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• Is a web-server (e.g servlets) a good solution for an IM server?

  - by John

  I'm looking at a new app, broadly speaking an IM application with a strong client-server model - all communications go through a server so they can be logged centrally. The server will be Java in some form, clients could at this point be anything from a .NET Desktop app to Flex/Silverlight, to a simple web-interface using JS/AJAX. I had anticipated doing the server using standard J2EE so I get a thread-safe, multi-user server for 'free'... to make things simple let's say using Servlets (but in practice SpringMVC would be likely). This all seemed very neat but I'm concerned if the stateless nature of Servlets is the best approach. If my memory of servlets (been a year or two) is right, each time a client sent a HTTP request, typically a new message entered by the user, the servlet could not assume it had the user/chat in memory and might have to get it from the DB... regardless it has to look it up. Then it either has to use some PUSH system to inform other members of the chat, or cache that there are new messages, for other clients who poll the server using AJAX or similar - and when they poll it again has to lookup the chat, including new messages, and send the new data. I'm wondering if a better system would be the server is running core Java, and implements a socket-based communication with clients. This allows much more immediate data transfer and is more flexible if say the IM client included some game you could play. But then you're writing a custom server and sockets don't sound very friendly to a browser-based client on current browsers. Am I missing some big piece of the puzzle here, it kind of feels like I am? Perhaps a better way to ask the question would simply be "if the client was browser-based using HTML/JS and had to run on IE7+,FF2+ (i.e no HTML5), how would you implement the server?" edit: if you are going to suggest using XMPP, I have been trying to get my head around this in another question, so please consider if that's a more appropriate place to discuss this specifically.

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• Canvas - @font-face doesn't work on IE9+

  - by iMoses

  I've created a widget which allows the user to locate free-text over an image using a textarea. When saving the image a background canvas application reads the text and calculates its position, then it draws the text to the canvas over the image and saves a new image file. The font I use for this widget is league-gothic which I am importing using the @font-face method. This seems to work great and without any issues on all browsers except (of-course) for IE9 and IE10. When using internet explorer you can clearly see that the font was indeed loaded since the textarea uses the same font, but when trying to draw the text onto the canvas the font-family reverts to one of its fallback, in this case Arial. I've searched quite a bit and found nothing. Unlike most font issues I found that concern the canvas element, I am completely sure that the font has indeed loaded as I am viewing it before saving the result. Anything at all will help me at the moment. If you have any insight, experience with similar bugs or whatever, please share :) Thanks in advance. P.S. I can't expose a code example at the moment, but if it becomes a problem I'll do my best to provide one.

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• Is there a faster way to access a property member of a class using reflection?

  - by Ross Goddard

  I am currently using the following code to access the property of an object using reflection: Dim propInfo As Reflection.PropertyInfo = myType.GetProperty(propName) Dim objValue As Object = propInfo.GetValue(myObject, Nothing) I am having some issues with the speed since this type of code is being called many times and is causing some slowdown. I have been looking into using Refelction.Emit or dynamic methods, but I am not sure exactly how to make use of them. Background Information: I am creating a list of a subset of the properties of the object, associating then with some meta information (such as if they can be loaded from the database or xml, if they are editable, can the user see them). This is for later consumption so we can write code such as : foreach prop as BaseWrapper in graphNode.NodeProperties prop.LoadFromDataRow(dr) next The application makes heavy use of having access to this list. The problem is that on the initial load of a project, a larger number of objects are being created that make use of this, so for each object created it is looping through this code a number of times. I initially tried adding each property to the list manually, but this ran into problems with not everything being initialized at the correct time and some other issues. If there is no other good way, then I may have to rethink some of the design and see what else can be done to improve the performance.

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• How do I read hex numbers into an unsigned int in C [Solved]

  - by sil3nt

  I'm wanting to read hex numbers from a text file into an unsigned integer so that I can execute Machine instructions. It's just a simulation type thing that looks inside the text file and according to the values and its corresponding instruction outputs the new values in the registers. For example, the instructions would be: 1RXY - Save register R with value in memory address XY 2RXY - Save register R with value XY BRXY - Jump to register R if xy is this and that etc.. ARXY - AND register R with value at memory address XY The text file contains something like this each in a new line. (in hexidecimal) 120F B007 290B My problem is copying each individual instruction into an unsigned integer...how do I do this? #include <stdio.h> int main(){ FILE *f; unsigned int num[80]; f=fopen("values.txt","r"); if (f==NULL){ printf("file doesnt exist?!"); } int i=0; while (fscanf(f,"%x",num[i]) != EOF){ fscanf(f,"%x",num[i]); i++; } fclose(f); printf("%x",num[0]); }

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• Copy object using pointer (templates)

  - by Azodious

  How the push_back of stl::vector is implemented so it can make copy of any datatype .. may be pointer, double pointer and so on ... I'm implementing a template class having a function push_back almost similar to vector. Within this method a copy of argument should be inserted in internal memory allocated memory. but the argument is a pointer. (an object pointer). Can you pls tell how to create copy from pointer. so that if i delete the pointer in caller still the copy exists in my template class? Code base is as follows: template<typename T> class Vector { public: void push_back(const T& val_in) { T* a = *(new T(val_in)); m_pData[SIZE++] = a; } } Caller: Vector<MyClass*> v(3); MyClass* a = new MyClass(); a->a = 0; a->b = .5; v.push_back(a); delete a; Thanks.

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• Bind an Incode DataTemplate in WPF

  - by Mike Bynum

  I have a WPF Application which is using MVVM. I know that there ways of doing this in XAML but I am working on a plugin architecture and came up with a solution where a plugin exposes it's viewmodel to my plugin host's viewmodel and it's datatemplate. I want to leave the lifetime management of the plugin view up to WPF. I have tried having the plugins expose a UserControl but ran into issues when WPF decided to dispose of my UserControl so I would not reattach it without weird hacky work arounds. I am having issues getting some sort of binding working to where i can bind a control to the data and it's template to my data template. I have a ViewModel which looks something like: public class MyViewModel { public DataTemplate SelectedTemplate{ get; set;} public object SelectedViewModel {get; set;} } The selected template and viewmodel are determined somewhere else in the code but are irrelevant to my question. My question is how i can bind to a DataTemplate so that I know how to display the data shown in the SelectedViewModel. The DataTemplate is a DataTemplate created incode which respresents: <DataTemplate DataType="{x:Type vm:MyViewModel}"> <v:MyUserControl /> </DataTemplate> I have tried: <UserControl Template="{Binding Path=SelectedTemplate}" Content="{Binding Path=SelectedViewModel"} /> But UserControl expects a control template and not a data template.

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• How to iteratively generate k elements subsets from a set of size n in java?

  - by Bea Metitiri

  Hi, I'm working on a puzzle that involves analyzing all size k subsets and figuring out which one is optimal. I wrote a solution that works when the number of subsets is small, but it runs out of memory for larger problems. Now I'm trying to translate an iterative function written in python to java so that I can analyze each subset as it's created and get only the value that represents how optimized it is and not the entire set so that I won't run out of memory. Here is what I have so far and it doesn't seem to finish even for very small problems: public static LinkedList<LinkedList<Integer>> getSets(int k, LinkedList<Integer> set) { int N = set.size(); int maxsets = nCr(N, k); LinkedList<LinkedList<Integer>> toRet = new LinkedList<LinkedList<Integer>>(); int remains, thresh; LinkedList<Integer> newset; for (int i=0; i<maxsets; i++) { remains = k; newset = new LinkedList<Integer>(); for (int val=1; val<=N; val++) { if (remains==0) break; thresh = nCr(N-val, remains-1); if (i < thresh) { newset.add(set.get(val-1)); remains --; } else { i -= thresh; } } toRet.add(newset); } return toRet; } Can anybody help me debug this function or suggest another algorithm for iteratively generating size k subsets? EDIT: I finally got this function working, I had to create a new variable that was the same as i to do the i and thresh comparison because python handles for loop indexes differently.

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• JavaScript array random index insertion and deletion

  - by Tomi

  I'm inserting some items into array with randomly created indexes, for example like this: var myArray = new Array(); myArray[123] = "foo"; myArray[456] = "bar"; myArray[789] = "baz"; ... In other words array indexes do not start with zero and there will be "numeric gaps" between them. My questions are: Will these numeric gaps be somehow allocated (and therefore take some memory) even when they do not have assigned values? When I delete myArray[456] from upper example, would items below this item be relocated? EDIT: Regarding my question/concern about relocation of items after insertion/deletion - I want to know what happens with the memory and not indexes. More information from wikipedia article: Linked lists have several advantages over dynamic arrays. Insertion of an element at a specific point of a list is a constant-time operation, whereas insertion in a dynamic array at random locations will require moving half of the elements on average, and all the elements in the worst case. While one can "delete" an element from an array in constant time by somehow marking its slot as "vacant", this causes fragmentation that impedes the performance of iteration.

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• Extensions methods and forward compatibilty of source code.

  - by TcKs

  Hi, I would like solve the problem (now hypothetical but propably real in future) of using extension methods and maginification of class interface in future development. Example: /* the code written in 17. March 2010 */ public class MySpecialList : IList<MySpecialClass> { // ... implementation } // ... somewhere elsewhere ... MySpecialList list = GetMySpecialList(); // returns list of special classes var reversedList = list.Reverse().ToList(); // .Reverse() is extension method /* now the "list" is unchanged and "reveresedList" has same items in reversed order */ /* --- in future the interface of MySpecialList will be changed because of reason XYZ*/ /* the code written in some future */ public class MySpecialList : IList<MySpecialClass> { // ... implementation public MySpecialList Reverse() { // reverse order of items in this collection return this; } } // ... somewhere elsewhere ... MySpecialList list = GetMySpecialList(); // returns list of special classes var reversedList = list.Reverse().ToList(); // .Reverse() was extension method but now is instance method and do something else ! /* now the "list" is reversed order of items and "reveresedList" has same items lake in "list" */ My question is: Is there some way how to prevent this case (I didn't find them)? If is now way how to prevent it, is there some way how to find possible issues like this? If is now way how to find possible issues, should I forbid usage of extension methods? Thanks.

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• Sorting a very large text file in Java

  - by Alice

  Hi, I have a large text file I need to sort in Java. The format is: word [tab] frequency [new line] The algorithm for sorting is: Read some of the file, filtering for purlely alphabetic words. Once you have X number of alphabetic words, call Collections.sort and write the result to a file. Repeat until you have finished reading the file. Start reading two sorted files, comparing line by line for the word with higher frequency, and writing at the same time to a new file as to not load much into your memory Repeat until all files are merged into one large file Right now I've divided the large file into smaller ones (sorted by descending frequency) with 10,000 lines each. I know I need to somehow merge these files back together, but I'm not sure how to go about this. I've created a LinkedList to keep track of all the files created. The algorithm says to compare each line in the two files, except I've tried a case where , say file1 = 8,6,5,3,1 and file2 = 9,8,8,8,8. Then if I compare them line by line I would get file3 = 9,8,8,6,8,5,8,3,8,1 which is incorrectly sorted (they should be in decreasing order). I think I'm misunderstanding some part of the algorithm. If someone could point out what I should do instead, I'd greatly appreciate it. Thanks. edit: Yes this is an assignment. We aren't allowed to increase memory unfortunately :(

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• !gcroot output leads nowhere

  - by Jeff Costa

  I am troubleshooting memory fragmentation in an app pool, as evidenced by a small number of Free objects consuming the most space on the heap: 0x000007ff00256728 6,543 3,890,208 System.Collections.Hashtable+bucket[] 0x000007ff002649a8 7,297 22,979,560 System.Byte[] 0x000007ff001e0d90 251,347 30,374,304 System.String 0x0000000001d0c830 373 48,036,816 Free Running the !dumpgen 3 command reveals the fragmentation; There is a repeating pattern of Free and System.Object objects of the same size: 000000017feb7350 24 **** FREE **** 000000017feb7368 8192 System.Object[] 000000017feb9368 24 **** FREE **** 000000017feb9380 8192 System.Object[] 000000017febb380 24 **** FREE **** 000000017febb398 8192 System.Object[] 000000017febd398 24 **** FREE **** 000000017febd3b0 8192 System.Object[] 000000017febf3b0 24 **** FREE **** 000000017febf3c8 8192 System.Object[] 000000017fec13c8 24 **** FREE **** 000000017fec13e0 8192 System.Object[] 000000017fec33e0 24 **** FREE **** 000000017fec33f8 8192 System.Object[] 000000017fec53f8 24 **** FREE **** 000000017fec5410 14024 System.Object[] 000000017fec8ad8 24 **** FREE **** 000000017fec8af0 8192 System.Object[] 000000017fecaaf0 24 **** FREE **** 000000017fecab08 8192 System.Object[] 000000017feccb08 24 **** FREE **** 000000017feccb20 8192 System.Object[] 000000017feceb20 24 **** FREE **** 000000017feceb38 8192 System.Object[] 000000017fed0b38 24 **** FREE **** 000000017fed0b50 8192 System.Object[] 000000017fed2b50 24 **** FREE **** 000000017fed2b68 8192 System.Object[] When I try to obtain the root of one of the System.Objects with !gcroot, I get a pinned handle, but no additional stack data: Scan Thread 41 OSThread 1044 DOMAIN(0000000001D51330):HANDLE(Pinned):15217e8:Root: 000000017fe60fe8(System.Object[]) As you can see, there is no additional data to go on. Running a !handle command also yields nothing: 0:041> !handle 000000017fe7a068 ff Handle 000000017fe7a068 Type <Error retrieving type> unable to query object information unable to query object information No object specific information available How can I trace out this memory leak when I cannot find what is rooting System.Object?

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• Automated generation of the "What's new in this version" reading

  - by lunohodov

  We all know it - this is the reading that lists the changes brought by each new version of our favorite software. Whenever it comes bundled as a file (Changes.txt, CHANGES, WhatsNew.txt, etc) or is presented within an installer this is usually the first thing we read before installing/updating. On a current project we have a Changelog.txt file that gets updated by hand every time a notable change occurs. However this often leads to "I forgot to update the changelog". So I am looking for a way to automate this. I am thinking of a script that extracts the changes from our commit messages (using a convention) and generates the file. For example a commit message like Updated json-glib to 0.7.6 [changes] - Fix crash on Windows - Fix issues with facebook contacts with very large UIDs. Would produce the following Changes.txt Version 1.9.18 (03/10/2010) Fix crash on Windows Fix issues with facebook contacts with very large UIDs. Does anyone know a better solution/tool for this or am I going to write my own? Thanks!

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• Objective-C Result from a Static Method saved to class instance variable giving "EXC_BAD_ACCESS" when used.

  - by KinGBin

  I am trying to store the md5 string as a class instance variable instead of the actual password. I have a static function that will return a md5 string which I'm trying to store in an instance variable instead of the actual password. I have the following setter for my class instance variable: -(void)setPassword:(NSString *)newpass{ if(newpass != password){ password = [utils md5HexDigest:newpass]; } } This will pass back the correct md5 string and save it to the password variable in my init function: [self setPassword:pword];. If I call another instance method and try to access self.password" I will get "EXC_BAD_ACCESS". I understand that the memory is getting released, but I have no clue to make sure it stays. I have tried alloc init with autorelease with no luck. This is the md5HexDigest function getting called during the init (graciously found in another stackoverflow question): + (NSString*)md5HexDigest:(NSString*)input { const char* str = [input UTF8String]; unsigned char result[CC_MD5_DIGEST_LENGTH]; CC_MD5(str, strlen(str), result); NSMutableString *ret = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH*2]; for(int i = 0; i<CC_MD5_DIGEST_LENGTH; i++) { [ret appendFormat:@"%02x",result[i]]; } return ret; } Any help/pointers would be greatly appreciated. I would rather have the md5 string saved in memory than the actual password calling the md5 every time I needed to use the password. Thanks in advance.

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• How does git fetches commits associated to a file ?

  - by liadan

  I'm writing a simple parser of .git/* files. I covered almost everything, like objects, refs, pack files etc. But I have a problem. Let's say I have a big 300M repository (in a pack file) and I want to find out all the commits which changed /some/deep/inside/file file. What I'm doing now is: fetching last commit finding a file in it by: fetching parent tree finding out a tree inside recursively repeat until I get into the file additionally I'm checking hashes of each subfolders on my way to file. If one of them is the same as in commit before, I assume that file was not changed (because it's parent dir didn't change) then I store the hash of a file and fetch parent commit finding file again and check if hash change occurs if yes then original commit (i.e. one before parent) was changing a file And I repeat it over and over until I reach very first commit. This solution works, but it sucks. In worse case scenario, first search can take even 3 minutes (for 300M pack). Is there any way to speed it up ? I tried to avoid putting so large objects in memory, but right now I don't see any other way. And even that, initial memory load will take forever :( Greets and thanks for any help!

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• HTML5 Local Storage of audio element source - is it possible?

  - by andrewdotcom

  Hi stackoverflow experts I've been experimenting with the audio and local storage features of html5 of late and have run into something that has me stumped. I'd like to be able to cache or store the source of the audio element locally to enable speedier and offline playback. The problem is I can't see how this is possible with the current implementation. I have tried the following using webkit: Creating a manifest file to set up local caching but the audio file appears not to be a cacheable item maybe due to the way it is stream or something I have also attempted to use javascript to put an audio object into local storage but the size of the mp3 makes this impossible due to memory issues (i think). I have tried to use the data uri and base64 to use the html as a audio transport that can be cached but again the filesize makes this prohibitive. Also the audio element does not seem to like this in webkit (works fine in mozilla) I have tried several methods of putting the data into the local database store. Again suffering the same issues as the other cases. I'd love to hear any other ideas anyone may have as to how I could achieve my goal of offline playback using caching/local storage in webkit.

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• Tools for debugging when debugger can't get you there?

  - by brian1001

  I have a fairly complex (approx 200,000 lines of C++ code) application that has decided to crash, although it crashes a little differently on a couple of different systems. The trick is that it doesn't crash or trap out in debugger. It only crashes when the application .EXE is run independently (either the debug EXE or the release EXE - both behave the same way). When it crashes in the debug EXE, and I get it to start debugging, the call stack is buried down into the windows/MFC part of things, and isn't reflecting any of my code. Perhaps I'm seeing a stack corruption of some sort, but I'm just not sure at the moment. My question is more general - it's about tools and techniques. I'm an old programmer (C and assembly language days), and a relative newcomer (couple/few years) to C++ and Visual Studio (2003 for this projecT). Are there tricks or techniques anyone's had success with in tracking down crashing issues when you cannot make the software crash in a debugger session? Stuff like permission issues, for example? The only thing I've thought of is to start plugging in debug/status messages to a logfile, but that's a long, hard way to go. Been there, done that. Any better suggestions? Am I missing some tools that would help? Is VS 2008 better for this kind of thing? Thanks for any guidance. Some very smart people here (you know who you are!). cheers.

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• restrict duplicate rows in specific columns in mysql

  - by JPro

  I have a query like this : select testset, count(distinct results.TestCase) as runs, Sum(case when Verdict = "PASS" then 1 else 0 end) as pass, Sum(case when Verdict <> "PASS" then 1 else 0 end) as fails, Sum(case when latest_issue <> "NULL" then 1 else 0 end) as issues, Sum(case when latest_issue <> "NULL" and issue_type = "TC" then 1 else 0 end) as TC_issues from results join testcases on results.TestCase = testcases.TestCase where platform = "T1_PLATFORM" AND testcases.CaseType = "M2" and testcases.dummy <> "flag_1" group by testset order by results.TestCase The result set I get is : testset runs pass fails issues TC_issues T1 66 125 73 38 33 T2 18 19 16 16 15 T3 57 58 55 55 29 T4 52 43 12 0 0 T5 193 223 265 130 22 T6 23 12 11 0 0 My problem is, this is a result table which has testcases running multiple times. So, I am able to restrict the runs using the distinct TestCases but when I want the pass and fails, since I am using case I am unable to eliminate the duplicates. Is there any way to achieve what I want? any help please? thanks.

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• PHP include taking too long

  - by wxiiir

  I have a php file with around 100mb which is full of arrays (only arrays). I've made a script that includes this file (for processing), first it exhausted the default Xampp 128mb memory limit, i've raised it to 1024mb but it just takes forever and doesn't do anything. I'm sure the problem is being created by the sheer size of the file because i've tried removing all lines of code and just leaving the include and an echo for me to know when it finishes executing, and it does the same thing (which is taking forever), i've also tried to run the 100mb file in separate and same thing again. A 10mb file is taking forever as well but a similar 1mb file is almost instantly read and executed so the problem must be more than just the file size. I was avoiding using c++ for a simple project as this and would rather not to as php is easier for me and the task that will be executed doesn't need to benefit from the added speed that it would have if it had been done in c++ but if i have no luck in solving this problem i guess i'll have to. EDIT Reasons for not using a database: 1-Whoever made it didn't used a database and it will be pretty hard to store this in an organized database if i'm not able to do something with it first, like just reading it, copying parts from it or putting in memory or something. 2-I don't have experience working with databases as pretty much all stuff i've ever done in php didn't needed large amounts of stored data, 50kb at best, if i was thinking about a big project or huge chunks of data as this one, i definitely would, but i didn't made this mess to start with and now i have to undo it. 3-The logic for having to store a small portion of data like 10mb in hard drive when now every computer has pretty much enough ram to fit the whole OS in it is pretty much incomprehensible unless someone gives a good explanation about it, if i had to access a lot of said files simultaneously i would understand but like i said, this is a simple project, this is the only file that will be accessed at a given time this isn't even to make some kind of website, it's to run a few times and be done with it.

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• Another thread safe queue implementation

  - by jensph

  I have a class, Queue, that I tried to make thread safe. It has these three member variables: std::queue<T> m_queue; pthread_mutex_t m_mutex; pthread_cond_t m_condition; and a push and pop implemented as: template<class T> void Queue<T>::push(T value) { pthread_mutex_lock( &m_mutex ); m_queue.push(value); if( !m_queue.empty() ) { pthread_cond_signal( &m_condition ); } pthread_mutex_unlock( &m_mutex ); } template<class T> bool Queue<T>::pop(T& value, bool block) { bool rtn = false; pthread_mutex_lock( &m_mutex ); if( block ) { while( m_queue.empty() ) { pthread_cond_wait( &m_condition, &m_mutex ); } } if( !m_queue.empty() ) { value = m_queue.front(); m_queue.pop(); rtn = true; } pthread_mutex_unlock( &m_mutex ); return rtn; } Unfortunately there are occasional issues that may be the fault of this code. That is, there are two threads and sometimes thread 1 never comes out of push() and at other times thread 2 never comes out of pop() (the block parameter is true) though the queue isn't empty. I understand there are other implementations available, but I'd like to try to fix this code, if needed. Anyone see any issues? The constructor has the appropriate initializations: Queue() { pthread_mutex_init( &mMutex, NULL ); pthread_cond_init( &mCondition, NULL ); } and the destructor, the corresponding 'destroy' calls.

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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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• Receiving "expected expression before" Error When Using A Struct

  - by Zach Dziura

  I'm in the process of creating a simple 2D game engine in C with a group of friends at school. I'd like to write this engine in an Object-Oriented way, using structs as classes, function pointers as methods, etc. To emulate standard OOP syntax, I created a create() function which allocates space in memory for the object. I'm in the process of testing it out, and I'm receiving an error. Here is my code for two files that I'm using to test: test.c: #include <stdio.h> int main() { typedef struct { int i; } Class; Class *test = (Class*) create(Class); test->i = 1; printf("The value of \"test\" is: %i\n", test->i); return 0; } utils.c: #include <stdio.h> #include <stdlib.h> #include "utils.h" void* create(const void* class) { void *obj = (void*) malloc(sizeof(class)); if (obj == 0) { printf("Error allocating memory.\n"); return (int*) -1; } else { return obj; } } void destroy(void* object) { free(object); } The utils.h file simply holds prototypes for the create() and destroy() functions. When I execute gcc test.c utils.c -o test, I'm receiving this error message: test.c: In function 'main': test.c:10:32: error: expected expression before 'Class' I know it has something to do with my typedef at the beginning, and how I'm probably not using proper syntax. But I have no idea what that proper syntax is. Can anyone help?

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• How do I display core data on second view controller?

  - by jon

  I am working on my first core data iPhone application. I am using a navigation controller, and the root view controller displays 4 rows. Clicking the first row takes me to a second table view controller. However, when I click the back button, repeat the row tap, click the back button again, and tap the row a third time, I get an error. I have been researching this for a week with no success. I can reproduce the error easily: Create a new Navigation-based Application, use Core Data for storage, call it MyTest which creates MyTestAppDelegate and RootViewController. Add new UIViewController subclass, with UITableViewController and xib, call it ListViewController. Copy code from RootViewController.h and .m to ListViewController.h and .m., changing the file names appropriately. To simplify the code, I removed the trailing “_” from all variables. In RootViewController, I added #import ListViewController.h, set up an array to display 4 rows and navigate to ListViewController when clicking the first row. In ListViewController.m, I added #import MyTestAppDelegate.h” and the following code: - (void)viewDidLoad { [super viewDidLoad]; if (managedObjectContext == nil) { managedObjectContext = [(MyTestAppDelegate *)[[UIApplication sharedApplication] delegate] managedObjectContext]; } .. } The sequence that causes the error is tap row, return, tap row, return, tap row - error. managedObjectContext is synthesized for the third time. I appreciate your patience and your help, as this makes no sense to me. ADDENDUM: I may have a partial solution. http://www.iphonedevsdk.com/forum/iphone-sdk-development/41688-accessing-app-delegates-managed-object-context.html If I do not release the managedObjectContext in the .m file, the error goes away. Is that ok or will that cause me issues? - (void)dealloc { [fetchedResultsController release]; // [managedObjectContext release]; [super dealloc]; } ADDENDUM 2: See solution below. Sorry for the formatting issues - this was my first post.

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• Java: multi-threaded maps: how do the implementations compare?

  - by user346629

  I'm looking for a good hash map implementation. Specifically, one that's good for creating a large number of maps, most of them small. So memory is an issue. It should be thread-safe (though losing the odd put might be an OK compromise in return for better performance), and fast for both get and put. And I'd also like the moon on a stick, please, with a side-order of justice. The options I know are: HashMap. Disastrously un-thread safe. ConcurrentHashMap. My first choice, but this has a hefty memory footprint - about 2k per instance. Collections.sychronizedMap(HashMap). That's working OK for me, but I'm sure there must be faster alternatives. Trove or Colt - I think neither of these are thread-safe, but perhaps the code could be adapted to be thread safe. Any others? Any advice on what beats what when? Any really good new hash map algorithms that Java could use an implementation of? Thanks in advance for your input!

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• Separating code logic from the actual data structures. Best practices?

  - by Patrick

  I have an application that loads lots of data into memory (this is because it needs to perform some mathematical simulation on big data sets). This data comes from several database tables, that all refer to each other. The consistency rules on the data are rather complex, and looking up all the relevant data requires quite some hashes and other additional data structures on the data. Problem is that this data may also be changed interactively by the user in a dialog. When the user presses the OK button, I want to perform all the checks to see that he didn't introduce inconsistencies in the data. In practice all the data needs to be checked at once, so I cannot update my data set incrementally and perform the checks one by one. However, all the checking code work on the actual data set loaded in memory, and use the hashing and other data structures. This means I have to do the following: Take the user's changes from the dialog Apply them to the big data set Perform the checks on the big data set Undo all the changes if the checks fail I don't like this solution since other threads are also continuously using the data set, and I don't want to halt them while performing the checks. Also, the undo means that the old situation needs to be put aside, which is also not possible. An alternative is to separate the checking code from the data set (and let it work on explicitly given data, e.g. coming from the dialog) but this means that the checking code cannot use hashing and other additional data structures, because they only work on the big data set, making the checks much slower. What is a good practice to check user's changes on complex data before applying them to the 'application's' data set?

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• Applying iterative algorithm to a set of rows from database

  - by Corvin

  Hello, this question may seem too basic to some, but please bear with be, it's been a while since I dealt with decent database programming. I have an algorithm that I need to program in PHP/MySQL to work on a website. It performs some computations iteratively on an array of objects (it ranks the objects based on their properties). In each iteration the algorithm runs through all collection a couple of times, accessing various data from different places of the whole collection. The algorithm needs several hundred iterations to complete. The array comes from a database. The straightforward solution that I see is to take the results of a database query and create an object for each row of the query, put the objects to an array and pass the array to my algorithm. However, I'm concerned with efficacy of such solution when I have to work with an array of several thousand of items because what I do is essentially mirror the results of a query to memory. On the other hand, making database query a couple of times on each iteration of the algorithm also seems wrong. So, my question is - what is the correct architectural solution for a problem like this? Is it OK to mirror the query results to memory? If not, which is the best way to work with query results in such an algorithm? Thanks!

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