Programming for Engineers in Python Lecture 13 Shit

Programming for Engineers in Python Lecture 13: Shit Happens Autumn 2011 -12 1

Lecture 12: Highlights • Dynamic programming • Overlapping subproblems • Optimal structure • Memoization • Fibonacci • Evaluating trader’s performance (maximum subarray sum) • Optimizing shipping cargo (Knapsack) 2

Optimizing Shipping Cargo (Knapsack) • • • 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 3

Solution (Recursive) 4

Solution (Memoization) – The Idea W M(N, W) N 5

Solution (Memoization) - Code 6

Solution (Iterative) – The Idea In Class “Bottom-Up”: start with solving small problems and gradually grow W M(N, W) N 7

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 8

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 10

Extensions • NP completeness http: //en. wikipedia. org/wiki/NP-complete • Pseudo polynomial http: //en. wikipedia. org/wiki/Pseudo-polynomial_time 11

Plan • • • (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 12

IO in Python • This is NOT exam material • • http: //www. penzilla. net/tutorials/python/fileio/ http: //docs. python. org/tutorial/inputoutput. html#reading-and-writing-files 13

Exceptions Handling in Python • This is NOT exam material • • http: //www. penzilla. net/tutorials/python/exceptions/ http: //docs. python. org/tutorial/errors. html#exceptions 14

Magic Source: http: //csu-il. blogspot. com/ 15

Mind Reading Card Trick • • 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 16

Israeli ID Error Detection • Israeli ID: unique person, 9 digits • Right most digit is control digit • How is the control checked? • Consider first 8 ID digits • For every 2 nd 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 17

Example: 053326187 0 5 3 3 2 6 1 8 7 0 + 1 + 3 + 6 + 2 + 3 + 1 + 7 = 23 (23 + control_digit) % 10 = 0 18

Raid • • Redundant array of independent disks http: //en. wikipedia. org/wiki/RAID Add XOR disk How to fix a flaw disk’s data? 19

Course Summary Time travel to week #1 20

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! 21

Course Objectives Develop basic computing skills (programming, algorithms, applications) 22

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… 23

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… 24

Syllabus Tentative, not in order, probably too ambitious • • Python programming basics • Using packages • Recursion • Sort & search algorithms, • runtime analysis • Dynamic programming Error handling Input/output Graphical user interface (GUI) Simulation Optimization Data analysis Control Signal processing 25

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 26

Python as a First Language • Based on Zelle’s paper from 1999 http: //mcsp. wartburg. edu/zelle/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) 27

Compiler (C, C++, Java (not exactly)) 28

Hello World (in C) ; -) 29

Interpreter 30

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”) 31

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 32

Python is Simple Python: C++: Java: 33

Python has a Simple Uniform Data Model 34

Python is Safe 35

Python Supports OOP Stack as an example: 36

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, … 37

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. , You. Tube, Bit. Torrent, Google, Yahoo!, IDF, NASA), https: //us. pycon. org/2012/sponsors/ 38

Some Obstacles (Real and imagined) • • • 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 39

Lack of Compile Time Checking 40

Lack of Compile Time Checking 41

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 43

Exam • One hand written double sided A 4 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) 44

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! 45

Exam Tips • Question parts ( )סעיפים are 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 46

Good Luck! 47