Programming for Engineers in Python

Transcription

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

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

3 Optimizing Shipping Cargo (Knapsack) Shipping capacity W = 1000 Offers from potential shippers n = 100 Each offer i has a weight w i and an offered reward v i 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

4 Solution (Recursive) 4

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

6 Solution (Memoization) - Code 6

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

8 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

9 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

10 Why Knapsack? בעיית הגנב 10

11 Extensions NP completeness Pseudo polynomial 11

12 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

13 IO in Python This is NOT exam material

14 Exceptions Handling in Python This is NOT exam material

15 Magic Source: 15

16 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

17 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 2 nd digit d: d < 5 write 2*d d > 5 write 2*d (sum of 2*d digits) The rest of the digits remain without change ID

18 Example: = 23 (23 + control_digit) % 10 = 0 18

19 Raid Redundant array of independent disks Add XOR disk How to fix a flaw disk s data? 19

20 Course Summary Time travel to week #1 20

21 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

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

23 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

24 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

25 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

26 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

27 Python as a First Language Based on Zelle s paper from 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

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

29 Hello World (in C) ;-) 29

30 Interpreter 30

31 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

32 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

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

34 Python has a Simple Uniform Data Model 34

35 Python is Safe 35

36 Python Supports OOP Stack as an example: 36

37 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

38 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), 38

39 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

40 Lack of Compile Time Checking 40

41 Lack of Compile Time Checking 41

42 סקר שביעות רצון (הרשמי ( 42

43 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

44 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) 44

45 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

46 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

47 Good Luck! 47