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  • Scan file contents into an array of a structure.

    - by ZaZu
    Hello, I have a structure in my program that contains a particular array. I want to scan a random file with numbers and put the contents into that array. This is my code : ( NOTE : This is a sample from a bigger program, so I need the structure and arrays as declared ) The contents of the file are basically : 5 4 3 2 5 3 4 2 #include<stdio.h> #define first 500 #define sec 500 struct trial{ int f; int r; float what[first][sec]; }; int trialtest(trial *test); main(){ trial test; trialtest(&test); } int trialtest(trial *test){ int z,x,i; FILE *fin; fin=fopen("randomfile.txt","r"); for(i=0;i<5;i++){ fscanf(fin,"%5.2f\t",(*test).what[z][x]); } fclose(fin); return 0; } But the problem is, whenever this I run this code, I get this error : (25) : warning 508 - Data of type 'double' supplied where a pointer is required I tried adding do{ for(i=0;i<5;i++){ q=fscanf(fin,"%5.2f\t",(*test).what[z][x]); } }while(q!=EOF); But that didnt work either, it gives the same error. Does anyone have a solution to this problem ?

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  • Array length is zero in jQuery.

    - by James123
    I wrote like this. After submit click loop is not excuting. But I saw value are there, But array lenght is showing '0'. (Please see picture). Why it is not going into loop? and $('#myVisibleRows').val(idsString); becoming 'empty'. $(document).ready(function() { $('tr[@class^=RegText]').hide().children('td'); var list_Visible_Ids = []; var idsString, idsArray; alert($('#myVisibleRows').val()); idsString = $('#myVisibleRows').val(); idsArray = idsString.split(','); $.each(idsArray, function() { if (this != "") { $('#' + this).siblings(('.RegText').toggle(true)); window['list_Visible_Ids'][this] = 1; } }); $('tr.subCategory1') .css("cursor", "pointer") .attr("title", "Click to expand/collapse") .click(function() { //this = $(this); $(this).siblings('.RegText').toggle(); list_Visible_Ids[$(this).attr('id')] = $(this).css('display') != 'none' ? 1 : null; alert(list_Visible_Ids[$(this).attr('id')]) }); $('#form1').submit(function() { idsString = ''; $.each(list_Visible_Ids, function(key, val) { alert(val); if (val) { idsString += (idsString != '' ? ',' : '') + key; } }); $('#myVisibleRows').val(idsString); form.submit(); }); });

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  • php parsing csv with ftell

    - by Robert82
    I have a 500mb csv file with over 500,000 lines, each with 80 fields. I am using fget to process the file line by line. $col1 = array(); while (($row = fgetcsv($handle, 1000, ",")) !== FALSE) { $col1[] = $row[0]; } Because of an execution time limit on the PHP file by my hosting provider (120 seconds), I can't process the whole file in one run. I tried using ftell() and fseek() to remember the last position for restart. The trouble is, sometimes the ftell() position is in the middle of a row, and resuming means missing the first half of the row. Is there an elegant way to know the last line successfully processed, and resume from the one after it? I realize I can do a simple counter, and then loop through to that point again, but that would produce diminishing returns on the rows I can process towards the end of the file. Is there something like ftell() and fseek() that would work in my case? Or a way to limit ftell() to return the pointer for the end of the previous line?

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  • Game design flaw, need help investigating

    - by Snake
    I am not sure if I will be able to get help here but I would give it a shot. The problem is I dont know where the problem is. I have a cards game, in which when you "human" play by dragging a card, then at the end of card being dragged, a handler using postExecute is called with delay of 0.5 sec to start the next player in turn (which is a bot) The bot chooses the color and plays it and at the end of the animation (the card moving to the middle) a handler is started for the next bot and so on. Once the play reaches again to the human player, it waits for his touchs to drag the crad and start the cycle again. The problem that in production, sometimes I am getting errors. The error is resulting in somehow messing up the sequence which ends up with players having more cards than others. After investigation, I found that the transition from human to bot is the problem. Somehow, the transition is happening twice (meaning handler calling post execute twice and the bot is playing twice and everything is messed up). Its been mutliple months and I can't reproduce it (to fix it) and I cna't figure out why this is happeneing? ANY IDEA how I can go after it? How can I get more info about or how can I solve something like that? any pointer would help me

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  • Can I make a derived class inherit a derived member from its base class in Java?

    - by Eric
    I have code that looks like this: public class A { public void doStuff() { System.out.print("Stuff successfully done"); } } public class B extends A { public void doStuff() { System.out.print("Stuff successfully done, but in a different way"); } public void doMoreStuff() { System.out.print("More advanced stuff successully done"); } } public class AWrapper { public A member; public AWrapper(A member) { this.member = member; } public void doStuffWithMember() { a.doStuff(); } } public class BWrapper extends AWrapper { public B member; public BWrapper(B member) { super(member); //Pointer to member stored in two places: this.member = member; //Not great if one changes, but the other does not } public void doStuffWithMember() { member.doMoreStuff(); } } However, there is a problem with this code. I'm storing a reference to the member in two places, but if one changes and the other does not, there could be trouble. I know that in Java, an inherited method can narrow down its return type (and perhaps arguments, but I'm not certain) to a derived class. Is the same true of fields?

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  • Refetching a previously visited page

    - by user613665
    All, I am having a field day with page refetching. Any help or pointer will be greatly appreciated!! The behavior is a bit specific to mobile browser. Problem: I have two pages and created a shortcut link to pg#1 in the home page. Through a form submit button, user is taken from pg#1 to pg#2. All that is working fine. Now once I am on pg#2. I will leave the browser and click the shortcut later. The browser will stay on pg#2 and won't go to pg#1 even though the path in URLS is different between the two views. It is almost like Django decides that since I have already visited view#1, it doesn't need to fetch it again. This problem or behavior doesn't happen if I move the same code that handle the two views and the templates to a bare bone test project. Setup: I am using django-registration, context session. I am not using any HTML caching tag. I already have DEBUG turned on in my settings.py. Are there other ways that I can tell what the server is doing. Thanks in advance. pdxMobile Update: Here is the code snippets. def sendmsg(request): if request.method =='POST': messages.add_message(request, messages.INFO, "Hello world") return redirect ('rcvmsg') return render_to_response('sendMsg.html',RequestContext(request)) def rcvmsg(request): '''view that receives the msg.''' printMsg ='Didnt get a message' if messages: thisMsg = messages.get_messages(request) for rcvMsg in thisMsg: printMsg = rcvMsg return render_to_response('rcvMsg.html',{'print_msg':printMsg},RequestContext(request)) URL: url(r'^rcvMsg/','mydomain.mainApp.views.rcvmsg',name='rcvmsg'), (r'^sendMsg/code','mydomain.mainApp.views.sendmsg'),

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  • Using unions to simplify casts

    - by Steven Lu
    I realize that what I am trying to do isn't safe. But I am just doing some testing and image processing so my focus here is on speed. Right now this code gives me the corresponding bytes for a 32-bit pixel value type. struct Pixel { unsigned char b,g,r,a; }; I wanted to check if I have a pixel that is under a certain value (e.g. r, g, b <= 0x10). I figured I wanted to just conditional-test the bit-and of the bits of the pixel with 0x00E0E0E0 (I could have wrong endianness here) to get the dark pixels. Rather than using this ugly mess (*((uint32_t*)&pixel)) to get the 32-bit unsigned int value, i figured there should be a way for me to set it up so I can just use pixel.i, while keeping the ability to reference the green byte using pixel.g. Can I do this? This won't work: struct Pixel { unsigned char b,g,r,a; }; union Pixel_u { Pixel p; uint32_t bits; }; I would need to edit my existing code to say pixel.p.g to get the green color byte. Same happens if I do this: union Pixel { unsigned char c[4]; uint32_t bits; }; This would work too but I still need to change everything to index into c, which is a bit ugly but I can make it work with a macro if i really needed to.

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  • Convert one delphi code line to c++

    - by user1332636
    How can I write that line in c++? This is the code in delphi type TSettings = record sFileName: String[50]; siInstallFolder: Byte; bRunFile: Boolean; ... end; .. var i: dword; sZdData: PChar; Settings :Tsettings; begin .... ZeroMemory(@Settings, sizeof(Tsettings)); settings := Tsettings(Pointer(@sZdData[i])^); // this code to c++ c++ code (hope the rest is OK) struct TSettings{ char sFileName[50]; byte siInstallFolder; bool bRunFile; ... } Settings; ... DWORD i; LPBYTE sZdData; ZeroMemory(&Settings, sizeof(TSettings)); Settings = ????? // im failing here i dunno what to do // i need same as in delphi code above

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  • Internal typedef and circular dependency

    - by bcr
    I have two classes whose functions take typedefed pointers to eachother as return values and parameters. I.e.: class Segment; class Location : public Fwk::NamedInterface { public: // ===== Class Typedefs ===== typedef Fwk::Ptr<Location const> PtrConst; typedef Fwk::Ptr<Location> Ptr; // ===== Class Typedefs End ===== void segmentIs(Segment::Ptr seg); /* ... */ } and class Location; class Segment : public Fwk::NamedInterface { public: // ===== Class Typedefs ===== typedef Fwk::Ptr<Segment const> PtrConst; typedef Fwk::Ptr<Segment> Ptr; // ===== Class Typedefs End ===== void locationIs(Location::Ptr seg); /* ... */ } This understandably generated linker errors...which the forward declarations of the respective classes don't fix. How can I forward declare the Ptr and PtrConst typedefs while keeping these typedefs internal to the class (i.e. I would like to write Location::Ptr to refer to the location pointer type)? Thanks folks!

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  • Search box with images strange issue in ie

    - by mebots
    Have the following searchbox image: With the following code and css: #searchcontainer { margin: 40px 0 0 0; } .search { float: right; margin: 0; padding: 0; height: 21px; width: 310px; } .search input, .search button { margin: 0; padding: 0; font-size: 0.9em; color: #A7A9AC; border: 0; outline: none; } .search input.box { margin: 0; padding: 2px 5px 0 5px; width:230px; height: 19px; background: #FFF url(images/search.gif) no-repeat top left; } .search input.btn { margin: 0 0 0 -5px; padding: 0; width:70px; height: 21px; cursor:pointer; cursor: hand; /* cross browser */ text-indent: -9999px; background: #FFF url(images/search.gif) no-repeat top right; } <div id="searchcontainer"> <form id="searchform" class="search" method="post" action="#"> <input name="box" type="text" value="zoek..." class="box" /> <input name="btn" type="button" class="btn" /> </form> </div> In firefox it looks ok, but in ie and chrome the button "Zoek" goes down a bit, see image In my opinion the css is ok. but can't find where it goes wrong.

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  • How do I know when to increase or decrease angle to get a specific angle?

    - by Thomas
    Hi. I am programming a game and I have come to a very hard spot. Basically I have a circle and I have 2 angles on this circle. Angle 1 (A) is a point I want angle 2 (B) to go to. During my game every frame I need to check weither or not to increase or decrease my angle value by a certain amount (speed) to eventually reach the first angle. My question is how do I do this? I tried doing this but I don't seem to be doing it right. bool increase = false; float B = [self radiansToDegrees:tankAngle]; float A = [self radiansToDegrees:tankDestinationAngle]; float newAngle = B; if(B < A) { float C = B - (360 - A); float D = A - B; if(C < D) increase = false; else increase = true; } else if(B A) { float C = B - A; float D = A - (360 - B); if(C < D) increase = false; else increase = true; } if(increase) { newAngle += 1.0; } else { newAngle -= 1.0; } if(newAngle 360.0) { newAngle = 0 + (newAngle - 360.0); } else if(newAngle < 0.0) { newAngle = 360 + newAngle; } if(newAngle == 0) newAngle = 360; newAngle = [self degreesToRadians:newAngle]; [self setTanksProperPositionRotation:newAngle]; The basic effect I am trying to achieve is when the user makes a new point, which would be angle 1, angle 2 would move towards angle 1 choosing the fastest direction. I think I have spent around 4 hours trying to figure this out.

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  • I am trying to access the individual bytes in a floating point number and I am getting unexpected results

    - by oweinh
    So I have this so far: #include <iostream> #include <string> #include <typeinfo> using namespace std; int main () { float f = 3.45; // just an example fp# char* ptr = (char*)&f; // a character pointer to the first byte of the fp#? cout << int(ptr[0]) << endl; // these lines are just to see if I get what I cout << int(ptr[1]) << endl; // am looking for... I want ints that I can cout << int(ptr[2]) << endl; // otherwise manipulate. cout << int(ptr[3]) << endl; } the result is: -51 -52 92 64 so obviously -51 and -52 are not in the byte range that I would expect for a char... I have taken information from similar questions to arrive at this code and from all discussions, a conversion from char to int is straightforward. So why negative values? I am trying to look at a four-byte number, therefore I would expect 4 integers, each in the range 0-255. I am using Codeblocks 13.12 with gcc 4.8.1 with option -std=C++11 on a Windows 8.1 device.

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  • Processor, OS : 32bit, 64 bit

    - by Sandbox
    I am new to programming and come from a non-CS background (no formal degree). I mostly program winforms using C#. I am confused about 32 bit and 64 bit.... I mean, have heard about 32 bit OS, 32 bit processor and based on which a program can have maximum memory. How it affects the speed of a program. There are lot more questions which keep coming to mind. I tried to go through some Computer Organization and Architecture books. But, either I am too dumb to understand what is written in there or the writers assume that the reader has some CS background. Can someone explain me these things in a plain simple English or point me to something which does that. EDIT: I have read things like In 32-bit mode, they can access up to 4GB memory; in 64-bit mode, they can access much much more....I want to know WHY to all such things. BOUNTY: Answers below are really good....esp one by Martin. But, I am looking at a thorough explanation, but in plain simple English.

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  • How can a B-tree node be represented?

    - by chronodekar
    We're learning B-trees in class and have been asked to implement them in code. The teacher has left choice of programming language to us and I want to try and do it in C#. My problem is that the following structure is illegal in C#, unsafe struct BtreeNode { int key_num; // The number of keys in a node int[] key; // Array of keys bool leaf; // Is it a leaf node or not? BtreeNode*[] c; // Pointers to next nodes } Specifically, one is not allowed to create a pointer to point to the structure itself. Is there some work-around or alternate approach I could use? I'm fairly certain that there MUST be a way to do this within the managed code, but I can't figure it out. EDIT: Eric's answer pointed me in the right direction. Here's what I ended up using, class BtreeNode { public List<BtreeNode> children; // The child nodes public static int MinDeg; // The Minimum Degree of the tree public bool IsLeaf { get; set; } // Is the current node a leaf or not? public List<int> key; // The list of keys ... }

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  • Can you target Google Chrome? (Yes, you can)

    - by Kyle Sevenoaks
    Hi, I need to position this update button on www.euroworker.no/order (you'll have to add an item to the cart, use the Kjøp button to add and Handlevogn to view the cart). Works in FF and IE. (Although there is another alignment problems with IE) but not in Chrome or Safari. I had it working before, but the only thing I can think of to do is target safari and Chrome. Is this possible? Here's the CSS and HTML(Smarty) for you. HTML(Smarty): {capture assign="cartUpdate"} <div id="cartUpdate"><!--<input type="submit" class="submit" value="{tn _update}" />--> <button type="submit" class="submit" id="oppdatersubmit" name="saveFields" title="Oppdater" value="">&nbsp;</button> </div> {/capture} {assign var="cartUpdate" value=$cartUpdate|@str_split:10000} {php}$GLOBALS['cartUpdate'] = $this->get_template_vars('cartUpdate'); $this->assign_by_ref('GLOBALS', $GLOBALS);{/php} {form action="controller=order action=update" method="POST" enctype="multipart/form-data" handle=$form id="cartItems"} CONTENT {/form} And the CSS: #oppdatersubmit { background-image:url(../../upload/oppdater.png); background-repeat:no-repeat; background-position:left; background-color:none; border:none; overflow:hidden; outline:none; white-space: nowrap; width:77px; height:25px; cursor:pointer; position:absolute; } #cartUpdate { position:absolute; width:160px; height:30px; left:580px; bottom:130px; } Need to change these for Chrome and Safari. Thanks.

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  • [C++] Parent-Child scheme

    - by rubenvb
    I'm writing a class that holds a pointer to a parent object of the same type (think Qt's QObject system). Each object has one parent, and the parent should not be destroyed when a child is destroyed (obviously). class MyClass { public: MyClass(const MyClass* ptr_parent): parent(parent){}; ~MyClass(){ delete[] a_children; }; private: const MyClass* ptr_parent; // go to MyClass above MyClass* a_children; // go to MyClass below size_t sz_numChildren; // for iterating over a_children } (Excuse my inline coding, it's only for brevity) Will destroying the "Master MyClass" take care of all children? No child should be able to kill it's parent, because I would then have pointers in my main program to destroyed objects, correct? Why might you ask? I need a way to "iterate" through all subdirectories and find all files on a platform independent level. The creation of this tree will be handled by native API's, the rest won't. Is this a good idea to start with? Thanks!

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  • Is this an error in "More Effective C++" in Item28?

    - by particle128
    I encountered a question when I was reading the item28 in More Effective C++ .In this item, the author shows to us that we can use member template in SmartPtr such that the SmartPtr<Cassette> can be converted to SmartPtr<MusicProduct>. The following code is not the same as in the book,but has the same effect. #include <iostream> class Base{}; class Derived:public Base{}; template<typename T> class smart{ public: smart(T* ptr):ptr(ptr){} template<typename U> operator smart<U>() { return smart<U>(ptr); } ~smart(){delete ptr;} private: T* ptr; }; void test(const smart<Base>& ) {} int main() { smart<Derived> sd(new Derived); test(sd); return 0; } It indeed can be compiled without compilation error. But when I ran the executable file, I got a core dump. I think that's because the member function of the conversion operator makes a temporary smart, which has a pointer to the same ptr in sd (its type is smart<Derived>). So the delete directive operates twice. What's more, after calling test, we can never use sd any more, since ptr in sd has already been delete. Now my questions are : Is my thought right? Or my code is not the same as the original code in the book? If my thought is right, is there any method to do this? Thanks very much for your help.

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  • In sync query calls, one query causing other query to run slower. Why?

    - by Irchi
    Sorry for the long question, but I think this is an interesting situation and I couldn't find any explanations for it: I was involved in optimization of an application that performed a large number of sequential SELECT and INSERT statements on a single dedicated SQL Server database. The process needs to INSERT a large number of records into a table, but for each of them there should be some value mappings, which performed using SELECT statements on another table in the same database. For a specific execution, it took 90 minutes to run. I used a profiler (JProfiler - the application is Java-based) to determine how much time does each part of the application take. It yields that 60% of the time was spent on INSERT method calls, and almost 20% on SELECT calls (the rest distributed in other parts). After some trials, I came to this situation: I commented out the INSERT query that took 60% of the time. I was expecting for the total run time to be around 35 minutes, as I have removed 60% of the 90 minutes. But the whole process took the same 90 minutes (doing only SELECTs and nothing else), but each SELECT took longer this time! Everything was running sync, there were no async calls. And there was only one single thread of execution. SELECT and INSERT queries are very simple, and don't have anything special, and they are on different tables, but on the same DB. I tested with both the DB on the application machine, and on a remote network machine. I can't think of any explanation for this, as the Profiler (Application profiler, not SQL Profiler) reported the changes in the method call times, and by removing INSERT statements SELECT statements took longer to run. Can anyone give me some kind of explanation of what could have happened? (there can't be cache / query optimization stuff, because the queries were run in sync, and in a single thread, and it was far from affecting the cache this much) I should note that the bottleneck of the speed was in SQL server, using most of the CPU time.

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  • how to pass vector of string to foo(char const *const *const)?

    - by user347208
    Hi, This is my first post so please be nice. I searched in this forum and googled but I still can not find the answer. This problem has bothered me for more than a day, so please give me some help. Thank you. I need to pass a vector of string to a library function foo(char const *const *const). I can not pass the &Vec[0] since it's a pointer to a string. Therefore, I have an array and pass the c_str() to that array. The following is my code (aNames is the vector of string): const char* aR[aNames.size()]; std::transform(aNames.begin(), aNames.end(), aR, boost::bind(&std::string::c_str, _1)); foo(aR); However, it seems it causes some undefined behavior: If I run the above code, then the function foo throw some warnings about illegal characters ('èI' blablabla) in aR. If I print aR before function foo like this: std::copy(aR, aR+rowNames.size(), std::ostream_iterator<const char*>(std::cout, "\n")); foo(aR); Then, everything is fine. My questions are: Does the conversion causes undefined behavior? If so, why? What is the correct way to pass vector of string to foo(char const *const *const)? Thank you very much for your help!

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  • Best Approach for Checking and Inserting Records

    - by nevets1219
    In one of our existing C programs which purpose is: Open connection to DB for record in all_record: if record contain certain data: if record is NOT in table A: // see #1 insert record information into table A and B // see #2 Close connection to DB select field from table where field=XXX 2 inserts This is typically done every X months to sync everything up or so I'm told. I've also been told that this process takes roughly a couple of days. There is (currently) at most 2.5million records (though not necessarily all 2.5m will be inserted). One of the table contains 10 fields and the other 5 fields. There isn't much to be done about iterating through the records since that part can't be changed at the moment. What I would like to do is speed up the part where I query MySQL. I'm not sure if I have left out any important details -- please let me know! I'm also no SQL expert so feel free to point out the obvious. I thought about: Putting all the inserts into a transaction (at the moment I'm not sure how important it is for the transaction to be all-or-none or if this affects performance) Using Insert X Where Not Exists Y LOAD DATA INFILE (but that would require I create a (possibly) large temp file) I read that (hopefully someone can confirm) I should drop indexes so they aren't re-calculated. mysql Ver 14.7 Distrib 4.1.22, for sun-solaris2.10 (sparc) using readline 4.3

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  • Class is not applying to submit button

    - by Mayur
    Hi All, I m Trying to apply a class to following submit button Code: <input type="submit" value="Submit" name="commit"> css : .confirm-button-submit { width : 79px; font : bold 12px sans-serif;; color : #000; background : url("../images/confirm-btn.png") 0 -33px no-repeat; text-decoration : none; margin-top :0px; text-align:center; border:0px; cursor : pointer; height:170px; } .confirm-button a { display : block; width : 79px; padding : 8px 0px 12px 0px; font : bold 12px sans-serif;; color : #000; background : url("../images/confirm-btn.png") 0 -33px no-repeat; text-decoration : none; margin-top :0px; text-align:center; } .confirm-button a:hover { display : block; width : 79px; padding : 8px 0px 12px 0px; font : bold 12px sans-serif;; color : #fff; background : url("../images/confirm-btn.png") 0 0 no-repeat; text-decoration : none; margin-top :0px; text-align:center; } But its not working proper what to do Thanks

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  • How to delete Drawn Line in java?

    - by Jeyjey
    Hello Folks, well this is my code: import javax.swing.; import javax.; import java.awt.; import java.awt.Color; import java.awt.Graphics.; import java.awt.event.*; import javax.swing.UIManager; public class SimpleGUI extends JFrame{ public SimpleGUI(){ this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE) ; } public void go(){ Drawpanel = new Mypanel(); JFrame frame = new JFrame("Chasing Line"); frame.getContentPane().add(BorderLayout.CENTER, Drawpanel); frame.setSize(300,300); frame.setVisible(true); Drawpanel.addMouseMotionListener(new java.awt.event.MouseMotionAdapter() { public void mouseMoved(java.awt.event.MouseEvent evt) { DrawpanelMouseMoved(evt); } }); } public void DrawpanelMouseMoved(java.awt.event.MouseEvent evt) { xpos=evt.getX(); ypos=evt.getY(); System.out.println("Coordinates : X :"+ xpos+"Y: "+ypos); Drawpanel.paintImage(xpos,ypos); } class Mypanel extends JPanel{ public void paintImage(int xpost,int ypost){ Graphics d = getGraphics(); d.setColor(Color.black); d.drawLine(xpost, 0, xpost, this.getHeight()); d.setColor(Color.red); d.drawLine(0, ypost, this.getWidth(),ypost); this.validate(); } } // end the inner class public static void main(String[] args){ try { UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel"); } catch(Exception e) { System.err.println("Look and feel not set"); } SimpleGUI win = new SimpleGUI(); win.go(); } Mypanel Drawpanel; private int xpos=0; private int ypos=0; } // close SimpleGUI class The problem is how can i delete the old lines?, i mea,make only the current x and y lines appear on the screen, make the intersection between both lines "follow" the mouse pointer. thanks for any reply.

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  • optimization math computation (multiplication and summing)

    - by wiso
    Suppose you want to compute the sum of the square of the differences of items: $\sum_{i=1}^{N-1} (x_i - x_{i+1})^2$, the simplest code (the input is std::vector<double> xs, the ouput sum2) is: double sum2 = 0.; double prev = xs[0]; for (vector::const_iterator i = xs.begin() + 1; i != xs.end(); ++i) { sum2 += (prev - (*i)) * (prev - (*i)); // only 1 - with compiler optimization prev = (*i); } I hope that the compiler do the optimization in the comment above. If N is the length of xs you have N-1 multiplications and 2N-3 sums (sums means + or -). Now suppose you know this variable: sum = $x_1^2 + x_N^2 + 2 sum_{i=2}^{N-1} x_i^2$ Expanding the binomial square: $sum_i^{N-1} (x_i-x_{i+1})^2 = sum - 2\sum_{i=1}^{N-1} x_i x_{i+1}$ so the code becomes: double sum2 = 0.; double prev = xs[0]; for (vector::const_iterator i = xs.begin() + 1; i != xs.end(); ++i) { sum2 += (*i) * prev; prev = (*i); } sum2 = -sum2 * 2. + sum; Here I have N multiplications and N-1 additions. In my case N is about 100. Well, compiling with g++ -O2 I got no speed up (I try calling the inlined function 2M times), why?

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  • Singleton pattern in C++

    - by skydoor
    I have a question about the singleton pattern. I saw two cases concerning the static member in the singleton class. First it is an object, like this class CMySingleton { public: static CMySingleton& Instance() { static CMySingleton singleton; return singleton; } // Other non-static member functions private: CMySingleton() {} // Private constructor ~CMySingleton() {} CMySingleton(const CMySingleton&); // Prevent copy-construction CMySingleton& operator=(const CMySingleton&); // Prevent assignment }; One is an pointer, like this class GlobalClass { int m_value; static GlobalClass *s_instance; GlobalClass(int v = 0) { m_value = v; } public: int get_value() { return m_value; } void set_value(int v) { m_value = v; } static GlobalClass *instance() { if (!s_instance) s_instance = new GlobalClass; return s_instance; } }; What's the difference between the two cases? Which one is correct?

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  • Sending a message to nil?

    - by Ryan Delucchi
    As a Java developer who is reading Apple's Objective-C 2.0 documentation: I wonder as to what sending a message to nil means - let alone how it is actually useful. Taking an excerpt from the documentation: There are several patterns in Cocoa that take advantage of this fact. The value returned from a message to nil may also be valid: If the method returns an object, any pointer type, any integer scalar of size less than or equal to sizeof(void*), a float, a double, a long double, or a long long, then a message sent to nil returns 0. If the method returns a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in registers, then a message sent to nil returns 0.0 for every field in the data structure. Other struct data types will not be filled with zeros. If the method returns anything other than the aforementioned value types the return value of a message sent to nil is undefined. Has Java rendered my brain incapable of grokking the explanation above? Or is there something that I am missing that would make this as clear as glass? Note: Yes, I do get the idea of messages/receivers in Objective-C, I am simply confused about a receiver that happens to be nil.

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