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• What are the lesser known but cool data structures ?

  - by f3lix

  There a some data structures around that are really cool but are unknown to most programmers. Which are they? Everybody knows linked lists, binary trees, and hashes, but what about Skip lists, Bloom filters for example. I would like to know more data structures that are not so common, but are worth knowing because they rely on great ideas and enrich a programmer's tool box. PS: I am also interested on techniques like Dancing links which make interesting use of the properties of a common data structure. EDIT: Please try to include links to pages describing the data structures in more detail. Also, try to add a couple of words on why a data structures is cool (as Jonas Kölker already pointed out). Also, try to provide one data-structure per answer. This will allow the better data structures to float to the top based on their votes alone.

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• Is it possible to have regexp that matches all valid regular expressions?

  - by Juha Syrjälä

  Is it possible to detect if a given string is valid regular expression, using just regular expressions? Say I have some strings, that may or may not be a valid regular expressions. I'd like to have a regular expression matches those string that correspond to valid regular expression. Is that possible? Or do I have use some higher level grammar (i.e. context free language) to detect this? Does it affect if I am using some extended version of regexps like Perl regexps? If that is possible, what the regexp matching regexp is?

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• Discrete Curve evolution algorithm

  - by konidela

  hai , i am trying to implement discrete curve evolution algorithm in c++ do any one help me with psudo code or c code or some simple steps of your understanding

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• Balancing a Binary Tree (AVL)

  - by Gustavo Carreno

  Ok, this is another one in the theory realm for the CS guys around. In the 90's I did fairly well in implementing BST's. The only thing I could bever get my head around was the intricacy of the algorithm to balance a Binary Tree (AVL). Can you guys help me on this?

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• What is the most easy way to get in advanced Type Theory.

  - by Bubba88

  Of course, by 'advanced' I mean here just something beyond what every programmer does know. I'm currently more-or-less comfortable with the basics and want to understand the most important, most elegant and most practically applicable achievements of modern type theory. I just do not have much time, desire and mental powers to study all the formalistics more thoroughly and that may change in the future. But there is something really attractive for me in that branch, that just forces to ask silly questions like this :) Thank you very much!

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• How do I get rid of this drive mount confirmation question when booting the computer?

  - by Dave M G

  With help from this site, I was able to set up an SSHFS connection between two computers on my LAN so that one auto mounts on the other at boot time. Everything works, but there is this annoying confirmation that comes up whenever I boot: An error occurred while mounting /home/dave/Mythbuntu. Press S to skip mounting or M or Manual recovery If I press S, then booting continues, and my drive is mounted as hoped, so it seems like even though I "skipped" it, maybe it tried again and succeeded later in the boot process. I followed the instructions here to set up "if up / if down" scripts, and here is my current /etc/fstab: sshfs#[email protected]:/home/mythbuntu /home/dave/Mythbuntu fuse auto,users,exec,uid=1000,gid=1000,allow_other,reconnect,transform_symlinks,BatchMode=yes 0 0 Although the mounting is working, this step of having to press S every time I boot is obviously kind of a hassle. How do I configure my computer so I don't have to do that, and so that my other computer will still automount?

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• permutation and/ or combination of california license plate

  - by stdnoit

  just wondering california's license plate is 0XXX0000 number, 3 alphabets and 4 numbers. how many permutation / combinations of total different license plate it could be ? i learned this in discrete structure but I have forgotten

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• Fitch Format Proofs - any resources around?

  - by devoured elysium

  I am currently studying Fitch Format first order logic proofs. My lecturer follows closely Language, Proof and Logic by Jon Barwise. I am trying to do some proofs but I am having some trouble getting to understand how to do these proofs. As I have already read what Language Proof and Logic has to offer, I'd like to know if there are any other books or resources around that use the Fitch format for their formal proofs. Plus, having solved exercises would be of great(!) help. Thanks

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• Learning material on SAT

  - by Jules

  What are good documents to read on SAT solvers. I have not been able to find good material via Google. The documents I found were either birds eye view, too advanced or corrupted PDF files... Which papers/documents do you recommend to learn about the algorithms in modern practical SAT solvers?

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• What makes people think that NNs have more computational power than existing models?

  - by Bubba88

  I've read in Wikipedia that neural-network functions defined on a field of arbitrary real/rational numbers (along with algorithmic schemas, and the speculative `transrecursive' models) have more computational power than the computers we use today. Of course it was a page of russian wikipedia (ru.wikipedia.org) and that may be not properly proven, but that's not the only source of such.. rumors Now, the thing that I really do not understand is: How can a string-rewriting machine (NNs are exactly string-rewriting machines just as Turing machines are; only programming language is different) be more powerful than a universally capable U-machine? Yes, the descriptive instrument is really different, but the fact is that any function of such class can be (easily or not) turned to be a legal Turing-machine. Am I wrong? Do I miss something important? What is the cause of people saying that? I do know that the fenomenum of undecidability is widely accepted today (though not consistently proven according to what I've read), but I do not really see a smallest chance of NNs being able to solve that particular problem. Add-in: Not consistently proven according to what I've read - I meant that you might want to take a look at A. Zenkin's (russian mathematician) papers after mid-90-s where he persuasively postulates the wrongness of G. Cantor's concepts, including transfinite sets, uncountable sets, diagonalization method (method used in the proof of undecidability by Turing) and maybe others. Even Goedel's incompletness theorems were proven in right way in only 21-st century.. That's all just to plug Zenkin's work to the post cause I don't know how widespread that knowledge is in CS community so forgive me if that did look stupid. Thank you!

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• A Simulator for a non-deterministic Push-Down Automaton

  - by shake

  Well, i need to make simulator for non-deterministic Push-Down Automaton. Everything is okey, i know i need to do recursion or something similar. But i do not know how to make that function which would simulate automaton. I got everything else under control, automaton generator, stack ... I am doing it in java, so this is maybe only issue that man can bump on, and i did it. So if anyone have done something similar, i could use advices. This is my current organisation of code: C lasses: class transit: list -contains non deterministic transitions state input sign stack sign class generator it generate automaton from file clas NPA public boolean start() - this function i am having trouble with Of course problem of separate stacks, and input for every branch. I tried to solve it with collection of objects NPA and try to start every object, but it doesn work :((..

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• How do I implement graphs and graph algorithms in a functional programming language?

  - by brad

  Basically, I know how to create graph data structures and use Dijkstra's algorithm in programming languages where side effects are allowed. Typically, graph algorithms use a structure to mark certain nodes as 'visited', but this has side effects, which I'm trying to avoid. I can think of one way to implement this in a functional language, but it basically requires passing around large amounts of state to different functions, and I'm wondering if there is a more space-efficient solution.

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• Specifying Language for a grammar

  - by darkie15

  Hi All, Is there any specific methodology followed to specify a language for given grammar ?? i.e. Is it necessary to run all the production rules given in a grammar to determine the language it represents? I don't have an example as such since the one I am working on is a homework question. Regards, darkie15

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• Calculating rotation and translation matrices between two odometry positions for monocular linear triangulation

  - by user1298891

  Recently I've been trying to implement a system to identify and triangulate the 3D position of an object in a robotic system. The general outline of the process goes as follows: Identify the object using SURF matching, from a set of "training" images to the actual live feed from the camera Move/rotate the robot a certain amount Identify the object using SURF again in this new view Now I have: a set of corresponding 2D points (same object from the two different views), two odometry locations (position + orientation), and camera intrinsics (focal length, principal point, etc.) since it's been calibrated beforehand, so I should be able to create the 2 projection matrices and triangulate using a basic linear triangulation method as in Hartley & Zissermann's book Multiple View Geometry, pg. 312. Solve the AX = 0 equation for each of the corresponding 2D points, then take the average In practice, the triangulation only works when there's almost no change in rotation; if the robot even rotates a slight bit while moving (due to e.g. wheel slippage) then the estimate is way off. This also applies for simulation. Since I can only post two hyperlinks, here's a link to a page with images from the simulation (on the map, the red square is simulated robot position and orientation, and the yellow square is estimated position of the object using linear triangulation.) So you can see that the estimate is thrown way off even by a little rotation, as in Position 2 on that page (that was 15 degrees; if I rotate it any more then the estimate is completely off the map), even in a simulated environment where a perfect calibration matrix is known. In a real environment when I actually move around with the robot, it's worse. There aren't any problems with obtaining point correspondences, nor with actually solving the AX = 0 equation once I compute the A matrix, so I figure it probably has to do with how I'm setting up the two camera projection matrices, specifically how I'm calculating the translation and rotation matrices from the position/orientation info I have relative to the world frame. How I'm doing that right now is: Rotation matrix is composed by creating a 1x3 matrix [0, (change in orientation angle), 0] and then converting that to a 3x3 one using OpenCV's Rodrigues function Translation matrix is composed by rotating the two points (start angle) degrees and then subtracting the final position from the initial position, in order to get the robot's straight and lateral movement relative to its starting orientation Which results in the first projection matrix being K [I | 0] and the second being K [R | T], with R and T calculated as described above. Is there anything I'm doing really wrong here? Or could it possibly be some other problem? Any help would be greatly appreciated.

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• Oracle Virtualization Friday Spotlight - November 8, 2013

  - by Monica Kumar

  Hands-on Private Cloud Simulator In One Hour Submitted by: Doan Nguyen, Senior Principal Product Marketing Director My aeronautics instructor used to say, "you can’t appreciate flying until you take flight." To clarify, this is not about gearing up in a flying squirrel suit and hopping off a cliff (topic for another blog!) but rather about flying an airplane. The idea is to get hands-on with the controls at the cockpit and experience flight before you actually fly a real plane. After the initial 40 hours of flight time, the concept sank in and it really made sense.This concept is what inspired our technical experts to put together the hands-on lab for a private cloud deployment and management self-service model. Yes, we are comparing the lab to a flight simulator! Let’s look at the parallels: To get trained to fly, starting in the simulator gets you off the ground quicker. There is no need to have a real plane to begin with. In a hands-on lab, there is no need for a real server, with networking and real storage installed. All you need is your laptop The simulator is pre-configured, pre-flight check done. Similarly, in a hands-on lab, Oracle VM and Oracle Enterprise Manager are pre-configured and assembled using Oracle VM VirtualBox as the container. Software installations are not needed. After time spent training at the controls, you can really appreciate the practical experience of flying. Along the same lines, the hands-on lab is a guided learning path, without the encumbrances of hardware, software installation, so you can learn about cloud deployment and management.  However, unlike the simulator training, your time investment with the lab is only about an hour and not 40 hours! This hands-on lab takes you through private cloud deployment and management using Oracle VM and  Oracle Enterprise Manager Cloud Control 12c in an Infrastructure as a service IaaS model. You will first configure the IaaS cloud as the cloud administrator and then deploy guest virtual machines (VMs) as a self-service user. Then you are ready to take flight into the cloud! Why not step into the cockpit now!

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• Why does my computer just keep restarting while trying to install Ubuntu 13.04 (x86) from USB alongside Windows 7?

  - by Sumit Chahal

  I have checked the other questions, but none of them match my problem (the ones that do don't have satisfactory answers). I downloaded the Ubuntu 13.04 iso and burned it to my USB device following the given instructions. I was also able to boot the Ubuntu setup from it by changing the boot sequence so that the USB device is now at the top. But I am only able to use the "trial" version of Ubuntu. Every time I tick "Install Ubuntu alongside Windows" and click "Continue", the computer just restarts and I see the same Try-Ubuntu-or-install-it menu. I have tried pressing any key when that little icon appears at the bottom, but it is not at all helpful. I also read somewhere that I should eject my USB device after I click "Continue", but doing so just makes the computer run Windows 7 instead.

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• What problems have you solved using constraint programming?

  - by knorv

  I'd like to know about specific problems you - the SO reader - have solved using constraint programming and what constraint logic language you used. Questions: What problems have you used constraint programming to solve? What constraint logic language did you use? I'm looking for first-hand experiences, so please do not answer unless you have that.

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• What is better for a student programming in C++ to learn for writing GUI: C# vs QT?

  - by flashnik

  I'm a teacher(instructor) of CS in the university. The course is based on Cormen and Knuth and students program algorithms in C++. But sometimes it is good to show how an algorithm works or just a result of task through GUI. Also in my opinion it's very imporant to be able to write full programs. They will have courses concerning GUI but a three years, later, in fact, before graduatuion. I think that they should be able to write simple GUI applications earlier. So I want to teach them it. How do you think, what is more useful for them to learn: programming GUI with QT or writing GUI in C# and calling unmanaged C++ library?

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• Do invariant assertions fit into C# programming?

  - by P.Brian.Mackey

  In the book coders at work, the author asks "How do you use invariants in your code". Please explain what this question means. I saw class invariants on wiki, but the example is in Java and I am not skilled enough in Java to relate this example to C#. .NET 4.0 introduces invariance, covariance, and contravariance and is well explained here. Invariance is so broad. The authors usage of the word seems unit test related. For those that read the book, what does the author mean? Are we talking about making an assumption and simply testing the validity after the unit test?

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• List of fundamental data structures - what am I missing?

  - by jboxer

  I've been studying my fundamental data structures a bunch recently, trying to make sure I've got them down cold. By "fundamental", I mean the real basic ones. Fancy ones like Red-Black Trees and Bloom Filters are clearly worth knowing, but they're usually either enhancements of fundamental ones (Red-Black Trees are binary search trees with special properties to keep them balanced) or they're only useful in very specific situations (Bloom Filters). So far, I'm "fluent" in the following data structures: Arrays Linked Lists Stacks/Queues Binary Search Trees Heaps/Priority Queues Hash Tables However, I feel like I'm missing something. Are there any fundamental ones that I'm forgetting about? EDIT: Added these after posting the question Strings (suggested by catchmeifyoutry) Sets (suggested by Peter) Graphs (suggested by Nick D and aJ) B-Trees (Suggested by tloach) I'm a little on-the-fence about whether these are too fancy or not, but I think they're different enough from the fundamental structures (and important enough) to be worth studying as fundamental.

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• Learning material on SAT (Boolean Satisfiability Problem)

  - by Jules

  What are good documents to read on SAT (Boolean satisfiability problem) solvers. I have not been able to find good material via Google. The documents I found were either birds eye view, too advanced or corrupted PDF files... Which papers/documents do you recommend to learn about the algorithms in modern practical SAT solvers?

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• Why are GPUs cheaper than CPUs? [closed]

  - by Lucas

  What are the technological reasons whereby GPUs are cheaper than CPUs?

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• minimum enclosing rectangle of fixed aspect ratio

  - by Ramya Narasimha

  I have an Image with many rectangles at different positions in the image and of different sizes (both overlapping and non-overlapping). I also have a non-negative scores associated with each of these rectangles. My problem now is to find one larger rectangle *of a fixed (given) aspect ratio* that encloses as many of these rectangles as possible. I am looking for an algorithm to do this, if anyone has a solution, even a partial one it would be helpful. Please note that the positions of the rectangles in the image is fixed and cannot be moved around and there is no orientation issue as all of them are upright.

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• Which is the best algorithm to "Estimate and Visulize 2d skeleton using Opencv" from the drawn conto

  - by kumar

  Which is the best algorithm to "Estimate and Visulize 2d skeleton using Opencv" from the drawn contour Is the Recursive Centroid algorithm the Best? any reference links or docs please provide

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• Good way to identify similar images?

  - by Nick

  I've developed a simple and fast algorithm in PHP to compare images for similarity. Its fast (~40 per second for 800x600 images) to hash and a unoptimised search algorithm can go through 3,000 images in 22 mins comparing each one against the others (3/sec). The basic overview is you get a image, rescale it to 8x8 and then convert those pixels for HSV. The Hue, Saturation and Value are then truncated to 4 bits and it becomes one big hex string. Comparing images basically walks along two strings, and then adds the differences it finds. If the total number is below 64 then its the same image. Different images are usually around 600 - 800. Below 20 and extremely similar. Are there any improvements upon this model I can use? I havent looked at how relevant the different components (hue, saturation and value) are to the comparison. Hue is probably quite important but the others? To speed up searches I could probably split the 4 bits from each part in half, and put the most significant bits first so if they fail the check then the lsb doesnt need to be checked at all. I dont know a efficient way to store bits like that yet still allow them to be searched and compared easily. I've been using a dataset of 3,000 photos (mostly unique) and there havent been any false positives. Its completely immune to resizes and fairly resistant to brightness and contrast changes.

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