Welcome to Informatik 2!

Transcription

1 Welcome to Informatik 2!

2 Our Subject this Term: Algorithms and Data Structures in Java and C++

3 Totally Different Problems!? What is the minimum number of people involved for a person to become his/her own grandparent by marriage? In order to set up a network, James has to connect desks A-F. As a first guess, he sums up all distances to order cable. How much better could he do? A 4m 3m C B 2m 3m 10m D 4m 5m F E Which 4 digits need to be removed? schmiedecke 09 Info2-1-Intro 3

4 The Algorithmic Perspective In Informatik 1, we dealt with creating objects combined them to form interacting systems with behaviour either active or reactive That was the object oriented paradigm for problem solving But there is another perspective: it is not about systems, but about computation processes i.e. solving mathematical problems in a stepwise process using the computer to do tedious, time- consuming computations The Algorithmic Perspective schmiedecke 09 Info2-1-Intro 4

5 The Cradle of Computer Science Greek mathematiciens invented the process of stepwise computing more than 2000 years ago. Present in other ancient cultures, too. In our days: the basis for mechanization of computing Brunsviga Calculator and of course of electronic computing. schmiedecke 09 Info2-1-Intro 5 Zuse Computer Z1 (1936) and Z64 (1964)

6 Is this an Algorithm? Let AB and CD be the given numbers,, such that the bigger one, AB, is not measured precisely by the smaller one,, CD. Keep taking the smaller number away from the bigger one, until the remainder measures the the number before it. This remainder is the greatest common measure of the two given numbers. Euclid, about 250 B.C. (abbreviated) An algorithm describes the solution of a problem as a set of consecutive simple steps. (Definition from Info 1) schmiedecke 09 Info2-1-Intro 6

7 Algorithms can be... fast or slow By all means, algorithms offer many opportunities for making mistakes! schmiedecke 09 Info2-1-Intro 7

8 Code Quality Matters! Aspects: efficiency / complexity next week correctness robustness readability and structure today schmiedecke 09 Info2-1-Intro 8

9 Contents and Learning Material Part 1: Algorithm and Data Structure Design until Witsun using Java Text Book: Duane Bailey, Java Structures available online! exam on part 1 60% Part 2: Introduction to C week introduction online course material (still to be selected) redoing java labs exam on part 2 40 % schmiedecke 09 Info2-1-Intro 9

10 Lab Work Reports: published as web pages (as soon as possible) include running programs as applets, when applicable Teamwork: groups of two pair programming: don't split the work, but share it! OS: Linux is mandatory for part 1 Grading: Part of the grade for your achievements during lab time. schmiedecke 09 Info2-1-Intro 10

11 Step 1: Code Quality (Book Chapter 2)

12 Programmers are Craftsmen Designing Algorithms established correctness known complexity and measured performance readable and managable code Designing Data Structures correctness, robustness, and efficiency support algorithm efficiency well-understood concept re-usable Using Programming Tools support code quality schmiedecke 09 Info2-1-Intro 12

13 Code Readability What makes code readable / hard to read? Believe it or not, there are well accepted rules for readbility: Code Style Guides, regulating: Naming and spelling conventions camelcase, only class names starting with capital, constants all capital,... Formatting: indentation, new line conventions, convnetions for control structures... Program sizes: max method length, max class length, max num of parameters, max nesting of control structures (often applied as code quality metrics) Comment conventions usage of javadoc comments, line comments, assertions schmiedecke 09 Info2-1-Intro 13

14 Code Style Guides 1. Formatting Conventions indentation new lines comment enforcement line length method length class length schmiedecke 09 Info2-1-Intro 14

15 from Geosoft JavaStyle schmiedecke 09 Info2-1-Intro 15 quoted from

16 Code Style Profile in Eclipse schmiedecke 09 Info2-1-Intro 16

17 Eclipse: Code Templates schmiedecke 09 Info2-1-Intro 17

18 Code Style Guides 2 Naming Conventions Case CamelCase starting capital for classes CamelCase starting small for all other names Capitals for constants all names in English... Grammar verbs for methods nouns for Types nouns in plural for collections positive is-terms for booleans type prefixes for constants no abbreviations... schmiedecke 09 Info2-1-Intro 18

19 from Geosoft JavaStyle schmiedecke 09 Info2-1-Intro 19

20 Implemented Style Guides Style guides ensure readability and testability Implementation in IDE is better than discipline! Style checking still necessary! Main advantage: switch dynamically between different styles: production and test slide presentation schmiedecke 09 Info2-1-Intro 20

21 Production Style schmiedecke 09 Info2-1-Intro 21

22 Presentation Style schmiedecke 09 Info2-1-Intro 22

23 Refactoring Refactoring means "restoring a clean program structure" - in succession to program modifications... Experience: Most programs originally "tidy" schmiedecke 09 Info2-1-Intro 23

24 Refactoring Tasks Style Guide Requirements Renaming Abstraction Requirements Changing Method Signatures Localizing / Delocalizing Variables Structure Requirements Splitting Methods Splitting Classes Moving Methods to other Classes Moving Methods to Superclasses / Subclasses Extracting Interfaces schmiedecke 09 Info2-1-Intro 24

25 Simple Editing vs Refactoring Simple editing, e.g. changing a method signature, will cause a lot of errors in different classes. Let eclipse help you with refactoring: Refactoring takes care of all references within the current working set. What's a working set? An arbitrary set of classes, usually a project, but sometimes a set of projects or even a free collection of classes in a workspace. schmiedecke 09 Info2-1-Intro 25

26 Structural Quality: Let's do the Number Riddle Major Principle: Good code has a clear structure! Steps towards structure: look for a good abstraction separate concerns find useful, supportive data structures Which 4 digits need to be removed? schmiedecke 09 Info2-1-Intro 26

27 Three Steps Towards Structure 1. Abstraction: we have to iterate through all valid combinations and evaluate them. 2. Separation of Concerns: separate iteration and evaluation 3. Data structure: implement the Enumeration interface to manage the iteration. public interface java.util.enumeration <T> { public boolean hasmoreelements(); public <T> nextelement(); } schmiedecke 09 Info2-1-Intro 27

28 Basic Enumeration Setup Sample solution (may be wrong...) = 548 Encoding: Combinations can be described as arrays of 4 integers, representing the number of digits removed { 0, 1, 1, 2 } Iteration: like numbers to the base of 4: 0000, 0001, 0002, 0003, 0010, 0011, 00012,... Valid combinations: sum of digits = , 0022, 0031, 0103,... Enumeration should produce sequence of valid combinations schmiedecke 09 Info2-1-Intro 28

29 Basic Evaluation Setup Assume Enumeration producing arrays of valid combinations For each combination, evaluate as sum of integers print solution, if sum fits Simple can be done in the main method... schmiedecke 09 Info2-1-Intro 29

30 Solver Code public class RiddleSolver { public static void main(string[] args) { int requiredsum = 548; int[] combination; RiddleCombinationEnumerator rce = new RiddleCombinationEnumerator(); while (rce.hasmoreelements()) { combination = rce.nextelement(); if (sumup(combination)==requiredsum) printformula(); } } private static String formula =""; private static void printformula () { System.out.println(formula); } schmiedecke 09 Info2-1-Intro 30

31 ctd. private static int sumup (int[] combination){ formula = ""; int factor=0, sum=0; } for (int i=0; i<combination.length; i++) { switch (combination[i]) { case 0: factor = 111; break; // no digit removed case 1: factor = 11; break; // 1 digit removed case 2: factor = 1; break; // 2 digits removed case 3: factor = 0; break; // all digits removed } int value = factor*(i+3); formula += " + " + value; // collect string sum += value; } formula += " = "+sum; return sum; schmiedecke 09 Info2-1-Intro 31

32 Enumeration, V 1.0 public class RiddleCombinationEnumerator implements Enumeration<int[]> { protected int[] current = { 0, 0, 1, 3 }; protected boolean finished = false; } public boolean hasmoreelements() { return!finished; } Too involved. refactor!! public int[] nextelement() { int [] result = current; do { //findnextcombination for (int index=3; index>=0; index--) { current[index] = (current[index]+1) % 4; if (current[index]!= 0) break; // no overflow } if (current[0]+current[1]+current[3]+current[3] == 0) finished = true; } while (!finished &&!combinationvalid()); return result; schmiedecke 09 Info2-1-Intro 32

33 Refactor Extract Method schmiedecke 09 Info2-1-Intro 33

34 schmiedecke 09 Info2-1-Intro 34

35 Enumeration Code public class RiddleCombinationEnumerator implements Enumeration<int[]> { protected int[] current = { 0, 0, 1, 3 }; protected boolean finished = false; public boolean hasmoreelements() { return!finished; } public int[] nextelement() { int [] result = current; do findnextcombination(); while (!finished &&!combinationvalid()); return result; } private void findnextcombination() {...} private boolean combinationvalid() { return current[0]+current[1]+current[2]+current[3] == 4; } schmiedecke 09 Info2-1-Intro 35

36 ctd.. private void findnextcombination() { for (int index=3; index>=0; index--) { current[index] = (current[index]+1) % 4; if (current[index]!= 0) return; // no overflow } if (current[0]==0) finished = true; } schmiedecke 09 Info2-1-Intro 36

37 Fine Structure, but... It's not commented add javadoc comments! Is it trustworthy? Is it correct? Is it robust? how could we find out? What can we do? schmiedecke 09 Info2-1-Intro 37

38 Pre- and Postconditions Pre- and postconditions can be used to (semi-)formally define the semantics of methods if formal, they can be used for correctness proofs Use pre- andd postconditions to specify the meaning before coding keep them as documentation schmiedecke 09 Info2-1-Intro 38

39 Enumeration Code with Pre and Post public class RiddleCombinationEnumerator implements Enumeration<int[]> { protected int[] current = { 0, 0, 1, 3 }; protected boolean finished = false; public boolean hasmoreelements() { // pre: // post: returns true, until all valid combinations have // been enumerated return!finished; } public int[] nextelement() { // pre: hasmoreelements // post: current combination is returned // either current holds the next valid combination, // and finished is false, // or current contains all zeros, and finished is true int [] result = current; do findnextcombination(); while (!finished &&!combinationvalid()); return result; schmiedecke } 09 Info2-1-Intro 39

40 ctd. private void findnextcombination() { // pre: hasmoreelements // post: current is updated to next value, or to zeros // finished is set to false if end is reached for (int index=3; index>=0; index--) { current[index] = (current[index]+1) % 4; if (current[index]!= 0) return; } if (current[0]==0) finished = true; } private boolean combinationvalid() { //pre: current is not null // post: validity check: sum of digits removed is 4 return (current[0]+current[1]+current[2]+current[3] == 4); } schmiedecke 09 Info2-1-Intro 40

41 Integrate into javadoc /** * findnextcombination * Enumerates combinations like base-4 numbers. * Sets current to the next combination. * On overflow (0000), finished is set true. hasmoreelements current is updated to next value, or to zeros * finished is set to false if end is reached */ private void findnextcombination() { for (int index=3; index>=0; index--) { current[index] = (current[index]+1) % 4; if (current[index]!= 0) return; } if (current[0]==0) finished = true; } schmiedecke 09 Info2-1-Intro 41

42 Enforcing Preconditions Parameter preconditions on public methods are the user's responsibility enforce using Exceptions /RuntimeExceptions Preconditions on private methods or preconditions on hidden state are the implementor's responsibility must be asserted failing is a severe programming error. use assertions as safeguard. schmiedecke 09 Info2-1-Intro 42

43 Enforcing Public Preconditions public int[] nextelement() { // pre: hasmoreelements // post: current combination is returned // either current holds the next valid combination, // and finished is false, // or current contains all zeros, and finished is true if (!hasmoreelements()) throw new EnumerationEndedException(); int [] result = current; do findnextcombination(); while (!finished &&!combinationvalid()); return result; } schmiedecke 09 Info2-1-Intro 43

44 Enforcing Private Preconditions private void findnextcombination() { // pre: finished is false // post: current is updated to next value, or to zeros // finished is set to true if end is reached assert!finished: "Enumeration has expired"; for (int index=3; index>=0; index--) { current[index] = (current[index]+1) % 4; if (current[index]!= 0) return; } if (current[0]==0) finished = true; } private boolean combinationvalid() { //pre: current is not null // post: validity check: sum of digits removed is 4 assert current!= null; return (current[0]+current[1]+current[2]+current[3] == 4); } schmiedecke 09 Info2-1-Intro 44

45 Assertions in Java Since Java1.4, assert is a keyword and has a meaning in Java: assert condition; if the contion fails, an AssertionError is thrown. Exception in thread "main" java.lang.assertionerror at enumerator.riddlecombinationenumerator.findnextcombin.... assert condition : messageexpression; if the condition fails, an AssertionError is thrown with the messageexpression as message parameter. Exception in thread "main" java.lang.assertionerror: Enumeration expired at enumerator.riddlecombinationenumerator.findnextcomb..... schmiedecke 09 Info2-1-Intro 45

46 Where to Put Assertions? You can dynamically assert any condition that you believe holds or has to hold true at runtime: preconditions of private methods loop invariants method invariants postconditions thread states Make use of them whenever a semantic comment allows to be stated as an assertion (keep the comment for more verbal explanation) schmiedecke 09 Info2-1-Intro 46

47 Assertion Don't-s Never: use an assertion condition with side effect (the assertion may be switched off at runtime) assert index++ == 100; use an assertion instead of a documented Exception to check the input parameter compliance of public methods. An assertion is not an interface check; a broken assertion marks a severe internal programming error. - assert inputparam!= null schmiedecke 09 Info2-1-Intro 47

48 Enabling Assertions Assertions can be switched on or off at VM start using the VM options -enableassertions, -ea -disableassertions, -da (default) If disabled, assert staements are ignored. In the shell, this means calling java ea MyClass In eclipse, enter ea as VM argument in your run cnfiguration. schmiedecke 09 Info2-1-Intro 48

49 Testing Correctness Postconditions can be used to determine test sets Design test sets before coding Run all tests after every code modification... The answer is automatic testing: You define the test sets They are run and evaluated automatically. We will look into the JUnit test framework some other time... schmiedecke 09 Info2-1-Intro 49

50 Design for Scalability Scalability: Ability to adapt to growing problem sizes: amount of data to be handled input frequency number of users or connections Limiting Factors: data structure limitations memory computer perfomance algorithm complexity schmiedecke 09 Info2-1-Intro 50

51 Example: Vector Vector is a flexible array initial size when exhausted, data copied to new enlarged array. How much growth at a time? Different strategies: constant increment doubling the size... with very different consequences! schmiedecke 09 Info2-1-Intro 51

52 Vector Extension Fixed increment take initial size 1, increment (n-1) = ½ n(n-1) copy operations for 1000 elements, that is ½ million copy operations for larger initial size and increment, you get ½ size²/increment², still a lot*) Double size take initial size 1 at the end, only n/2 elements have to be copied n/2 = n-1 copy operations for 1000 elements, that is 1000 copy operations *)Note: The initial size and increment become neglegible for large vectors schmiedecke 09 Info2-1-Intro 52

53 Design for Efficiency What does it mean? Efficiency means optimized resource consumption for a given task Resources: Time and space (memory) There may be an optimum and a worst case Efficiency is a property of the algorithm, not of implementation details! schmiedecke 09 Info2-1-Intro 53

54 Does it matter? Watch them sort at schmiedecke 09 Info2-1-Intro 54

55 Appendix: Linux schmiedecke 09 Info2-1-Intro 55

56 Switching OS......for those of you who have been using Windows last term. Reasons: Linux is open source Is a true multi-user OS Most servers are Unix-derivatives Big changes? First week is "shell week" But then: use eclipse as before... Mandatory? Yes, at least in the lab. You can use a virtual linux on your own machine, if you do not want to install another or second OS (VMplayer) It is your own responsibility to get familiar with Linux you will definitely need it - and hopefully like it schmiedecke 09 Info2-1-Intro 56

57 Basic Linux Terminology Command Shell: Shell Interactive Process: Terminal Bourne-Shell, C-Shell, bash (born again shell, Linux only) Graphical User Interfaces: KDE, Gnome Graphics support: X-Server X-Terminal is a command shell with graphics support: font size and colouring, copy-paste using mouse,... You should already be familiar with some Unix networking tools: ssh secure network command shell VNC secure networking shared GUI schmiedecke 09 Info2-1-Intro 57

58 First bash steps Open bash shell (or X-Terminal) Type commands: ls -lh contents of current directory pwd name of current directory mkdir name create new directory cd change directory mv datei verz move or rename file cp datei verz copy file rm datei delete file echo text or echo $file type text / file contents more datei type one page at a time with search option cmd1 cmd2 "pipe": output of cmd1 is used as input to cmd2 man cmd or info cmd manual pages for cmd man cmd more show one page at a time schmiedecke 09 Info2-1-Intro 58

59 First vi steps vi is a sophisticated command line text editor which runs on every Unix machine (use it for ssh connections) Special feature: 2 modi edit mode and input mode input commands change to input mode return to edit mode using ESC Editing: Start, Save, Quit: vi file opens file, creates it if necessary :q quit vi without saving :w save ZZ save and quit schmiedecke 09 Info2-1-Intro 59

60 vi Editing Cursor Control: use cursor keys or ghjk for cursor control Screen up, Screen down, Pos1 and End 23G goto line 23 G goto end-of-file Deleting Text : x delete character after cursor dw delete word from cursor position dd delete line Modifying Text (Implicit change to input mode!) r replace current character cw replace word from cursor position cd replace line from cursor position (Don't forget to return to edit mode, using ESC!) schmiedecke 09 Info2-1-Intro 60

61 vi input Change to Input Mode i insert before cursor a insert after cursor o insert on new line after cursor Return to Edit Mode Escape Hint: use two bash shells: one vi shell (save without leaving, :w) one shell for compilation and execution schmiedecke 09 Info2-1-Intro 61

62 ... so let the adventure start NOW! Of course, there are some really good introductions to the bash shell and the vi editor in the internet. Go and find one that suits you best!