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Programming for
Engineers in
Python
Lecture 13: Shit Happens
Autumn 2011-12
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Lecture 12: Highlights
• Dynamic programming
• Overlapping subproblems
• Optimal structure
• Memoization
• Fibonacci
• Evaluating trader’s performance (maximum
subarray sum)
• Optimizing shipping cargo (Knapsack)
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Optimizing Shipping Cargo (Knapsack)
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Shipping capacity W = 1000
Offers from potential shippers n = 100
Each offer i has a weight wi and an offered reward vi
Maximize the reward given the W tonnage limit
A(n,W) - the maximum value that can be attained from
considering the first n items weighting at most W tons
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Solution (Recursive)
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Solution (Memoization) – The Idea
W
M(N,W)
N
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Solution (Memoization) - Code
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Solution (Iterative) – The Idea
In Class
“Bottom-Up”: start with solving small problems and
gradually grow
W
M(N,W)
N
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DP: Iterative VS. Memoization
• Same Big O computational complexity
• If all subproblems must be solved at least
once, iterative is better by a constant factor
due to no recursive involvement
• If some subproblems may not need to be
solved, Memoized algorithm may be more
efficient, since it only solve these
subproblems which are definitely required
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Steps in Dynamic Programming
1. Characterize structure of an optimal solution
2. Define value of optimal solution recursively
3. Compute optimal solution values either topdown (memoization) or bottom-up (in a table)
4. Construct an optimal solution from computed
values
Why Knapsack?
‫בעיית הגנב‬
http://en.wikipedia.org/wiki/Knapsack_problem
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Extensions
• NP completeness http://en.wikipedia.org/wiki/NP-complete
• Pseudo polynomial http://en.wikipedia.org/wiki/Pseudo-polynomial_time
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Plan
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(Quick overview on) Reading and writing files
(Quick overview on) Exceptions handling
Error correction / error detection
(Brief) Course summary
Python as a first language
HW, exams and exam tips
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IO in Python
• This is NOT exam material
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http://www.penzilla.net/tutorials/python/fileio/
http://docs.python.org/tutorial/inputoutput.html#reading-and-writing-files
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Exceptions Handling in Python
• This is NOT exam material
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http://www.penzilla.net/tutorials/python/exceptions/
http://docs.python.org/tutorial/errors.html#exceptions
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Magic
Source: http://csu-il.blogspot.com/
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Mind Reading Card Trick
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Error correction / error identification
Error correcting for one card flip
What if 2 cards flip? 3? 4?
Applications:
• Messages between computers
• Hard disk drive
• CD
• Barcode
• Spelling corraction
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Israeli ID Error Detection
• Israeli ID: unique per person, 9 digits
• Right most digit is control digit
• How is the control checked?
• Consider first 8 ID digits
• For every 2nd digit d:
• d < 5  write 2*d
• d > 5  write 2*d + 1 – 10 (sum of 2*d digits)
• The rest of the digits remain without change
• ID 053326187
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Example: 053326187
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0 + 1 + 3 + 6 + 2 + 3 + 1 + 7 = 23
(23 + control_digit) % 10 = 0
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Raid
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Redundant array of independent disks
http://en.wikipedia.org/wiki/RAID
Add XOR disk
How to fix a flaw disk’s data?
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Course Summary
Time travel to week #1
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Welcome!
• You are about to take a new programming
course in Python
• This is the first run ever of this course
• The idea is to 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 computing skills
(programming, algorithms, applications)
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Practical Sessions
• In a standard classroom
• Purposes:
• Practice topics that were presented in class
• “Preparations” for next week’s class
• Background for homework assignments
• Learn practical tools
• Lectures will be harder to understand, and it is ok…
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A Personal Note on HW
• It will take you a lot of time and frustration
• It is an engineering difficulty: figuring out
what's wrong with a system and how to fix it
• You're engineers: make it work!
• There is no other way to learn how to program
• Believe me…
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Syllabus
Tentative, not in order, probably too ambitious
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Python programming basics •
Using packages
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Recursion
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Sort & search algorithms,
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runtime analysis
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Dynamic programming
Error handling
Input/output
Graphical user interface (GUI)
Simulation
Optimization
Data analysis
Control
Signal processing
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My Motivation
• What computational capabilities engineers “need”?
• Be able to write simple scripts
• Be able to use existing tools (modules)
• Problems solving capabilities
• Understand what and how CS people do
• For better collaboration
• General knowledge
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Python as a First Language
• Based on Zelle’s paper from 1999
http://mcsp.wartburg.edu/zelle/python/python-first.html
• Popular alternatives: C, C++, Java
• The advantages of using a scripting* language as a first
language, specifically Python
* - Zelle’s words, never let someone tell you python isn't good because
it's a scripting language (we’ll see why soon)
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Compiler
(C, C++, Java (not exactly))
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Hello World (in C) ;-)
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Interpreter
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Criteria for a First Language
• Requirements from a first programming course:
• Learning how to program
• Problem solving, design
• Implications for Programming Language Choice
• Simple syntax and semantics (simple problems should be solved
simply)
• Hands on: high level and flexible languages allow designs to be
expressed with minimal overhead  encourages experimentation
• Supports modern approaches to design and abstraction (e.g., OOP)
• Widely available (not for “teaching only”, used in the “real world”)
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The Case for Scripting Languages
• System programming languages (C, C++, Pascal, Java)
• Statically typed
• Usually compiled
• Modest abstraction from the underlying machine
• Scripting languages:
• Dynamically typed
• Usually interpreted
• Very high level
• Modern scripting languages are not “toy”/”glue” languages!
• Pros
• Very flexible encourage experimentation
• Allow students to build more sophisticated and interesting projects
with less implementation effort
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Python is Simple
Python:
C++:
Java:
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Python has a Simple Uniform Data
Model
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Python is Safe
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Python Supports OOP
Stack as an example:
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Python is Fun
• Simplicity makes it fun to learn
• Data structures (dynamic array, hash table, immutable lists)
and class mechanism can be used to quickly build
• No type declarations  less code, more flexible
• Huge libraries of modules
• E.g., for GUI, client-server applications, html
viewers, databases, animation
• E.g., PIL, pygame, scipy, numpy, swampy,
matplotlib,…
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Python is Practical
• Scripting languages are gaining popularity. By some
accounts, more software is written in scripting than
system languages
• Python is free
• Python is a mature language
• Widely industrial used (e.g., YouTube, BitTorrent, Google,
Yahoo!, IDF, NASA), https://us.pycon.org/2012/sponsors/
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Some Obstacles (Real and imagined)
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Lack of compile time checking – next slide
Scripting languages are too inefficient
Python is unfamiliar (to lecturers…)
Our students want languages X
There aren’t any textbooks
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Lack of Compile Time Checking
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Lack of Compile Time Checking
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‫סקר שביעות רצון (הרשמי(‬
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Course’s HW
• Total of 11 hw assignments
• You must pass at least 7 assignments to be
eligible to pass the course
• HW will count for 20% of course’s grade
• 10 highest assignments will count for the
final grade
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Exam
• One hand written double sided A4 page is
allowed. Each student has to prepare his/her
own page, photo-copying is not allowed
• Two weeks before the exam we will publish
sample questions in the spirit of the exam
• A Q&A session will be held with the TAs a
few days before the exam
• Meanwhile, you can “feel” how it looks like
by checking previous years exams (in C)
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Exam Tips
• The exam material includes all that was taught in
class, tirgul, and hw assignments. Yes, including
today’s class
• You can get 20% of each part by answering “I
don’t know”  knowing that you do not know is
much better than don’t knowing
• I like to give questions that are highly similar to
examples that were shown during the course
• Read the questions carefully. Understanding what
you are asked about is critical!
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Exam Tips
• Question parts (‫ )סעיפים‬are usually dependent.
Most times you should use an earlier part when
solving later parts in a given question.
• You may (and should) solve a question part even if
you have not solved previous parts
• How to prepare?
• Dealing with exam stress
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Good Luck!
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