Problem of the Day  Why are manhole covers round?

Problem of the Day  Why are manhole covers round? No matter how circle rotated, radius is constant so… …only shape that could not fall into the hole

Announcements  If you need more review of Java…  I have lots of good resources – talk to me  Use “Additional Help” link on webpage  Weekly assignments problems due before class  Remove rust from summer and get back into coding  During lectures will also add some new ideas

Style Guide  Good style helps when writing code  Program logic becomes easier to follow  Easier to read and fix errors  Starting & stopping work on project is far easier  Prevents  Prevents many common errors  Style includes two parts:  Formatting  Documentation

Indentation  Traditionally indent code within set of braces ({})  Be consistent with indentation size (I use 2 spaces)

Indentation  Traditionally indent code within set of braces ({})  Be consistent with indentation size (I use 2 spaces)  Nothing is

Indentation  Traditionally indent code within set of braces ({})  Be consistent with indentation size (I use 2 spaces)  Nothing is more

Indentation  Traditionally indent code within set of braces ({})  Be consistent with indentation size (I use 2 spaces)  Nothing is more annoying

Indentation  Traditionally indent code within set of braces ({})  Be consistent with indentation size (I use 2 spaces)  Nothing is more annoying than

Indentation  Traditionally indent code within set of braces ({})  Be consistent with indentation size (I use 2 spaces)  Nothing is more annoying than looking for the next line.

Indentation char _ [3141 ],__3141[3141];_314159[31415],_3141[31415];main(){register char* _3_141,*_3_1415, *_3__1415; register int _314,_31415,__31415,*_31, _3_14159,__3_1415;*_ =__31415=2,_ [0][_ ]=1[__3141]=5;__3_1415=1;do{_3_14159=_314=0,__ ;for( _31415 =0;_31415<(3,14-4)*__31415;_ )_31415[_3141]=_314159[_31415]= - 1;_3141[*_314159=_3_14159]=_314;_3_141=_ __3_1415;_3_1415= __3_1415 +__3141;for(_31415 = __3_1415 ;_31415;_ ,_3_141 ++,_3_1415++){_314 +=_314<<2 ;_314<<=1;_314+= *_3_1415;_31 =_ _314; if(!(*_31+1) )* _31 =_314 / __31415,_314 [_3141]=_314 % __31415 ;* ( _3__1415=_3_141 )+= *_3_1415 = *_31;while(* _3__1415 >= 31415/3141 ) * _3__1415+= - 10,(*--_3__1415 )++;_314=_314 [_3141]; if ( ! _3_14159 && * _3_1415)_3_14159 =1,__3_1415 = 3141-_31415;}if( _314+(__31415 >>1)>=__31415 ) while ( ++ * _3_141==3141/314 )*_3_141--=0 ;}while(_3_14159 ) ; { char * __3_14= "3.1415"; write((3,1) (--*__3_14,__3_14 ),(_3_ ,++_3_14159)) ; } for ( _31415 = 1; _31415< ;_ )write( 31415% 314-( 3,14),_ [ _31415 ] + " ","314" [ 3]+1)-_314; puts((*_ =0,_ )) ;_314= *" ";}

Braces  Always use braces:  Always use braces: for (int i = 0; i < n; i++); sum = sum + i; sumSquare = sumSquare + (i*i);

Braces  Always use braces:  Always use braces: for (int i = 0; i < n; i++); sum = sum + i; sumSquare = sumSquare + (i*i);  Using braces vs. fixing a bug 10 seconds 30 minutes

Increment/Decrement Operators  Only use ++ & -- on own line (or for loops) // What is result of each line? while (++k < n) { a[i++] = (2 * i) + 6; b[j] = (j++ * j) - 1; c[j] = a[j] +++ b[j]; }

Statements  Use variables to break up complex ideas // What is this computing? return ( (year % 4 == 0) && (year % 100 != 0)) || ( (year % 100 == 0) && (year % 400 == 0));

Statements  Use variables to break up complex ideas divisibleBy4 = ((year % 4) == 0); divisible100 = ((year % 100) == 0); divisible400 = ((year % 400) == 0); return (divisible4 && !divisible100) || (divisible100 && divisible400);

Comments  We program using “code”  Not easy to read even simple projects  Review your CSC 111 projects  (even better, review a friend’s CSC 111 project)  Comments break up this code  Provide simple English descriptions  State assumptions and preconditions  Describe the outcome of a section

Comments private static void compileResults(String server, String handle) throws IOException { for (Experiment e : Experiment.values()) { Benchmark[] benches = e.getResults(); File newFile = getOutFile(server, e.name(), handle); newFile.createNewFile(); PrintStream out = new PrintStream(newFile); Double[] baselines = computeBaselineMeans(server, e.getResults(), handle, BASELINE, benches); for (Build b : Build.values()) { File[] files = new File[benches.length]; Scanner[] scanners = new Scanner[benches.length]; for (int i = 0; i < files.length; i++) { files[i] = getInFile(server, benches[i], b, handle); scanners[i] = new Scanner(files[i]); } String[] lines = new String[scanners.length]; int heapSize = 0; while (scanners[0].hasNextLine()) { out.print(b.name() + "\t"); for (int i = 0; i < scanners.length; i++) { lines[i] = scanners[i].nextLine(); out.print(lines[i] + "\t"); }

Ever Have A Bug?  At some point, everyone must fix bug in code  What should you do first after discovering the bug?

Ever Have A Bug?  At some point, everyone must fix bug in code  What should you do first after discovering the bug?

Ever Have A Bug?  At some point, everyone must fix bug in code  What should you do first after discovering the bug?

Ever Have A Bug?  At some point, everyone must fix bug in code  What should you do first after discovering the bug?

Ever Have A Bug?  At some point, everyone must fix bug in code  What should you do first second after discovering it?

Fixing Bugs  Three common ways students fix bugs:  Guess where error is and make random change  Use Eclipse debugger to run program & find source  Get out your pencil & paper and draw a memory trace

Normal Method of Debugging  Also called "debugging-via-drunken-walk" SMALL  Can work for SMALL programs with limited choices  Guessing for millions of lines of code harder  Not at all effective at actually fixing errors  Entire point is to avoid understanding bug's cause  "Fix" may cause other errors later on in program  Cannot prevent future errors using this approach

Better Method of Debugging  Could use Eclipse debugger to find and fix bug  Set breakpoints to stop program where bug likely  Use step filters to walk through the program bit by bit  Spend a few hours learning how to make it work  Can get to root of problem once it is executed  But requires executing entire program to trigger bug  Cannot work backward to try and understand why

Best Method of Debugging  Generating memory trace has many benefits  Could check algorithm – don’t need any code to try  Can make up situations and just trace a method  Look back through trace to see root cause of bug  Trace code by running it like you were computer  DON’T SKIP STEPS  DON’T SKIP STEPS – easy to (wrongly) assume things  As you go along update variables’ values  GIGO effects limit tracing’s effectiveness

Best Method of Debugging  Generating memory trace has many benefits  Could check algorithm – don’t need any code to try  Can make up situations and just trace a method  Look back through trace to see root cause of bug  Trace code by running it like you were computer  DON’T SKIP STEPS  DON’T SKIP STEPS – easy to (wrongly) assume things  As you go along update variables’ values  GIGO effects limit tracing’s effectiveness

Benefits of Memory Traces  Draw what actually occurs in memory  Pictures used to debug rather than bare text  Much easier to understand what a method does  Excellent way to learn new programming topics  Past studies found improves student grades  Debugging time decreased greatly at same time  Greater support for these traces in Eclipse  Drawing of static trace integrated with debugger  Over this term should see tools improve

Starting a Memory Trace  Paper holds 3 areas separated by vertical lines  Program stack drawn on left part of page  Right side of page holds the program heap  Program output shown outside the stack & heap  Objects allocated during run drawn in the heap  Will discuss how this is done next week  Draw frame on top of stack each method call  Fancy name for box labeled with method name  Each parameter & local variable needs space in box

Data Types  8+1 primitive data types  Examples: boolean, byte, char, int, double, String *  Only types that work with Java operators  Operators include: +, -, %, &&, ||, >=, <, !  Primitives used natively by computers  Means using them is very quick  Implies greater support within the language

Primitive Types  Primitive variables are simple to use  Each variable is “xerox” holding a value  Assignment copies value  Update assigned variable only; changes not reflected

Starting a Memory Trace public static void main(String[] args) { String s = “”; for (int i = 0; i < 5; i+=2) { s = s + i; System.out.println(s); } }

Writing a Memory Trace public static int fibber(int i, int j) { return i + j; } public static void main(String[] args) { int f = 0; int s = 1; for (int i = 0; i < 5; i+=4) { int oldF = f; f = s; s = fibber(oldF, f); } }

Your Turn  Get into groups and complete activity

For Next Lecture  Reading AF Chapter 3 & 7.13 for Friday  Continues Java review & looks at references  Introduces enum s – first really new concept of term  Use language template to take notes on enum s!  There is weekly assignment problem on Angel  Due before Friday’s lecture (via )  Get back into the swing of writing Java code