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  • Difference between an LL and Recursive Descent parser?

    - by Noldorin
    I've recently being trying to teach myself how parsers (for languages/context-free grammars) work, and most of it seems to be making sense, except for one thing. I'm focusing my attention in particular on LL(k) grammars, for which the two main algorithms seem to be the LL parser (using stack/parse table) and the Recursive Descent parser (simply using recursion). As far as I can see, the recursive descent algorithm works on all LL(k) grammars and possibly more, whereas an LL parser works on all LL(k) grammars. A recursive descent parser is clearly much simpler than an LL parser to implement, however (just as an LL one is simply than an LR one). So my question is, what are the advantages/problems one might encounter when using either of the algorithms? Why might one ever pick LL over recursive descent, given that it works on the same set of grammars and is trickier to implement? Hopefully this question makes some amount of sense. Sorry if it doesn't - I blame my the fact that this entire subject is almost entirely new to me.

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  • Curiosity’s Descent to Mars in HD [Video]

    - by Jason Fitzpatrick
    Thanks to the Mars Descent Imager (MARDI) mounted on the fore-port side of Curiosity we’re treated to a high-resolution video of Curiosity’s descent to Mars. The video from MARDI, combined with the video editing of YouTube user DLFitch, yields the above video. He writes: This is a full-resolution version of the NASA Curiosity rover descent to Mars, taken by the MARDI descent imager. As of August 20, all but a dozen 1600×1200 frames have been uploaded from the rover, and those missing were interpolated using thumbnail data. The result was applied a heavy noise reduction, color balance, and sharpening for best visibility. The video plays at 15fps, or 3x realtime. The heat shield impacts in the lower left frame at 0:21, and is shown enlarged at the end of the video. Image source. Complete MSL Curiosity Descent – Full Quality Enhanced 1080p + Heat Shield impact [YouTube] HTG Explains: Is UPnP a Security Risk? How to Monitor and Control Your Children’s Computer Usage on Windows 8 What Happened to Solitaire and Minesweeper in Windows 8?

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  • Good grammar for date data type for recursive descent parser LL(1)

    - by Totophil
    I'm building a custom expression parser and evaluator for production enviroment to provide a limited DSL to the users. The parser itself as the DSL, need to be simple. The parser is going to be built in an exotic language that doesn't support dynamic expression parsing nor has any parser generator tools available. My decision is to go for recursive descent approach with LL(1) grammar, so that even programmers with no previous experience in evaluating expression could quickly learn how the code works. It has to handle mixed expressions made up of several data types: decimals, percentages, strings and dates. And dates in the format of dd/mm/yyyy are easy to confuse with a string of devision ops. Is where a good solution to this problem? My own solution that is aimed at keeping the parser simple involves prefixing dates with a special symbol, let's say apostrophe: <date> ::= <apostr><digit><digit>/<digit><digit>/<digit><digit><digit><digit> <apostr> ::= ' <digit> ::= '0'..'9'

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  • Is it easier to write a recursive-descent parser using an EBNF or a BNF?

    - by Vivin Paliath
    I've got a BNF and EBNF for a grammar. The BNF is obviously more verbose. I have a fairly good idea as far as using the BNF to build a recursive-descent parser; there are many resources for this. I am having trouble finding resources to convert an EBNF to a recursive-descent parser. Is this because it's more difficult? I recall from my CS theory classes that we went over EBNFs, but we didn't go over converting them into a recursive-descent parser. We did go over converting BNF's into a recursive-descent parser. The reason I'm asking is because the EBNF is more compact. From looking at the EBNF's in general, I notice that terms enclosed between { and } can be converted into a while loop. Are there any other guidelines or rules?

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  • Wrong bbox or descent with @font-face and opentype font

    - by wowpatrick
    Hey all, I embedded a opentype font with @font-face. Works fine, but the bbox or descent (or baseline/descent line?) is wrong on certain browsers/OSs. The problem is e.g. on a Mac the bbox is not right, that means to center the text in it's bbox I have to set a padding to the paragraph. I guess the font was created on Windows and was not tested on Linux and OS X, and the font rendering engines in these OSs work diffrently. Any idea how to solve withs problem? Screenshot of the font in Windows, Linux and Mac. Form left to right: Ubuntu Firefox 3, Chromium, Windows XP Firefox 3/Chrome, Mac OS X Safari 5/Firefox 4 Beta 8. Screenshot of the fonts

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  • How does a segment-based rendering engine (as in Descent) work?

    - by Calmarius
    As far as I know Descent was one of the first games that featured a fully 3D environment, and it used a segment based rendering engine. Its levels are built from cubic segments (these cubes may be deformed as long as it remains convex and sides remain roughly flat). These cubes are connected by their sides. The connected sides are traversable (maybe doors or grids can be placed on these sides), while the unconnected sides are not traversable walls. So the game is played inside of this complex. Descent was software rendered and it had to be very fast, to be playable on those 10-100MHz processors of that age. Some latter levels of the game are huge and contain thousands of segments, but these levels are still rendered reasonably fast. So I think they tried to minimize the amount of cubes rendered somehow. How to choose which cubes to render for a given location? As far as I know they used a kind of portal rendering, but I couldn't find what was the technique used in this particular kind of engine. I think the fact that the levels are built from convex quadrilateral hexahedrons can be exploited.

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  • Converting EBNF to BNF

    - by Vivin Paliath
    It's been a few years since my computer-language class and so I've forgotten the finer points of BNF's and EBNF's and I don't have a textbook next to me. Specifically, I've forgotten how to convert an EBNF into BNF. From what little I remember, I know that one of the main points is to convert { term } into <term> | <many-terms>. But I don't remember the other rules. I've tried to look this up online but I can only find links to either homework questions, or a small comment about converting terms with curly braces. I can't find an exhaustive list of rules that define the translation.

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  • boost::Spirit Grammar for unsorted schema

    - by Hassan Syed
    I have a section of a schema for a model that I need to parse. Lets say it looks like the following. { type = "Standard"; hostname="x.y.z"; port="123"; } The properties are: The elements may appear unordered. All elements that are part of the schema must appear, and no other. All of the elements' synthesised attributes go into a struct. (optional) The schema might in the future depend on the type field -- i.e., different fields based on type -- however I am not concerned about this at the moment.

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  • Can Haskell's Parsec library be used to implement a recursive descent parser with backup?

    - by Thor Thurn
    I've been considering using Haskell's Parsec parsing library to parse a subset of Java as a recursive descent parser as an alternative to more traditional parser-generator solutions like Happy. Parsec seems very easy to use, and parse speed is definitely not a factor for me. I'm wondering, though, if it's possible to implement "backup" with Parsec, a technique which finds the correct production to use by trying each one in turn. For a simple example, consider the very start of the JLS Java grammar: Literal: IntegerLiteral FloatingPointLiteral I'd like a way to not have to figure out how I should order these two rules to get the parse to succeed. As it stands, a naive implementation like this: literal = do { x <- try (do { v <- integer; return (IntLiteral v)}) <|> (do { v <- float; return (FPLiteral v)}); return(Literal x) } Will not work... inputs like "15.2" will cause the integer parser to succeed first, and then the whole thing will choke on the "." symbol. In this case, of course, it's obvious that you can solve the problem by re-ordering the two productions. In the general case, though, finding things like this is going to be a nightmare, and it's very likely that I'll miss some cases. Ideally, I'd like a way to have Parsec figure out stuff like this for me. Is this possible, or am I simply trying to do too much with the library? The Parsec documentation claims that it can "parse context-sensitive, infinite look-ahead grammars", so it seems like something like I should be able to do something here.

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  • Is there any descent open-source search engine solutions?

    - by Nazariy
    Few weeks ago my friend asked me how hard is it to launch your own search engine service with list of websites that suppose to be crawled time to time. First what come at my mind was Google Custom Search however pricing policy is quite tricky and would drain your budget if you reach 500K queries per year. Another solution I found here was SearchBlox, which can be compared to Google Mini service. It's quite good solution if you planing to cover search over small amount of websites but for larger projects it is not very handy. I also found few other search platforms like Lucene, Hadoop and Xapian which seems to be quite powerful solutions to reach Google search quality, and Nutch as a web crawler. As most of open-source projects they share same problem, luck of comprehensive guidance of usage, examples and it's expected that you are expert in this subject. I'm wondering if any of you using this solutions, which of them would you recommend, and what should I be aware of?

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  • Where should I initialize variables for an OO Recursive Descent Parse Tree?

    - by Vasto
    I'd like to preface this by stating that this is for a class, so please don't solve this for me. One of my labs for my cse class is creating an interpreter for a BNF that was provided. I understand most of the concepts, but I'm trying to build up my tree and I'm unsure where to initialize values. I've tried in both the constructor, and in the methods but Eclipse's debugger still only shows the left branch, even though it runs through completely. Here is my main procedure so you can get an idea of how I'm calling the methods. public class Parser { public static void main(String[] args) throws IOException { FileTokenizer instance = FileTokenizer.Instance(); FileTokenizer.main(args); Prog prog = new Prog(); prog.ParseProg(); prog.PrintProg(); prog.ExecProg(); } Now here is My Prog class: public class Prog { private DeclSeq ds; private StmtSeq ss; Prog() { ds = new DeclSeq(); ss = new StmtSeq(); } public void ParseProg() { FileTokenizer instance = FileTokenizer.Instance(); instance.skipToken(); //Skips program (1) // ds = new DeclSeq(); ds.ParseDS(); instance.skipToken(); //Skips begin (2) // ss = new StmtSeq(); ss.ParseSS(); instance.skipToken(); } I've tried having Prog() { ds = null; ss = null; } public void ParseProg() { FileTokenizer instance = FileTokenizer.Instance(); instance.skipToken(); //Skips program (1) ds = new DeclSeq(); ds.ParseDS(); ... But it gave me the same error. I need the parse tree built up so I can do a pretty print and an execute command, but like I said, I only get the left branch. Any help would be appreciated. Explanations why are even more so appreciated. Thank you, Vasto

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  • Python, web log data mining for frequent patterns

    - by descent
    Hello! I need to develop a tool for web log data mining. Having many sequences of urls, requested in a particular user session (retrieved from web-application logs), I need to figure out the patterns of usage and groups (clusters) of users of the website. I am new to Data Mining, and now examining Google a lot. Found some useful info, i.e. querying Frequent Pattern Mining in Web Log Data seems to point to almost exactly similar studies. So my questions are: Are there any python-based tools that do what I need or at least smth similar? Can Orange toolkit be of any help? Can reading the book Programming Collective Intelligence be of any help? What to Google for, what to read, which relatively simple algorithms to use best? I am very limited in time (to around a week), so any help would be extremely precious. What I need is to point me into the right direction and the advice of how to accomplish the task in the shortest time. Thanks in advance!

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  • Neural Network with softmax activation

    - by Cambium
    This is more or less a research project for a course, and my understanding of NN is very/fairly limited, so please be patient :) ============== I am currently in the process of building a neural network that attempts to examine an input dataset and output the probability/likelihood of each classification (there are 5 different classifications). Naturally, the sum of all output nodes should add up to 1. Currently, I have two layers, and I set the hidden layer to contain 10 nodes. I came up with two different types of implementations 1) Logistic sigmoid for hidden layer activation, softmax for output activation 2) Softmax for both hidden layer and output activation I am using gradient descent to find local maximums in order to adjust the hidden nodes' weights and the output nodes' weights. I am certain in that I have this correct for sigmoid. I am less certain with softmax (or whether I can use gradient descent at all), after a bit of researching, I couldn't find the answer and decided to compute the derivative myself and obtained softmax'(x) = softmax(x) - softmax(x)^2 (this returns an column vector of size n). I have also looked into the MATLAB NN toolkit, the derivative of softmax provided by the toolkit returned a square matrix of size nxn, where the diagonal coincides with the softmax'(x) that I calculated by hand; and I am not sure how to interpret the output matrix. I ran each implementation with a learning rate of 0.001 and 1000 iterations of back propagation. However, my NN returns 0.2 (an even distribution) for all five output nodes, for any subset of the input dataset. My conclusions: o I am fairly certain that my gradient of descent is incorrectly done, but I have no idea how to fix this. o Perhaps I am not using enough hidden nodes o Perhaps I should increase the number of layers Any help would be greatly appreciated! The dataset I am working with can be found here (processed Cleveland): http://archive.ics.uci.edu/ml/datasets/Heart+Disease

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  • How does a segment based rendering engine work?

    - by Calmarius
    As far as I know Descent was one of the first games that featured a fully 3D environment, and it used a segment based rendering engine. Its levels are built from cubic segments (these cubes may be deformed as long as it remains convex and sides remain roughly flat). These cubes are connected by their sides. The connected sides are traversable (maybe doors or grids can be placed on these sides), while the unconnected sides are not traversable walls. So the game is played inside of this complex. Descent was software rendered and it had to be very fast, to be playable on those 10-100MHz processors of that age. Some latter levels of the game are huge and contain thousands of segments, but these levels are still rendered reasonably fast. So I think they tried to minimize the amount of cubes rendered somehow. How to choose which cubes to render for a given location? As far as I know they used a kind of portal rendering, but I couldn't find what was the technique used in this particular kind of engine. I think the fact that the levels are built from convex quadrilateral hexahedrons can be exploited.

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  • Cannot get right height of text in java.awt.BufferdImage/Graphics2D

    - by Tommy
    Im creating a servlet that renders a jpg/png with a given text. I want the text to be centered on the rendered image. I can get the width, but the height i'm getting seems to be wrong Font myfont = new Font(Font.SANS_SERIF, Font.BOLD, 400); BufferedImage image = new BufferedImage(500, 500, BufferedImage.TYPE_INT_ARGB); Graphics2D g = image.createGraphics(); g.setFont(myfont); g.setColor(Color.BLACK); FontMetrics fm = g.getFontMetrics(); Integer textwidth = fm.stringWidth(imagetext); Integer textheight = fm.getHeight(); FontRenderContext fr = g.getFontRenderContext(); LineMetrics lm = myfont.getLineMetrics("5", fr ); float ascent = lm.getAscent(); float descent = lm.getDescent(); float height = lm.getHeight(); g.drawString("5", ((imagewidth - textwidth) / 2) , y?); g.dispose(); ImageIO.write(image, "png", outputstream); These are the values I get: textwidth = 222 textheight = 504 ascent = 402 descent = 87 height = 503 Anyone know how to get the exact height om the "5" ? The estimated height should be around 250

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  • Can We Survive the Sun’s Death?

    - by Akemi Iwaya
    In the distant future, our sun will begin its descent into death after using up all of the hydrogen fuel in its core. When that happens, the inner parts of our solar system will suffer horrible consequences. But what will happen at that point in time and how quickly will things ‘deteriorate’? Is there anything that could be done to help our planet survive? AsapSCIENCE looks at this ‘hot’ topic in their latest video. Can We Survive The Sun’s Death?     

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  • What should developers know about Windows executable binary file compression?

    - by Peter Turner
    I'd never heard of this before, so shame on me, but programs like UPX can compress my files by 80% which is totally sweet, but I have no idea what the the disadvantages are in doing this. Or even what the compressor does. Website linked above doesn't say anything about dynamically linking DLLs but it mentions about compressing DESCENT 2 and about compressing Netscape 4.06. Also, it doesn't say what the tradeoffs are, only the benefits. If there weren't tradeoffs why wouldn't my linker compress the file? If I have an environment where I have one executable and 20-30 DLL's, some of which are dynamically loaded an unloaded fairly arbitrarily, but not in loops (hopefully), do I take a big hit in processing time decompressing these DLL's when they're used?

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  • Lag compensation of projectile shooting game

    - by Denis Ermolin
    I'm thinking about an algorithm for firing projectiles with lag compensation. Now I did find only one descent solution: Player hits fire button. Client sends input "fire". Client waits for server response. Server generates bullet then sends response to client. Client recieves response and finally fires projectile. Is this solution only "trueway"? I find it the only one that can be fair to all of the clients. Valve in this case, doesn't compensate lag from rocket shots. I am feeling that I will not compensate it, too. I think that with today's bandwidth I can close my eyes on this problem, because I don't see any solutions with fair logic. What do you think?

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  • How to survive if you can only do things your way as a programmer?

    - by niceguyjava
    I hate hibernate, I hate spring and I am the kind of programmer who likes to do things his way. I hate micro-management and other people making decisions for me about what framework I should use, what patterns I should apply (hate patterns too) and what architecture I should design. I consider myself a successful programmer and have a descent financial situation due to my performance in past jobs, but I just can't take the standard Java jobs out there. I really love to design things from scratch and hate when I have to maintain other people's bad code, design and architecture, which is the majority you find out there for sure. Does anybody relate to that? What do you guys recommend me? Open up my on company, do consulting, or just keep looking hard until I find a job that suits my preferences, as hard as this may look like with all the hibernate and spring crap out there?

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  • Dijkstra’s algorithm and functions

    - by baris_a
    Hi guys, the question is: suppose I have an input function like sin(2-cos(3*A/B)^2.5)+0.756*(C*D+3-B) specified with a BNF, I will parse input using recursive descent algorithm, and then how can I use or change Dijkstra’s algorithm to handle this given function? After parsing this input function, I need to execute it with variable inputs, where Dijkstra’s algorithm should do the work. Thanks in advance. EDIT: May be I should ask also: What is the best practice or data structure to represent given function?

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  • Prevent image cropping when making sprites from gif

    - by OSaad
    Hey guys I've tried several tools (imagemagic, gif2png, Nconverter) to extract frames to make sprites from a .gif image that i have. I get the .pngs just fine, but they'r not the same size, some are 50x65 some 43x65 some 50x70, Which really screws any attempts at a descent animation. So is there a way to prevent this cropping or programmatically add extra transparent space to smaller ones and make them all the same size? Thanks.

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  • Efficiently solving sparse matrices

    - by anon
    For solving spare matrices, in general, how big does the matrix have to be (as a rule of thumb) for methods like congraduate descent to be faster than brute force solvers (that do not take advantage o sparsity)?

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  • Is this grammar SLR?

    - by Mike
    E - A | B A - a | c B - b | c My answer is no because it has a reduce/reduce conflict, can anyone else verify this? Also I gained my answer through constructing the transition diagram, is there a simpler way of finding this out? Thanks for the help! P.S Would a Recursive Descent be able to parse this?

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