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• Polygon area calculation using Latitude and Longitude generated from Cartesian space and a world fil

  - by Heath

  Given a series of GPS coordinate pairs, I need to calculate the area of a polygon (n-gon). This is relatively small (not larger than 50,000 sqft). The geocodes are created by applying an affine transform with data from a world file. I have tried to use a two step approach by doing converting the geocodes to cartesian coordinates: double xPos = (lon-lonAnchor)*( Math.toRadians( 6378137 ) )*Math.cos( latAnchor ); double yPos = (lat-latAnchor)*( Math.toRadians( 6378137 ) ); then I use a cross product calculation to determine the area. The issue is that the results are a bit off in accuracy (around 1%). Is there anything I can look into to improve this? Thanks.

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• Function pointer arrays in Fortran

  - by Eduardo Dobay

  I can create function pointers in Fortran 90, with code like real, external :: f and then use f as an argument to another function/subroutine. But what if I want an array of function pointers? In C I would just do double (*f[])(int); to create an array of functions returning double and taking an integer argument. I tried the most obvious, real, external, dimension(3) :: f but gfortran doesn't let me mix EXTERNAL and DIMENSION. Is there any way to do what I want? (The context for this is a program for solving a system of differential equations, so I could input the equations without having a million parameters in my subroutines.)

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• Problem with building tree bottom up

  - by Esmond

  Hi, I have problems building a binary tree from the bottom up. THe input of the tree would be internal nodes of the trees with the children of this node being the leaves of the eventual tree. So initially if the tree is empty the root would be the first internal node. Afterwards, The next internal node to be added would be the new root(NR), with the old root(OR) being one of the child of NR. And so on. The problem i have is that whenever i add a NR, the children of the OR seems to be lost when i do a inOrder traversal. This is proven to be the case when i do a getSize() call which returns the same number of nodes before and after addNode(Tree,Node) Any help with resolving this problem is appreciated edited with the inclusion of node class code. both tree and node classes have the addChild methods because i'm not very sure where to put them for it to be appropriated. any comments on this would be appreciated too. The code is as follows: import java.util.*; public class Tree { Node root; int size; public Tree() { root = null; } public Tree(Node root) { this.root = root; } public static void setChild(Node parent, Node child, double weight) throws ItemNotFoundException { if (parent.child1 != null && parent.child2 != null) { throw new ItemNotFoundException("This Node already has 2 children"); } else if (parent.child1 != null) { parent.child2 = child; child.parent = parent; parent.c2Weight = weight; } else { parent.child1 = child; child.parent = parent; parent.c1Weight = weight; } } public static void setChild1(Node parent, Node child) { parent.child1 = child; child.parent = parent; } public static void setChild2(Node parent, Node child) { parent.child2 = child; child.parent = parent; } public static Tree addNode(Tree tree, Node node) throws ItemNotFoundException { Tree tree1; if (tree.root == null) { tree.root = node; } else if (tree.root.getSeq().equals(node.getChild1().getSeq()) || tree.root.getSeq().equals(node.getChild2().getSeq())) { Node oldRoot = tree.root; oldRoot.setParent(node); tree.root = node; } else { //form a disjoint tree and merge the 2 trees tree1 = new Tree(node); tree = mergeTree(tree, tree1); } System.out.print("addNode2 = "); if(tree.root != null ) { Tree.inOrder(tree.root); } System.out.println(); return tree; } public static Tree mergeTree(Tree tree, Tree tree1) { String root = "root"; Node node = new Node(root); tree.root.setParent(node); tree1.root.setParent(node); tree.root = node; return tree; } public static int getSize(Node root) { if (root != null) { return 1 + getSize(root.child1) + getSize(root.child2); } else { return 0; } } public static boolean isEmpty(Tree Tree) { return Tree.root == null; } public static void inOrder(Node root) { if (root != null) { inOrder(root.child1); System.out.print(root.sequence + " "); inOrder(root.child2); } } } public class Node { Node child1; Node child2; Node parent; double c1Weight; double c2Weight; String sequence; boolean isInternal; public Node(String seq) { sequence = seq; child1 = null; c1Weight = 0; child2 = null; c2Weight = 0; parent = null; isInternal = false; } public boolean hasChild() { if (this.child1 == null && this.child2 == null) { this.isInternal = false; return isInternal; } else { this.isInternal = true; return isInternal; } } public String getSeq() throws ItemNotFoundException { if (this.sequence == null) { throw new ItemNotFoundException("No such node"); } else { return this.sequence; } } public void setChild(Node child, double weight) throws ItemNotFoundException { if (this.child1 != null && this.child2 != null) { throw new ItemNotFoundException("This Node already has 2 children"); } else if (this.child1 != null) { this.child2 = child; this.c2Weight = weight; } else { this.child1 = child; this.c1Weight = weight; } } public static void setChild1(Node parent, Node child) { parent.child1 = child; child.parent = parent; } public static void setChild2(Node parent, Node child) { parent.child2 = child; child.parent = parent; } public void setParent(Node parent){ this.parent = parent; } public Node getParent() throws ItemNotFoundException { if (this.parent == null) { throw new ItemNotFoundException("This Node has no parent"); } else { return this.parent; } } public Node getChild1() throws ItemNotFoundException { if (this.child1 == null) { throw new ItemNotFoundException("There is no child1"); } else { return this.child1; } } public Node getChild2() throws ItemNotFoundException { if (this.child2 == null) { throw new ItemNotFoundException("There is no child2"); } else { return this.child2; } } }

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• What is the equivalent of <composite-element> in Castle ActiveRecord?

  - by Daniel T.

  I have a TrackLog that has a collection of TrackPoints: public class TrackLog { public string Name { get; set; } public ISet<TrackPoint> TrackPoints { get; set; } } public class TrackPoint { public DateTime Timestamp { get; set; } public double Latitude { get; set; } public double Longitude { get; set; } } I'd like to map the track points as a collection of components, as this makes the most sense. According to the book NHibernate in Action, on page 187: Collections of components are mapped similarily to other collections of value type instances. The only difference is the use of <composite-element> in place of the familiar <element> tag. How would I do this using Castle ActiveRecord attributes?

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• Any way to get TStringList.CommaText to not escape commas with quotes?

  - by Mason Wheeler

  I'm doing some work with code generation, and one of the things I need to do is create a function call where one of the parameters is a function call, like so: result := Func1(x, y, Func2(a, b, c)); TStringList.CommaText is very useful for generating the parameter lists, but when I traverse the tree to build the outer function call, what I end up with looks like this: result := Func1(x, y, "Func2(a, b, c)"); It's quoting the third argument because it contains commas, and that produced invalid code. But I can't do something simplistic like StringReplace all double quotes with empty strings, because it's quite possible that a function argument could be a string with double quotes inside. Is there any way to make it just not escape the lines that contain commas?

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• MySQL - how long to create an index?

  - by user293594

  Can anyone tell me how adding a key scales in MySQL? I have 500,000,000 rows in a database, trans, with columns i (INT UNSIGNED), j (INT UNSIGNED), nu (DOUBLE), A (DOUBLE). I try to index a column, e.g. ALTER TABLE trans ADD KEY idx_A (A); and I wait. For a table of 14,000,000 rows it took about 2 minutes to execute on my MacBook Pro, but for the whole half a billion, it's taking 15hrs and counting. Am I doing something wrong, or am I just being naive about how indexing a database scales with the number of rows?

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• Help with complex sql query

  - by eugeneK

  To make story short, i'm building self-learning banner management system. Users will be able to insert these banners to their site when banners will be shown based on sales/impressions ratio. I have 4 tables Banners bannerID int bannerImage varchar.... SmartBanners smartBannerID int smartBannerArrayID int bannerID int impressionsCount int visibility tinyint (percents) SmartBannerArrays smartBannerArrayID int userID int Statistics bannerID int saleAmountPerDay decimal... Each night i need to generate new "visibility" for each SmartBanner based on whole SmartBannerArray that same user has. So i need to get sum of impressions and sales for each bannerID in SmartBannerArray. All comes to my mind is to use double cursor, one will loop thought SmartBannerArrays get needed values for sum of impressions and sales and then inner loop which will access each SmartBanner and change it's "visibility" percentage based on (sales/impressions)/(sumOfSales/sumOfImpressions)*100 Hope you get the picture... Is there any other way to design better tables or not to use double cursor to avoid server overload ?

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• jQuery / jqgrid / Editing form events

  - by MiBol

  I'm working with the jqGrid and I want to know if exists an event to read a double click in the Editing Form? Has example: I have a grid with ColumnA and ColumnB. I want read the event when the user perform a double click under ColumnB (In the Editing Form). Thanks! I found the solution of my problem ^^ Here is the code, to this example I use the alert "TEST!!!"... [Thanks to Oleg to wake up my mind :P] In the colModel { name: 'Total_uploads', index: 'Total_uploads', width: '100', editable: true, edittype: 'text', editoptions: { size: 10, maxlength: '20', dataInit: function (el) { $(el).click(function () { alert("TEST!!!"); }); } }, editrules: { required: true }, formoptions: { label: 'Total uploads: ', elmsuffix: '&nbsp;&nbsp; <span style="color : #0C66BE; font-family: Calibri">(*)</span>' } }

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• How do you implement position-sensitive zooming inside a JScrollPane?

  - by tucuxi

  I am trying to implement position-sensitive zooming inside a JScrollPane. The JScrollPane contains a component with a customized 'paint' that will draw itself inside whatever space it is allocated - so zooming is as easy as using a MouseWheelListener that resizes the inner component as required. But I also want zooming into (or out of) a point to keep that point as central as possible within the resulting zoomed-in (or -out) view (this is what I refer to as 'position-sensitive' zooming), similar to how zooming works in google maps. I am sure this has been done many times before - does anybody know the "right" way to do it under Java Swing?. Would it be better to play with Graphic2D's transformations instead of using JScrollPanes? Sample code follows: package test; import java.awt.*; import java.awt.event.*; import java.awt.geom.*; import javax.swing.*; public class FPanel extends javax.swing.JPanel { private Dimension preferredSize = new Dimension(400, 400); private Rectangle2D[] rects = new Rectangle2D[50]; public static void main(String[] args) { JFrame jf = new JFrame("test"); jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); jf.setSize(400, 400); jf.add(new JScrollPane(new FPanel())); jf.setVisible(true); } public FPanel() { // generate rectangles with pseudo-random coords for (int i=0; i<rects.length; i++) { rects[i] = new Rectangle2D.Double( Math.random()*.8, Math.random()*.8, Math.random()*.2, Math.random()*.2); } // mouse listener to detect scrollwheel events addMouseWheelListener(new MouseWheelListener() { public void mouseWheelMoved(MouseWheelEvent e) { updatePreferredSize(e.getWheelRotation(), e.getPoint()); } }); } private void updatePreferredSize(int n, Point p) { double d = (double) n * 1.08; d = (n > 0) ? 1 / d : -d; int w = (int) (getWidth() * d); int h = (int) (getHeight() * d); preferredSize.setSize(w, h); getParent().doLayout(); // Question: how do I keep 'p' centered in the resulting view? } public Dimension getPreferredSize() { return preferredSize; } private Rectangle2D r = new Rectangle2D.Float(); public void paint(Graphics g) { super.paint(g); g.setColor(Color.red); int w = getWidth(); int h = getHeight(); for (Rectangle2D rect : rects) { r.setRect(rect.getX() * w, rect.getY() * h, rect.getWidth() * w, rect.getHeight() * h); ((Graphics2D)g).draw(r); } } }

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• Argument passing regarding ivars

  - by StoneBreaker

  I am passing an iVar into a function. The iVar is a double. The value of the iVar inside the function is correct, but the value of the iVar outside the function is not changed. This must have something to do with the way that I am receiving the iVar into the function. How do I pass a double iVar in so that its value is changed in the object and not only in the function? I do the same thing with pointers and the results are as expected, so I think I am not understanding scalar argument passing in c/objective-c.

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• Declarative JDOQL vs Single-String JDOQL : performance

  - by DrDro

  When querying with JDOQL is there a performance difference between using the declarative version and the Single-String version: Example from the JDOQL doc: //Declarative JDOQL : Query q = pm.newQuery(org.jpox.Person.class, "lastName == \"Jones\" && age < age_limit"); q.declareParameters("double age_limit"); List results = (List)q.execute(20.0); //Single-String JDOQL : Query q = pm.newQuery("SELECT FROM org.jpox.Person WHERE lastName == \"Jones\"" + " && age < :age_limit PARAMETERS double age_limit"); List results = (List)q.execute(20.0); Other then performance, are there any reasons for which one is better to use then the other or is it just about the one with which we feel more comfortable.

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• bin_at in dlmalloc

  - by chunhui

  In glibc malloc.c or dlmalloc It said "repositioning tricks"As in blew, and use this trick in bin_at. bins is a array,the space is allocated when av(struct malloc_state) is allocated.doesn't it? the sizeof(bin[i]) is less then sizeof(struct malloc_chunk*)? Who can describe this trick for me? I can't understand the bin_at macro.why they get the bins address use this method?how it works? Very thanks,and sorry for my poor English. /* To simplify use in double-linked lists, each bin header acts as a malloc_chunk. This avoids special-casing for headers. But to conserve space and improve locality, we allocate only the fd/bk pointers of bins, and then use repositioning tricks to treat these as the fields of a malloc_chunk*. */ typedef struct malloc_chunk* mbinptr; /* addressing -- note that bin_at(0) does not exist */ #define bin_at(m, i) \ (mbinptr) (((char *) &((m)->bins[((i) - 1) * 2])) \ - offsetof (struct malloc_chunk, fd)) The malloc_chunk struct like this: struct malloc_chunk { INTERNAL_SIZE_T prev_size; /* Size of previous chunk (if free). */ INTERNAL_SIZE_T size; /* Size in bytes, including overhead. */ struct malloc_chunk* fd; /* double links -- used only if free. */ struct malloc_chunk* bk; /* Only used for large blocks: pointer to next larger size. */ struct malloc_chunk* fd_nextsize; /* double links -- used only if free. */ struct malloc_chunk* bk_nextsize; }; And the bin type like this: typedef struct malloc_chunk* mbinptr; struct malloc_state { /* Serialize access. */ mutex_t mutex; /* Flags (formerly in max_fast). */ int flags; #if THREAD_STATS /* Statistics for locking. Only used if THREAD_STATS is defined. */ long stat_lock_direct, stat_lock_loop, stat_lock_wait; #endif /* Fastbins */ mfastbinptr fastbinsY[NFASTBINS]; /* Base of the topmost chunk -- not otherwise kept in a bin */ mchunkptr top; /* The remainder from the most recent split of a small request */ mchunkptr last_remainder; /* Normal bins packed as described above */ mchunkptr bins[NBINS * 2 - 2]; /* Bitmap of bins */ unsigned int binmap[BINMAPSIZE]; /* Linked list */ struct malloc_state *next; #ifdef PER_THREAD /* Linked list for free arenas. */ struct malloc_state *next_free; #endif /* Memory allocated from the system in this arena. */ INTERNAL_SIZE_T system_mem; INTERNAL_SIZE_T max_system_mem; };

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• question about VHDL coding tips and tricks

  - by davit-datuashvili

  please explain me how this algorithm works ? http://vhdlguru.blogspot.com/2010/04/8-bit-binary-to-bcd-converter-double.html

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• How to best integrate generated code

  - by Arne

  I am evaluating the use of code generation for my flight simulation project. More specifically there is a requirement to allow "the average engineer" (no offense I am one myself) to define the differential equations that describe the dynamic system in a more natural syntax than C++ provides. The idea is to devise a abstract descriptor language that can be easily understood and edited to generate C++ code from. This descriptor is supplied by the modeling engineer and used by the ones implementing and maintaining the simulation evironment to generate code. I've got something like this in mind: model Aircraft has state x1, x2; state x3; input double : u; input bool : flag1, flag2; algebraic double : x1x2; model Engine : tw1, tw2; model Gear : gear; model ISA : isa; trim routine HorizontalFight; trim routine OnGround, General; constant double : c1, c2; constant int : ci1; begin differential equations x1' = x1 + 2.*x2; x2' = x2 + x1x2; begin algebraic equations x1x2 = x1*x2 + x1'; end model It is important to retain the flexibility of the C language thus the descriptor language is meant to only define certain parts of the definition and implementation of the model class. This way one enigneer provides the model in from of the descriptor language as examplified above and the maintenance enigneer will add all the code to read parameters from files, start/stop/pause the execution of the simulation and how a concrete object gets instatiated. My first though is to either generate two files from the descriptor file: one .h file containing declarations and one .cpp file containing the implementation of certain functions. These then need to be #included at appropriate places [File Aircarft.h] class Aircraft { public: void Aircraft(..); // hand-written constructor void ReadParameters(string &file_name); // hand-written private: /* more hand wirtten boiler-plate code */ /* generate declarations follow */ #include "Aircraft.generated.decl" }; [File Aircraft.cpp] Aircarft::Aircraft(..) { /* hand-written constructer implementation */ } /* more hand-written implementation code */ /* generated implementation code follows */ #include "Aircraft.generated.impl" Any thoughts or suggestions?

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• quantity of measurable units design pattern

  - by Berryl

  Hello I am thinking through a nice pattern to be useful across domains of measurable units (ie, Length, Time) and came up with the following use case and initial classes, and of course, questions! 1) Does a Composite pattern help or complicate? 2) Should the Convert method(s) in the ComposityNode be a separate converter class? All comments appreciated. Cheers, Berryl Example Use Case: var inch = new ConvertableUnit("inch", 1) var foot = new ConvertableUnit("foot", 12) var imperialUnits = new CompositeConvertableUnit("imperial units", .024) imperialUnits.AddChild(inch) imperialUnits.AddChild(foot) var meter = new ConvertableUnit("meter", 1) var millimeter = new ConvertableUnit("millimeter ", .001) var imperialUnits = new CompositeConvertableUnit("metric units", 1) imperialUnits.AddChild(meter) imperialUnits.AddChild(millimeter) var oneInch = new Quantity(1, inch); var oneFoot = new Quantity(1, foot); oneFoot.ToBase() // "12 inches" var oneMeter = new Quantity(1, meter); oneInch.ToBase() // .024 meters Possible Solution ConvertableUnit : Node double Rate string Name Quantity ConvertableUnit Unit double Amount CompositeConvertableUnit : Node ISet<ConvertableUnit> _children ConvertableUnit BaseUnit {get{ return _children.Where(c=>c.Rate == 1).First() } } Quantity ConvertTo(Quantity from, Quantity to) Quantity ToBase(Quantity from);

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• Memory in Eclipse

  - by user247866

  I'm getting the java.lang.OutOfMemoryError exception in Eclipse. I know that Eclipse by default uses heap size of 256M. I'm trying to increase it but nothing happens. For example: eclipse -vmargs -Xmx16g -XX:PermSize=2g -XX:MaxPermSize=2g I also tried different settings, using only the -Xmx option, using different cases of g, G, m, M, different memory sizes, but nothing helps. Does not matter which params I specify, the heap exception is thrown at the same time, so I assume there's something I'm doing wrong that Eclipse ignores the -Xmx parameter. I'm using a 32GB RAM machine and trying to execute something very simple such as: double[][] a = new double[15000][15000]; It only works when I reduce the array size to something around 10000 on 10000. I'm working on Linux and using the top command I can see how much memory the Java process is consuming; it's less than 2%. Thanks!

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• How to generate GIR files from the Vala compiler?

  - by celil

  I am trying to create python bindings to a vala library using pygi with gobject introspection. However, I am having trouble generating the GIR files (that I am planning to compile to typelib files subsequently). According to the documentation valac should support generating GIR files. Compiling the following helloworld.vala public struct Point { public double x; public double y; } public class Person { public int age = 32; public Person(int age) { this.age = age; } } public int main() { var p = Point() { x=0.0, y=0.1 }; stdout.printf("%f %f\n", p.x, p.y); var per = new Person(22); stdout.printf("%d\n", per.age); return 0; } with the command valac helloworld.vala --gir=Hello-1.0.gir doesn't create the Hello-1.0.gir file as one would expect. How can I generate the gir file?

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• Java Regex for matching quoted string with escaped quotes

  - by kayahr

  I know there are already many questions like mine but I found no answer which works in Java. So I write a new question. I have text files with content like this: key1 = "This is a \"test\" text with escapes using '\\' characters"; key2 = 'It must work with \'single\' quotes and "double" quotes'; I need a regular expression which matches the values in the double-quotes (or single-quotes). This regular expression must support the escaped quotes and escaped backslashes. The regular expression must work with Java standard Pattern/Matcher classes.

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• Arraylist is null; I cannot access books in the arraylist

  - by user3701380

  I am a beginner-intermediate java programmer and I am getting a null pointer exception from my arraylist. I am writing a bookstore program for APCS and when i add the book, it is supposed to add to the arraylist in the inventory class. But when i call a method to search for a book (e.g. by title), it shows that there isn't anything in the arraylist. //Here is my inventory class -- it has all methods for adding the book or searching for one The searching methods are in getBookByTitle, getBookByAuthor, and getBookByISBN and the method for adding a book is addBook package webbazonab; //Inventory Class //Bharath Senthil //Ansh Sikka import java.util.ArrayList; public class Inventory{ private ArrayList<Book> allBooks = new ArrayList<Book>(); private String bookTitles; private String bookAuthors; private String bookPrices; private String bookCopies; private String ISBNs; public Inventory() { } //@param double price, int copies, String bookTitle, String Author, String isbnNumber public void addBooks(Book addedBook){ allBooks.add(addedBook); } public boolean isAvailable(){ for(Book myBook : allBooks){ if(myBook.copiesLeft() == 0) return false; } return true; } public String populateTitle(){ for (Book titleBooks : allBooks){ bookTitles = titleBooks.getTitle() + "\n"; return bookTitles; } return bookTitles; } public String populateAuthor(){ for(Book authorBooks : allBooks){ bookAuthors = authorBooks.getAuthor() + "\n"; return bookAuthors; } return bookAuthors; } public String populatePrice(){ for (Book pricedBooks : allBooks){ bookPrices = String.valueOf(pricedBooks.getPrice()) + "\n"; } return "$" + bookPrices; } /** * * @return */ public String populateCopies(){ for (Book amtBooks : allBooks){ bookCopies = String.valueOf(amtBooks.copiesLeft()) + "\n"; return bookCopies; } return bookCopies; } public String populateISBN(){ for (Book isbnNums : allBooks){ ISBNs = isbnNums.getIsbn() + "\n"; return ISBNs; } return ISBNs; } @SuppressWarnings("empty-statement") public Book getBookByTitle(String titleSearch) { for(Book titleBook : allBooks) { if (titleBook.getTitle().equals(titleSearch)) { return titleBook; } } return null; } public Book getBookByISBN(String isbnSearch){ for(Book isbnBookSearches : allBooks){ if(isbnBookSearches.getIsbn().equals(isbnSearch)){ return isbnBookSearches; } } return null; } public Book getBookByAuthor(String authorSearch){ for(Book authorBookSearches : allBooks){ if(authorBookSearches.getAuthor().equals(authorSearch)){ return authorBookSearches; } } return null; } public void sort(){ for(int i = 0; i < allBooks.size(); i++) { for(int k = 0; k < allBooks.size(); k++) { if(((Book) allBooks.get(i)).getIsbn().compareTo(((Book) allBooks.get(k)).getIsbn()) < 1) { Book temp = (Book) allBooks.get(k); allBooks.set(k, allBooks.get(i)); allBooks.set(i, temp); } else if(((Book) allBooks.get(i)).getIsbn().compareTo(((Book) allBooks.get(k)).getIsbn()) > 1) { Book temp = (Book) allBooks.get(i); allBooks.set(i, allBooks.get(k)); allBooks.set(k, temp); } } } } public ArrayList<Book> getBooks(){ return allBooks; } } //The exception occurs when i call the method here (in another class): Inventory lib = new Inventory(); jTextField12.setText(lib.getBookByAuthor(authorSearch).getTitle()); Here is my book class if you need it package webbazonab; //Webbazon AB //Project By: Ansh Sikka and Bharath Senthil public class Book { private double myPrice; private String myTitle; private String bookAuthor; private String isbn; private int myCopies; public Book(double price, int copies, String bookTitle, String Author, String isbnNumber) { myPrice = price; myCopies = copies; myTitle = bookTitle; bookAuthor = Author; isbn = isbnNumber; } public double getPrice() { return myPrice; } public String getIsbn() { return isbn; } public String getTitle() { return myTitle; } public String getAuthor() { return bookAuthor; } public int copiesLeft(){ return myCopies; } public String notFound(){ return "The book you searched for could not be found!"; } public String toString() { return "Title: " + getTitle() + "\nAuthor: " + getAuthor() + "\nNumber of Available Books: " + copiesLeft() + "\nPrice: $" + getPrice(); } } Thanks!

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• Is there any reason to throw a DivideByZeroException?

  - by Atomiton

  Are there any cases when it's a good idea to throw errors that can be avoided? I'm thinking specifically of the DivideByZeroException and NullReferenceException For example: double numerator = 10; double denominator = getDenominatorFromUser(); if( denominator == 0 ){ throw new DivideByZeroException("You can't divide by Zero!"); } Are there any reasons for throwing an error like this? NOTE: I'm not talking about catching these errors, but specifically in knowing if there are ever good reasons for throwing them.

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• Can you use back references in the pattern part of a regular expression?

  - by Camsoft

  I there a way to back reference in the regular expression pattern? Example input string: Here is "quoted text" some quoted text. Say I want to pull out the quoted text, I could create the following expression: "([^"]+)" This regular expression would match quoted text. Say I want it to also support single quotes, I could change the expression to: ["']([^"']+)["'] But what if the input string has a mixture of quotes say Here is 'quoted text" some quoted text. I would not want the regex to match. Currently the regex in the second example would still match. What I would like to be able to do is if the first quote is a double quote then the closing quote must be a double. And if the start quote is single quote then the closing quote must be single. Can I use a back reference to achieve this?

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• Accelerometer Values from Android/iPhone device

  - by mrlinx

  I'm trying to map my movements with a android device into an OpenGL scene. I've recorded accelerometer values for a simples movement: Moving the phone (lies flat on a table) 10cm forward (+x), and then 10cm backward (-x). The problem is that this values when used to calculate velocity and position, makes only the opengl cube go forward. Seems like the negative acceleration recorded was not enough to reduce the speed and invert its movement. What can be the problem? This is my function that updates the velocity and position every time new data comes in: void updatePosition(double T2) { double T = 0.005; Vec3 old_pos = position.clone(), old_vel = velocity.clone(); velocity = old_vel.plus(acceleration.times(T)); position = old_pos.plus(old_vel.times(T).plus(acceleration.times(0.5 * Math.pow(T, 2)))); } This is the X,Y,Z accelerometer values over the entire captured time:

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• MPI and C structs

  - by hanno

  I have to admit, I was quite shocked to see how many lines of code are required to transfer one C struct with MPI. Under what circumstances will it work to simply transmit a struct using the predefined dataype MPI_CHAR? Consider the following example: struct particle { double x; double y; long i; }; struct particle p; MPI_Isend(&p, sizeof(particle), MPI_CHAR, tag, MPI_COMM_WORLD, &sendr); In my case, all processes run on the same architecture. Is padding the only issue?

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• Does DataType DataAnnotation Check the Expression?

  - by Jason

  I am currently using DataAnnotations within my ASP.NET MVC website to ensure data is properly validated. One question I wanted to verify (I think I know the answer, but I can't find verification online) - does the DataType DataAnnotation perform regular expression checks to ensure that you have received a valid e-mail/phone/currency/etc? [Required(ErrorMessage = "Price required")] [DataType(DataType.Currency, ErrorMessage = "Not a valid price")] [Range(0, double.MaxValue, ErrorMessage = "Price must be greater than 0.")] public decimal Price { get; set; } I believe the answer is no (meaning I have to provide my own, custom, RegularExpressionAttribute), but I wanted to double check before I do that for various field types.

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• C++ Using a class template argument as a template argument for another type

  - by toefel

  Hey Everyone, I'm having this problem while writing my own HashTable. It all works, but when I try to templatize the thing, it gave me errors. I recreated the problem as follows: THIS CODE WORKS: typedef double Item; class A { public: A() { v.push_back(pair<string, Item>("hey", 5.0)); } void iterate() { for(Iterator iter = v.begin(); iter != v.end(); ++iter) cout << iter->first << ", " << iter->second << endl; } private: vector<pair<string, double> > v; typedef vector< pair<string, double> >::iterator Iterator; }; THIS CODE DOES NOT: template<typename ValueType> class B { public: B(){} void iterate() { for(Iterator iter = v.begin(); iter != v.end(); ++iter) cout << iter->first << ", " << iter->second << endl; } private: vector<pair<string, ValueType> > v; typedef vector< pair<string, ValueType> >::iterator Iterator; }; the error messages: g++ -O0 -g3 -Wall -c -fmessage-length=0 -omain.o ..\main.cpp ..\main.cpp:50: error: type std::vector<std::pair<std::string, ValueType>, std::allocator<std::pair<std::string, ValueType> > >' is not derived from typeB' ..\main.cpp:50: error: ISO C++ forbids declaration of `iterator' with no type ..\main.cpp:50: error: expected `;' before "Iterator" ..\main.cpp: In member function `void B::iterate()': ..\main.cpp:44: error: `Iterator' was not declared in this scope ..\main.cpp:44: error: expected `;' before "iter" ..\main.cpp:44: error: `iter' was not declared in this scope Does anybody know why this is happening? Thanks!

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