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Bugs

Bugs. how to prevent them, how to find them and how to terminate them. CS100. Bugs. Programming Errors First bug A moth stuck in a Harvard Mark II mainframe in 1947. Bugs are bad. 1990 – AT&T long distance service failed for 9 hours and was traced to a single faulty line of code

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Bugs

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  1. Bugs how to prevent them, how to find them and how to terminate them CS100

  2. Bugs • Programming Errors • First bug • A moth stuck in a Harvard Mark II mainframe in 1947.

  3. Bugs are bad • 1990 – AT&T long distance service failed for 9 hours and was traced to a single faulty line of code • 1991 – Scud missile killed 28 soldiers because a bug caused the Patriot defense system to be off by 0.34 seconds • 2000 – Y2K

  4. Today’s Lecture • How to prevent bugs ? • Understand the problem • Understand Java • Follow good programming practices • How to find bugs ? • Testing • How to kill bugs

  5. Program Development Design Implement Testing

  6. Program Design : Classes Studentdouble averageGradeint grades[6]double weight[6]calcAverageGrade()getAverageGrade() Coursedouble averageGradedouble maxdouble mindouble sumint numOfStudentStudent students[]calcAverageGrade()getAverageGrade() LetterGradeStudentprintGrade() PassFailStudentprintGrade()

  7. Program Design : Pseudocode Average grade for studentsfor ( i = 0 .. 6) average += grade[i]*weight[i]return average Get inputget number of studentsfor i = 1 .. number of students get name get enrollment status get all six grades if enroll as pass fail then create a PassFailStudent else create a LetterGradeStudent Average Grade for coursefor each student sum += student’s weight averagereturn sum/number of students;

  8. pass/fail ? name 6 grades create a student Program Design : Data Flow final grade get final grade Student obj calc student’s average grade average grade

  9. i = 0 i == # of students ? get nameget statusget grades increment i Program Design : Control Flow

  10. Good Program vs. Bad Program • Easy to Read • Good Comments • Meaningful Names • Properly Indented • Blank Lines • Well-Designed • Covered all cases • Anticipate Changes • Reusable

  11. Good Design • Anticipate Changes • Reusable • Encapsulation • Think “LEGO”

  12. What if .. • create histogram for data between 1 .. 200 ? • tally data for smaller intervals ? 1 – 5 | ** 6 – 10 | ***** 11 – 15 | * : : 196 – 200 | *** • draw a histogram for average grades of all students for this class ?

  13. Bug Prevention • code reuse • fewer lines of code to write, fewer bugs • anticipate changes • fewer lines of code to change,fewer bugs • encapsulation • bugs are confined to one place, easier to detect and fix.

  14. Good Programs • Easy to read • blank lines • indentation • comments • meaningful names • Easy to read, easy to spot bugs !

  15. Finding Bugs • Wrong attitude : “My program works ! I am done.” • Did you test it with all possible inputs ? • negative numbers ? • zero ? • Did you test all possible path of execution ?

  16. Component Testing • Another motivation for encapsulations ! • Test each component separately • Make sure they worked before using them

  17. Debugging Techniques : Think “high-level” • scan from left to right • swap two adjacent elements if they are out of order • repeat until everything is in order

  18. Debugging Techniques :Printout • Print out your code • Spread it on a large table • Walkthrough your code • Draw diagrams • Make notes

  19. Debugging Techniques : Explain it to Someone Else • Old Chinese Proverb : “Onlookers see most of the game; Players see very little”

  20. Debugging Techniques :System.err.println • Let you inspect the intermediate value of variables 53 2147483647 034 2147483647 010 2147483647 0 82 2147483647 0 72 2147483647 0 • Can you guess what is wrong now ?

  21. Debugging Techniques : assert() • a method to make sure that your invariants are true. void assert(boolean condition, String errorMessage) {if (!condition)thrownew Error(errorMessage); }

  22. Debugging Techniques : assert() • TheError exception will cause the stack trace to be printed. java.lang.Error: data 364 is out of rangeat java.lang.Throwable.<init>at java.lang.Error.<init>at Histogram.assertat Histogram.addData at P2Q4.createHistogramat P2Q4.main

  23. Debugging Techniques :Debugger

  24. Summary • Bug Prevention • understand the problem and language • design before sit in front of computer • design for change/reuse • Bug Discovery • test all flow of controls • test small components separately before using it

  25. Summary • Bug Termination • re-think your algorithm from a higher-level • manually trace through your program • explain your program to others • System.err.print • assert() • use a Debugger

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