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  • Perf: Viewing thousands of images in Silverlight 3 on a 3D Wall

    - by Bob Holland
    I currently work on a very cool Silverlight app that displays photos in a 3D wall space like the Wall3D demo that is thrown in with Blend 3. The problem I am currently facing is performance. The app works like this: As you scroll right or left the 3d photo wall rotates As each movement is made, the next column of photos are downloaded, decoded into a BitmapImage and thrown into a 3D Wall Node. As you can imagine users (if you let them) will want to flip through the photos really quickly, but the problem I have is I cannot display the photos quick enough. In most cases it's a beautiful app that works really well, but when an album contains over 300 photos, you can imagine the sort of memory taken up by all the BitmapImage classes and how moving the slider can jump from photo 20 to photo 120 in a second. Of course we have algorithms in place to not download every photo in between, but I still can't work out a fast way to get the photos displayed. It may be a case that we need to throw away the 'great for show' 3D wall and go to a flat DeepZoom like wall like the Playboy archive one that Vertigo did. Still not sure, let me know your thoughts. P.S. We are using Kit3D for all the 3D work, it's using PerspectiveCamera, Model3DGroup, ModelVisual3D, RotateTransform3D & TranslateTransform3D. Cheers, Bob.

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  • Instantiating a list of parameterized types, making beter use of Generics and Linq

    - by DanO
    I'm hashing a file with one or more hash algorithms. When I tried to parametrize which hash types I want, it got a lot messier than I was hoping. I think I'm missing a chance to make better use of generics or LINQ. I also don't like that I have to use a Type[] as the parameter instead of limiting it to a more specific set of type (HashAlgorithm descendants), I'd like to specify types as the parameter and let this method do the constructing, but maybe this would look better if I had the caller new-up instances of HashAlgorithm to pass in? public List<string> ComputeMultipleHashesOnFile(string filename, Type[] hashClassTypes) { var hashClassInstances = new List<HashAlgorithm>(); var cryptoStreams = new List<CryptoStream>(); FileStream fs = File.OpenRead(filename); Stream cryptoStream = fs; foreach (var hashClassType in hashClassTypes) { object obj = Activator.CreateInstance(hashClassType); var cs = new CryptoStream(cryptoStream, (HashAlgorithm)obj, CryptoStreamMode.Read); hashClassInstances.Add((HashAlgorithm)obj); cryptoStreams.Add(cs); cryptoStream = cs; } CryptoStream cs1 = cryptoStreams.Last(); byte[] scratch = new byte[1 << 16]; int bytesRead; do { bytesRead = cs1.Read(scratch, 0, scratch.Length); } while (bytesRead > 0); foreach (var stream in cryptoStreams) { stream.Close(); } foreach (var hashClassInstance in hashClassInstances) { Console.WriteLine("{0} hash = {1}", hashClassInstance.ToString(), HexStr(hashClassInstance.Hash).ToLower()); } }

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  • Reinforcement learning toy project

    - by Betamoo
    My toy project to learn & apply Reinforcement Learning is: - An agent tries to reach a goal state "safely" & "quickly".... - But there are projectiles and rockets that are launched upon the agent in the way. - The agent can determine rockets position -with some noise- only if they are "near" - The agent then must learn to avoid crashing into these rockets.. - The agent has -rechargable with time- fuel which is consumed in agent motion - Continuous Actions: Accelerating forward - Turning with angle I need some hints and names of RL algorithms that suit that case.. - I think it is POMDP , but can I model it as MDP and just ignore noise? - In case POMDP, What is the recommended way for evaluating probability? - Which is better to use in this case: Value functions or Policy Iterations? - Can I use NN to model environment dynamics instead of using explicit equations? - If yes, Is there a specific type/model of NN to be recommended? - I think Actions must be discretized, right? I know it will take time and effort to learn such a topic, but I am eager to.. You may answer some of the questions if you can not answer all... Thanks

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  • Is there any open source tool that automatically 'detects' email threading like Gmail?

    - by Chris W.
    For instance, if the original message (message 1) is... Hey Jon, Want to go get some pizza? -Bill And the reply (message 2) is... Bill, Sorry, I can't make lunch today. Jonathon Parks, CTO Acme Systems On Wed, Feb 24, 2010 at 4:43 PM, Bill Waters wrote: Hey John, Want to go get some pizza? -Bill In Gmail, the system (a) detects that message 2 is a reply to message 1 and turns this into a 'thread' of sorts and (b) detects where the replied portion of the message actually is and hides it from the user. (In this case the hidden portion would start at "On Wed, Feb..." and continue to the end of the message.) Obviously, in this simple example it would be easy to detect the "On <Date, <Name wrote:" or the "" character prefixes. But many email systems have many different style of marking replies (not to mention HTML emails). I get the feeling that you would have to have some damn smart string parsing algorithms to get anywhere near how good GMail's is. Does this technology already exist in an open source project somewhere? Either in some library devoted to this exclusively or perhaps in some open source email client that does similar message threading? Thanks.

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  • Language+IDE for teaching high school students?

    - by daveagp
    I'm investigating languages and IDEs for a project involving teaching high-school students (around grade 11). It will teach basics of programming as an introduction to computer science (e.g., including how numbers/strings/characters are represented, using procedures and arrays, control flow, a little bit of algorithms, only very basic I/O). The non-negotiable requirements for this project are: a free up-to-date cross-platform IDE (Win & Mac incl. 64-bit) with debug a compiler where it's easy to learn from your mistakes together with the IDE, a gentle installation+learning curve So far, the best options I see are the following. Are there others I should know about? I am giving a short explanation with each one to generally show what I am looking for. In order from most to least promising: Pascal + FreePascal IDE (it seems a little buggy but actively developed?) Python + Eclipse + PyDev (good but features are overwhelming/hard to navigate) Groovy + Eclipse ('') Python + IDLE (looks unnatural to do debugging, to me) Pascal + Lazarus (IDE overwhelming, e.g. not obvious how to "start from scratch") Preferably, as a rule of thumb, the language should be direct enough that you don't need to wrap every program in a class, don't need to reference a System object to println, etc. I tried a little bit to see if there is something in JavaScript or (non-Visual) Basic along the lines of what I want, but found nothing so far. I would say that C/C++/C#, Java, Ruby, Lisp, VB do not fit my criteria for languages for this project. To reiterate my questions: are any of those 5 options really awesome or un-awesome? Are there other options which are even MORE awesome? Anything for Basic or JavaScript which meets all of the criteria? Thanks!

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  • Optimized graph drawing for the web

    - by Andreas Petersson
    Having seen some suggestions for graphs, I wonder what's the optimum for my problem. I want to render a directed graph to a servlet/picture that is displayed in the browser. There should be some kind of optimization of position. No dependency to Swing would be preferred. Algorithms are not important, since the structure of the graph is determined by business logic. It would be desired to be able add labels to edges as well. it would be optimal if i can serve this as png/svg. Which library/service would you recommend? clarifications: 1) The question is all about Graphs - like Directed Acyclic Graph - NOT - Charts. 2) flot, Google Charts - cannot plot graphs, only charts, or have i missed something? 3) no i do not need interactivity 4) graphviz would be nice, but the grappa java library is quite outdated and is built upon swing/awt. while it may be theoretically possible to render swing to images, it would not be my favorite way to to so in a server-app. 5) it would be fine to use an online service where the images are not hosted locally. edit: added links to Wikipedia to clarify graph/chart term

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  • Computer science textbooks

    - by Barrett Conrad
    I would like to try the book question a little bit differently. My goal is to know what the community thinks are the quintessential computer science textbooks. <beginsadstory>I lost all of my computer science and math books from college in Hurricane Katrina in 2005. I greatly miss having my familiar tomes to refer to when topics and problems come up, so I am looking to rebuild my library.<endsadstory> What are your recommendations for the best examples of academic caliber books for the field of computer science and its associated mathematics? I am looking for books on subjects like computational theory, programming languages, compilers, operating systems, algorithms and so on. Don't limit your suggestions to your textbooks only. If there is a book you have read that covers computer science or a related math in a formal way, but is not strictly a textbook, I would be love to hear about them as well. Finally, for the sake of creating a good reference for all of us, may I suggest posting one book per answer so they can be rated individually.

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  • Unexpected result in C algebra for search algorithm.

    - by Rhys
    Hi, I've implemented this search algorithm for an ordered array of integers. It works fine for the first data set I feed it (500 integers), but fails on longer searches. However, all of the sets work perfectly with the other four search algorithms I've implemented for the assignment. This is the function that returns a seg fault on line 178 (due to an unexpected negative m value). Any help would be greatly appreciated. CODE: 155 /* perform Algortihm 'InterPolationSearch' on the set 156 * and if 'key' is found in the set return it's index 157 * otherwise return -1 */ 158 int 159 interpolation_search(int *set, int len, int key) 160 { 161 int l = 0; 162 int r = len - 1; 163 int m; 164 165 while (set[l] < key && set[r] >= key) 166 { 167 168 printf ("m = l + ((key - set[l]) * (r - l)) / (set[r] - set[l])\n"); 169 170 printf ("m = %d + ((%d - %d) * (%d - %d)) / (%d - %d);\n", l, key, set[l], r, l, set[r], set[l]); 171 m = l + ((key - set[l]) * (r - l)) / (set[r] - set[l]); 172 printf ("m = %d\n", m); 173 174 #ifdef COUNT_COMPARES 175 g_compares++; 176 #endif 177 178 if (set[m] < key) 179 l = m + 1; 180 else if (set[m] > key) 181 r = m - 1; 182 else 183 return m; 184 } 185 186 if (set[l] == key) 187 return l; 188 else 189 return -1; 190 } OUTPUT: m = l + ((key - set[l]) * (r - l)) / (set[r] - set[l]) m = 0 + ((68816 - 0) * (100000 - 0)) / (114836 - 0); m = -14876 Thankyou! Rhys

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  • Getting started with character and text processing (encoding, regular expressions)

    - by TK
    I'd like to learn foundations of encodings, characters and text. Understanding these is important for dealing with a large set of text whether that are log files or text source for building algorithms for collective intelligence. My current knowledge is pretty basic: something like "As long as I use UTF-8, I'm okay." I don't say I need to learn about advanced topics right away. But I need to know: Bit and bytes level knowledge of encodings. Characters and alphabets not used in English. Multi-byte encodings. (I understand some Chinese and Japanese. And parsing them is important.) Regular expressions. Algorithm for text processing. Parsing natural languages. I also need an understanding of mathematics and corpus linguistics. The current and future web (semantic, intelligent, real-time web) needs processing, parsing and analyzing large text. I'm looking for some resources (maybe books?) that get me started with some of the bullets. (I find many helpful discussion on regular expressions here on Stack Overflow. So, you don't need to suggest resources on that topic.)

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  • An alternative to reading input from Java's System.in

    - by dvanaria
    I’m working on the UVa Online Judge problem set archive as a way to practice Java, and as a way to practice data structures and algorithms in general. They give an example input file to submit to the online judge to use as a starting point (it’s the solution to problem 100). Input from the standard input stream (java.lang.System.in) is required as part of any solution on this site, but I can’t understand the implementation of reading from System.in they give in their example solution. It’s true that the input file could consist of any variation of integers, strings, etc, but every solution program requires reading basic lines of text input from System.in, one line at a time. There has to be a better (simpler and more robust) method of gathering data from the standard input stream in Java than this: public static String readLn(int maxLg) { byte lin[] = new byte[maxLg]; int lg = 0, car = -1; String line = “”; try { while (lg < maxLg) { car = System.in.read(); if ((car < 0) || (car == ‘\n’)) { break; } lin[lg++] += car; } } catch (java.io.IOException e) { return (null); } if ((car < 0) && (lg == 0)) { return (null); // eof } return (new String(lin, 0, lg)); } I’m really surprised by this. It looks like something pulled directly from K&R’s “C Programming Language” (a great book regardless), minus the access level modifer and exception handling, etc. Even though I understand the implementation, it just seems like it was written by a C programmer and bypasses most of Java’s object oriented nature. Isn’t there a better way to do this, using the StringTokenizer class or maybe using the split method of String or the java.util.regex package instead?

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  • How does lucene index documents?

    - by Mehdi Amrollahi
    Hello, I read some document about Lucene; also I read the document in this link (http://lucene.sourceforge.net/talks/pisa). I don't really understand how Lucene indexes documents and don't understand which algorithms Lucene uses for indexing? On the above link, it says Lucene uses this algorithm for indexing: incremental algorithm: maintain a stack of segment indices create index for each incoming document push new indexes onto the stack let b=10 be the merge factor; M=8 for (size = 1; size < M; size *= b) { if (there are b indexes with size docs on top of the stack) { pop them off the stack; merge them into a single index; push the merged index onto the stack; } else { break; } } How does this algorithm provide optimized indexing? Does Lucene use B-tree algorithm or any other algorithm like that for indexing - or does it have a particular algorithm? Thank you for reading my post.

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  • Graphviz or Dynagraph for Graph-manipulation Program?

    - by noahlavine
    I'm looking into writing a program that will show a graph to the user. The graph will change over time (the user should be able to right-click on a graph item and ask for more detail, which will pop out new bits of the graph), and the user might be able to drag parts of the graph around. I would ideally also like to be able to specify the relative layout of certain parts of the graph myself while leaving the overall layout up to a library, but that's not essential. I'm trying to decide on a graph layout library to use. As far as I can tell, the two leading candidates are Graphviz and Dynagraph. The Dynagraph website suggests that Graphviz is for drawing static graphs, and that Dynagraph was forked from Graphviz and contains algorithms for graphs that will be updated. It has a sample program called Dynasty that does exactly what I want. However, the Graphviz site contains an example program called Lefty which seems to do exactly what I want. Graphviz also seems to be much more widely used, judging by Google (and SO) results. Finally, I'd like to code the GUI part in a language like Python or Scheme, which makes me a bit hesitant to use C++ because I understand it's harder to interface that to interpreters. So my question is, which library is better for what I'm trying to do? Do they both have strong and weak points? Has one of them actually ceased development and is just leaving its website up to confuse me? (I've seen http://stackoverflow.com/questions/464000/simple-dynamic-graph-display-for-c and http://stackoverflow.com/questions/2376987/open-source-libraries-to-design-directed-graphs, but I can't tell whether they're right about the Graphviz or Dynagraph choice because of Lefty and also the language issue.)

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  • Getting a job in the games industry as a developer, just knowing a game engine

    - by numerical25
    I recently enrolled in a community college for games developement. But I am skeptical about the curriculum. I have no experience in the gaming industry so I wouldn't be able to tell whether it's a good investment or not. So I am asking you. I don't want to get too much into the details of all the classes I am taking so I will try to be brief. By the time I graduate, I should have a understanding of how a game engine works. I will be working with the Unreal Engine to develop a Multiplayer game from scratch. So in the process of my final project, I will learn how to work within the Unreal Engine, learn Python and learn how to use its API to connect to a remote server and build game mechanics. Overall I will also recieve an associates degree in game development. I learn C++ but not C. The director said he was trying to implement C in the program as well. What I notice is I will not learn how to build a 3D game engine from scratch. They do not teach any artificial intelligence (AI). I will not learn how to work with the graphics card using a graphics API such as DirectX or OpenGL. I know building a game engine from scratch is a little complex, but at the same time the track is requiring me to take some advanced mathematics courses such as calculus and geometry 1 and 2. I also got to take a physics class. I just think that's a little much for just learning how to use the Unreal Engine but not actually build one or try to learn the anatomy of a games engine. Is this good enough to possibly land my a job in the industry? If I left anything out or was not detail, please feel free to ask more questions. Edit: I do learn data structures and algorithms.

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  • Scalable / Parallel Large Graph Analysis Library?

    - by Joel Hoff
    I am looking for good recommendations for scalable and/or parallel large graph analysis libraries in various languages. The problems I am working on involve significant computational analysis of graphs/networks with 1-100 million nodes and 10 million to 1+ billion edges. The largest SMP computer I am using has 256 GB memory, but I also have access to an HPC cluster with 1000 cores, 2 TB aggregate memory, and MPI for communication. I am primarily looking for scalable, high-performance graph libraries that could be used in either single or multi-threaded scenarios, but parallel analysis libraries based on MPI or a similar protocol for communication and/or distributed memory are also of interest for high-end problems. Target programming languages include C++, C, Java, and Python. My research to-date has come up with the following possible solutions for these languages: C++ -- The most viable solutions appear to be the Boost Graph Library and Parallel Boost Graph Library. I have looked briefly at MTGL, but it is currently slanted more toward massively multithreaded hardware architectures like the Cray XMT. C - igraph and SNAP (Small-world Network Analysis and Partitioning); latter uses OpenMP for parallelism on SMP systems. Java - I have found no parallel libraries here yet, but JGraphT and perhaps JUNG are leading contenders in the non-parallel space. Python - igraph and NetworkX look like the most solid options, though neither is parallel. There used to be Python bindings for BGL, but these are now unsupported; last release in 2005 looks stale now. Other topics here on SO that I've looked at have discussed graph libraries in C++, Java, Python, and other languages. However, none of these topics focused significantly on scalability. Does anyone have recommendations they can offer based on experience with any of the above or other library packages when applied to large graph analysis problems? Performance, scalability, and code stability/maturity are my primary concerns. Most of the specialized algorithms will be developed by my team with the exception of any graph-oriented parallel communication or distributed memory frameworks (where the graph state is distributed across a cluster).

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  • sample java code for approximate string matching or boyer-moore extended for approximate string matc

    - by Dolphin
    Hi I need to find 1.mismatch(incorrectly played notes), 2.insertion(additional played), & 3.deletion (missed notes), in a music piece (e.g. note pitches [string values] stored in a table) against a reference music piece. This is either possible through exact string matching algorithms or dynamic programming/ approximate string matching algos. However I realised that approximate string matching is more appropriate for my problem due to identifying mismatch, insertion, deletion of notes. Or an extended version of Boyer-moore to support approx. string matching. Is there any link for sample java code I can try out approximate string matching? I find complex explanations and equations - but I hope I could do well with some sample code and simple explanations. Or can I find any sample java code on boyer-moore extended for approx. string matching? I understand the boyer-moore concept, but having troubles with adjusting it to support approx. string matching (i.e. to support mismatch, insertion, deletion). Also what is the most efficient approx. string matching algorithm (like boyer-moore in exact string matching algo)? Greatly appreciate any insight/ suggestions. Many thanks in advance

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  • Optimal Serialization of Primitive Types

    - by Greg Dean
    We are beginning to roll out more and more WAN deployments of our product (.Net fat client w/ IIS hosted Remoting backend). Because of this we are trying to reduce the size of the data on the wire. We have overridden the default serialization by implementing ISerializable (similar to this), we are seeing anywhere from 12% to 50% gains. Most of our efforts focus on optimizing arrays of primitive types. I would like to know if anyone knows of any fancy way of serializing primitive types, beyond the obvious? For example today we serialize an array of ints as follows: [4-bytes (array length)][4-bytes][4-bytes] Can anyone do significantly better? The most obvious example of a significant improvement, for boolean arrays, is putting 8 bools in each byte, which we already do. Note: Saving 7 bits per bool may seem like a waste of time, but when you are dealing with large magnitudes of data (which we are), it adds up very fast. Note: We want to avoid general compression algorithms because of the latency associated with it. Remoting only supports buffered requests/responses(no chunked encoding). I realize there is a fine line between compression and optimal serialization, but our tests indicate we can afford very specific serialization optimizations at very little cost in latency. Whereas reprocessing the entire buffered response into new compressed buffer is too expensive.

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  • P6 Architecture - Register renaming aside, does the limited user registers result in more ops spent

    - by mrjoltcola
    I'm studying JIT design with regard to dynamic languages VM implementation. I haven't done much Assembly since the 8086/8088 days, just a little here or there, so be nice if I'm out of sorts. As I understand it, the x86 (IA-32) architecture still has the same basic limited register set today that it always did, but the internal register count has grown tremendously, but these internal registers are not generally available and are used with register renaming to achieve parallel pipelining of code that otherwise could not be parallelizable. I understand this optimization pretty well, but my feeling is, while these optimizations help in overall throughput and for parallel algorithms, the limited register set we are still stuck with results in more register spilling overhead such that if x86 had double, or quadruple the registers available to us, there may be significantly less push/pop opcodes in a typical instruction stream? Or are there other processor optmizations that also optimize this away that I am unaware of? Basically if I've a unit of code that has 4 registers to work with for integer work, but my unit has a dozen variables, I've got potentially a push/pop for every 2 or so instructions. Any references to studies, or better yet, personal experiences?

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  • How Random is System.Guid.NewGuid()? (Take two)

    - by Vilx-
    Before you start marking this as a duplicate, read me out. The other question has a (most likely) incorrect accepted answer. I do not know how .NET generates its GUIDs, probably only Microsoft does, but there's a high chance it simply calls CoCreateGuid(). That function however is documented to be calling UuidCreate(). And the algorithms for creating an UUID are pretty well documented. Long story short, be as it may, it seems that System.Guid.NewGuid() indeed uses version 4 UUID generation algorithm, because all the GUIDs it generates matches the criteria (see for yourself, I tried a couple million GUIDs, they all matched). In other words, these GUIDs are almost random, except for a few known bits. This then again raises the question - how random IS this random? As every good little programmer knows, a pseudo-random number algorithm is only as random as its seed (aka entropy). So what is the seed for UuidCreate()? How ofter is the PRNG re-seeded? Is it cryptographically strong, or can I expect the same GUIDs to start pouring out if two computers accidentally call System.Guid.NewGuid() at the same time? And can the state of the PRNG be guessed if sufficiently many sequentially generated GUIDs are gathered?

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  • ai: Determining what tests to run to get most useful data

    - by Sai Emrys
    This is for http://cssfingerprint.com I have a system (see about page on site for details) where: I need to output a ranked list, with confidences, of categories that match a particular feature vector the binary feature vectors are a list of site IDs & whether this session detected a hit feature vectors are, for a given categorization, somewhat noisy (sites will decay out of history, and people will visit sites they don't normally visit) categories are a large, non-closed set (user IDs) my total feature space is approximately 50 million items (URLs) for any given test, I can only query approx. 0.2% of that space I can only make the decision of what to query, based on results so far, ~10-30 times, and must do so in <~100ms (though I can take much longer to do post-processing, relevant aggregation, etc) getting the AI's probability ranking of categories based on results so far is mildly expensive; ideally the decision will depend mostly on a few cheap sql queries I have training data that can say authoritatively that any two feature vectors are the same category but not that they are different (people sometimes forget their codes and use new ones, thereby making a new user id) I need an algorithm to determine what features (sites) are most likely to have a high ROI to query (i.e. to better discriminate between plausible-so-far categories [users], and to increase certainty that it's any given one). This needs to take into balance exploitation (test based on prior test data) and exploration (test stuff that's not been tested enough to find out how it performs). There's another question that deals with a priori ranking; this one is specifically about a posteriori ranking based on results gathered so far. Right now, I have little enough data that I can just always test everything that anyone else has ever gotten a hit for, but eventually that won't be the case, at which point this problem will need to be solved. I imagine that this is a fairly standard problem in AI - having a cheap heuristic for what expensive queries to make - but it wasn't covered in my AI class, so I don't actually know whether there's a standard answer. So, relevant reading that's not too math-heavy would be helpful, as well as suggestions for particular algorithms. What's a good way to approach this problem?

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  • Simulating pass by reference for an array in Java

    - by Leif Andersen
    I was wondering, in java, is it possible to in anyway, simulate pass by reference for an array? Yes, I know the language doesn't support it, but is there anyway I can do it. Say, for example, I want to create a method that reverses the order of all the elements in an array. (I know that this code snippet isn't the best example, as there is a better algorithms to do this, but this is a good example of the type of thing I want to do for more complex problems). Currently, I need to make a class like this: public static void reverse(Object[] arr) { Object[] tmpArr = new Object[arr.length]; count = arr.length - 1; for(Object i : arr) tmpArr[count--] = i; // I would like to do arr = tmpArr, but that will only make the shallow // reference tmpArr, I would like to actually change the pointer they passed in // Not just the values in the array, so I have to do this: count = arr.length - 1; for(Object i : tmpArr) arr[count--] = i; return; } Yes, I know that I could just swap the values until I get to the middle, and it would be much more efficient, but for other, more complex purposes, is there anyway that I can manipulate the actual pointer? Again, thank you.

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  • How can I load file into web app through certain periods?

    - by Elena
    Hi all! I have next task: I need to load the same file into my web app several times, for example - twice a day! Suppose in that file I have information, that changes, and I need to load this info into my app to change the statistics for example. How can I load file several times (twice an hour, or twice a day)? What should I use? Is any algorithm to do that? I am not allowed to use external libraries like Quartz Scheduler. So I need to do it with Thread and/or Timer. Can anybody give me some example or algorithm how to do it. Where can I create the entry point to my Thread, can I do it in managed bean or I need some sort of filter/listener/servlet. I works with jsf and richFaces. Maybe in this technologies there are some algorithms to solve my problem. Any ideas? Thanks very much for help!

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  • Simulating pass by reference for an array reference (i.e. a reference to a reference) in Java

    - by Leif Andersen
    I was wondering, in java, is it possible to in anyway, simulate pass by reference for an array? Yes, I know the language doesn't support it, but is there anyway I can do it. Say, for example, I want to create a method that reverses the order of all the elements in an array. (I know that this code snippet isn't the best example, as there is a better algorithms to do this, but this is a good example of the type of thing I want to do for more complex problems). Currently, I need to make a class like this: public static void reverse(Object[] arr) { Object[] tmpArr = new Object[arr.length]; count = arr.length - 1; for(Object i : arr) tmpArr[count--] = i; // I would like to do arr = tmpArr, but that will only make the shallow // reference tmpArr, I would like to actually change the pointer they passed in // Not just the values in the array, so I have to do this: count = arr.length - 1; for(Object i : tmpArr) arr[count--] = i; return; } Yes, I know that I could just swap the values until I get to the middle, and it would be much more efficient, but for other, more complex purposes, is there anyway that I can manipulate the actual pointer? Again, thank you.

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  • What is a good standard exercise to learn the OO features of a language?

    - by FarmBoy
    When I'm learning a new language, I often program some mathematical functions to get used to the control flow syntax. After that, I like to implement some sorting algorithms to get used to the array/list constructs. But I don't have a standard exercise for exploring the languages OO features. Does anyone have a stock exercise for this? A good answer would naturally lend to inheritance, polymorphism, etc., for a programmer already comfortable with these concepts. An ideal answer would be one that could be communicated in a few words, without ambiguity, in the way that "implement mergesort" is completely unambiguous. (As an example, answering "design a game" is so vague as to be useless.) Any ideas? EDIT: I have to remark that the results here are somewhat ironic. 10 upvotes and (originally) 5 favorites suggest that this is a question others are interested in. Yet the most upvoted answer is one that says there is no good answer. Oh well. I think I'll look at the textbook below, I've found games useful in the past for OO.

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  • Better ways to implement a modulo operation (algorithm question)

    - by ryxxui
    I've been trying to implement a modular exponentiator recently. I'm writing the code in VHDL, but I'm looking for advice of a more algorithmic nature. The main component of the modular exponentiator is a modular multiplier which I also have to implement myself. I haven't had any problems with the multiplication algorithm- it's just adding and shifting and I've done a good job of figuring out what all of my variables mean so that I can multiply in a pretty reasonable amount of time. The problem that I'm having is with implementing the modulus operation in the multiplier. I know that performing repeated subtractions will work, but it will also be slow. I found out that I could shift the modulus to effectively subtract large multiples of the modulus but I think there might still be better ways to do this. The algorithm that I'm using works something like this (weird pseudocode follows): result,modulus : integer (n bits) (previously defined) shiftcount : integer (initialized to zero) while( (modulus<result) and (modulus(n-1) != 1) ){ modulus = modulus << 1 shiftcount++ } for(i=shiftcount;i>=0;i++){ if(modulus<result){result = result-modulus} if(i!=0){modulus = modulus << 1} } So...is this a good algorithm, or at least a good place to start? Wikipedia doesn't really discuss algorithms for implementing the modulo operation, and whenever I try to search elsewhere I find really interesting but incredibly complicated (and often unrelated) research papers and publications. If there's an obvious way to implement this that I'm not seeing, I'd really appreciate some feedback.

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  • Can you force a crash if a write occurs to a given memory location with finer than page granularity?

    - by Joseph Garvin
    I'm writing a program that for performance reasons uses shared memory (alternatives have been evaluated, and they are not fast enough for my task, so suggestions to not use it will be downvoted). In the shared memory region I am writing many structs of a fixed size. There is one program responsible for writing the structs into shared memory, and many clients that read from it. However, there is one member of each struct that clients need to write to (a reference count, which they will update atomically). All of the other members should be read only to the clients. Because clients need to change that one member, they can't map the shared memory region as read only. But they shouldn't be tinkering with the other members either, and since these programs are written in C++, memory corruption is possible. Ideally, it should be as difficult as possible for one client to crash another. I'm only worried about buggy clients, not malicious ones, so imperfect solutions are allowed. I can try to stop clients from overwriting by declaring the members in the header they use as const, but that won't prevent memory corruption (buffer overflows, bad casts, etc.) from overwriting. I can insert canaries, but then I have to constantly pay the cost of checking them. Instead of storing the reference count member directly, I could store a pointer to the actual data in a separate mapped write only page, while keeping the structs in read only mapped pages. This will work, the OS will force my application to crash if I try to write to the pointed to data, but indirect storage can be undesirable when trying to write lock free algorithms, because needing to follow another level of indirection can change whether something can be done atomically. Is there any way to mark smaller areas of memory such that writing them will cause your app to blow up? Some platforms have hardware watchpoints, and maybe I could activate one of those with inline assembly, but I'd be limited to only 4 at a time on 32-bit x86 and each one could only cover part of the struct because they're limited to 4 bytes. It'd also make my program painful to debug ;)

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