Programming for Engineers in Python Sawa 2015 Lecture 1: Introduction to Python 1.

Programming for Engineers

in Python

Sawa 2015

Lecture 1: Introduction to Python1

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Welcome to Programming for Engineers course!

• We will learn to program in Python.

• Goal: enable you to use programming as a tool to solve "real world" problems.

• Hard work is required!

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Course Objectives

Develop basic programming and algorithmic skills

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Administration

Lectures

Lena Dankin lenadank at tau.post.ac.il

Recitations

Aviad Panhi

Web Sites:

Moodle

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Administration

Website via Moodle: http://moodle.tau.ac.il

Everything is there:• Lectures• Practical sessions• Homework assignments, submission, grading• Code examples• Forum• Announcements

Make sure you are updated.

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Recitations

• Purposes:• Practice topics presented in class.• Learn practical tools.• Help in overcoming trivial difficulties

• Lectures will be harder to understand, and it is OK.

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Home Assignments

• Short assignments every day.• 3-4 home assignments that should be

submitted. • Some more assignments to practice the

material, no need to submit.

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Homework • Submission in singles via Moodle • It is allowed (and encouraged) to talk, share

ideas, and help friends. • No code-sharing!

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A Personal Note on HW

It will take time and effort to make the code work.

But

There is no other way to learn how to program

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Exam

• Written exam• Includes all course material• You must pass the exam to pass the course.

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Working Environment

• Lab 008• Home versus lab

VS.

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Syllabus

• Python programming basics• Recursion• Sort & Search algorithms• Runtime Analysis• Error Handling• Object-Oriented

Programming• Image Processing

• Scientific Calculations• Simulations • Data analysis

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Resources

• Course slides and pointers to relevant bibliography.• Many Python references, tutorials or manuals.• Recommended resources:

• Book: Think Python, by Allen B. Downey (http://greenteapress.com/thinkpython/thinkpython.html)

• Manual: Python 2.x documentation http://docs.python.org// (the official language manual)

• Course’s website

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Questions?

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Preface

• We assume no prior knowledge in programming.

• 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

Machine Code (Language)

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Programming Languages Basics

• A computer program is a sequence of instructions (text) 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.

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There are Many Programming Languages

Over 500 different computer languages are listed by Wikipedia

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Language Selection and Python

Consider: Goal Runtime vs. Development time Operating systems

Python: Quick development, small to medium tasks:Experiments / Data analysis / Small projects Short development-execution rounds

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Computer Program(more technically)

• A sequence of processor instructions designed to achieve a specific purpose

• Unless otherwise specified, the instructions are executed sequentially, in the order they’re written in the program.

• In this course, we assume that no two instructions can execute simultaneously.

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Installing and Running Python 2.7• Python 2.7 is already installed in the computers’

classroom• Install:

• Anaconda: https://store.continuum.io/cshop/anaconda/

• A “Anaconda" folder will be created in you Start Menu:• IDLE (Python GUI) - recommended working environment• Python (command line) - textual Python interpreter (faster, less

convenient)• A Working with IDLE video:

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

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Hello World!

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My First Python Program: Hello World!

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Hands On

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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

What are Variables ?

A location in the computer’s memory.

A variable:- has a name - holds a value- has type – according to its

value (“duck typing”)

This is how data is handled28

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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

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

Arithmetic operations: +, -, *, / , % (modulo), ** (power)

What type is 8/5 ? And 8/5.0 ?

Let’s check…

Variables and Assignments

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

The variable's name is a sequence of letters

and digits, starting with a letter.

The interpreter:

1. evaluates the expression

2. assigns its value to the variable.

10 70n m

Variable

Expression

Variables and Assignments: An Example

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:

>>> pi = 3.14159

>>> pi * 2 + 1

7.28318

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 Computes the remainder of dividing x by y

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Playing with Variables >>> a = 3

>>> a

3

>>> b = 5

5

>>> c = a + b

>>> c

8

>>> c = c * 2

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>>> b**a

125

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

>>> second = a + b * 2

>>> first, second

(16, 13)

>>> b / a

1

>>> b % a

2

Strings

• Strings are text sequences. • An ordered list of characters

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String Type

Strings Access>>> 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, 2014, cHello World 2014 !

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Strings Structure

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Strings Built In Methodshttp://docs.python.org/release/2.5.2/lib/string-methods.html

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Comparison Operators

Operator Name Description

x < y Less than true if x is less than y, otherwise false.

x > y Greater than true if x is greater than y, otherwise false.

x <= y Less than or equal to

true if x is less than or equal to y, otherwise false.

x >= y Greater than or equal to

true if x is greater than or equal to y, otherwise false.

x == y Equal true if x equals y, otherwise false.

x != y Not Equal true if x is not equal to y, otherwise false.

Compares two variables and returns a Boolean type result/variable

Comparison Operators

>>> 5 == 5.0

True

>>> 6 != 2*3

False

>>> -2 >= 1

False

>>> 3 <= 3

True

>>> x = 3 < 3

>>> x

False

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Logical Operators

Operates on two Booleans and returns a Boolean

Operator

Description

x and y

Both True: True, otherwise: False.

x or y

At least one is rue: True, Otherwise: False.

not x

x is False True, x is True False

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And, or, not

0 1

0 0 01 0 1

0 1

0 0 11 1 1

0 1

1 0

and or not

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Flow Control

Different inputs Different execution order–Computer games– Illegal input

Control structures– if-else– for loop–while loop

http://xkcd.com/1195/

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Conditional Statement: if

Used to execute statements conditionally

Syntax

if condition:

statement1

statement2

…• If condition is True, statements are executed

Condition = expression that evaluates to a Boolean

Indentation = determines the scope of the if block

Conditional Statements

>> num = 54 # choose a number

>> if num % 18 == 0: # num is a multiplication of 18

…print num, “is divisible by 18“

…res = num / 18

>>> print “Goodbye”

54 is divisible by 18

Goodbye

Conditional Statements - Examples

Conditional Statements

Indentation: • Following the if statement:

Open a new scope = one tab to the right. • Indicates the commands within the scope

of this if.

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if-else

if condition1:statement1

elif condition2:statement2

else:statement3

condition1 is true execute statement1

condition1 false and condition2 true execute statement2 both conditions are false execute statement3

if-else

if-else

if width == height:

print “found a square”

else:

print “found a rectangle”

width = height

print “now it is a square”

Indentation: else is not part of the if scope!

The commands under else are indented.

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if-elsea = 4

b = 5

c = 6

if a + b >= c and a + c >= b and b + c >= a:

print "Building a triangle"

else:

print "Cannot build a triangle"

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elif = else-if

if price < 100:

print "too cheap"

elif price > 200:

print "too expensive“

else:

print "reasonable price"

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Program Readability

• Real computer programs include thousands of code lines, lots of variables.

• Readability is very important for code maintenance,it is a requirement in this course!

• The next slide list some programming conventions that are crucial for readability.

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Program Readability – Cont.

• Choose informative variable names:

– Are i, j, m , n informative names? Depends on context.

• Comments:– option 1: begin with # (until end of line)– option 2: surrounded by """ (3 double quotes) when spanned

over multiple lines

• Documentation is most important before ‘logical units’ or complex implementation.

w sm a_string Not informative:

word_count sum_of_expenses student_name Informative:

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Class ExampleDonuts:

Input: Define a variable named donuts and assign to it some number (of donuts).

Output:• If donuts is no more than 5, print 'Number of donuts: donuts.'• Otherwise, if donuts is no more than 10, print

'Number of donuts: donuts...'

• Otherwise, print 'Number of donuts: a lot!'

Note: the trailing ‘.’ / ‘…’ / ‘!’ must be printed without any space before them.

Run your program. Examples are in the next slide.

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Class Example – Cont.Examples:

For 5 donuts you should get:

Number of donuts: 5.For 9 donuts you should get:

Number of donuts: 9...For 23 donuts you should get:

Number of donuts: a lot!

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Solution to Class Exampledonuts = 7if donuts <= 5: print 'Number of donuts:', str(donuts) + '.'elif donuts <= 10: print 'Number of donuts:', str(donuts) + '...'else: print 'Number of donuts: a lot!'

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“My Code doesn’t Work!”

• Go over your code carefully.• Consult course slides.

• Google (a useful and legitimate source).• Good forums to ask questions online:

stackoverflow.com, underwar.co.il

• Check the public forum in Moodle.• Submit a question to forum (reply within 48 hours).

– Please never paste solution code in the forum.

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Understanding Errors

Traceback (most recent call last): File "C:\Users\John\Desktop\blah.py", line 2, in <module> b = 2 / aZeroDivisionError: integer division or modulo by zero

Where in your program?

The last line of the error is most informative.