - Gives the characters from the given

range

a[1:4] will give ell

in Membership - Returns true if a character exists in

the given string

H in a will give 1

not in Membership - Returns true if a character does not

exist in the given string

M not in a will give 1

% Format - Performs String formatting See at next section

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Strings Built In Methodshttp://www.tutorialspoint.com/python/python_strings.htm

• String Formatting Operator

print("My name is %s and my weight is %d kg "!% ('Zara', 12))My name is Zara and my weight is 12 kg!

• Useful String methods:– len

– find, startswith, endswith

– isalpha, isdigit, islower,…

– join, replace

– strip, rstrip

– split