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  • What if we run out of stack space in C# or Python?

    - by dotneteer
    Supposing we are running a recursive algorithm on a very large data set that requires, say, 1 million recursive calls. Normally, one would solve such a large problem by converting recursion to a loop and a stack, but what if we do not want to or cannot rewrite the algorithm? Python has the sys.setrecursionlimit(int) method to set the number of recursions. However, this is only part of the story; the program can still run our of stack space. C# does not have a equivalent method. Fortunately, both C# and Python have option to set the stack space when creating a thread. In C#, there is an overloaded constructor for the Thread class that accepts a parameter for the stack size: Thread t = new Thread(work, stackSize); In Python, we can set the stack size by calling: threading.stack_size(67108864) We can then run our work under a new thread with increased stack size.

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  • Count unique visitors by group of visited places

    - by Mathieu
    I'm facing the problem of counting the unique visitors of groups of places. Here is the situation: I have visitors that can visit places. For example, that can be internet users visiting web pages, or customers going to restaurants. A visitor can visit as much places as he wishes, and a place can be visited by several visitors. A visitor can come to the same place several times. The places belong to groups. A group can obviously contain several places, and places can belong to several groups. Given that, for each visitor, we can have a list of visited places, how can I have the number of unique visitors per group of places? Example: I have visitors A, B, C and D; and I have places x, y and z. I have these visiting lists: [ A -> [x,x,y,x], B -> [], C -> [z,z], D -> [y,x,x,z] ] Having these number of unique visitors per place is quite easy: [ x -> 2, // A and D visited x y -> 2, // A and D visited y z -> 2 // C and D visited z ] But if I have these groups: [ G1 -> [x,y,z], G2 -> [x,z], G3 -> [x,y] ] How can I have this information? [ G1 -> 3, // A, C and D visited x or y or z G2 -> 3, // A, C and D visited x or z G3 -> 2 // A and D visited x or y ] Additional notes : There are so many places that it is not possible to store information about every possible group; It's not a problem if approximation are made. I don't need 100% precision. Having a fast algorithm that tells me that there were 12345 visits in a group instead of 12543 is better than a slow algorithm telling the exact number. Let's say there can be ~5% deviation. Is there an algorithm or class of algorithms that addresses this type of problem?

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  • yield – Just yet another sexy c# keyword?

    - by George Mamaladze
    yield (see NSDN c# reference) operator came I guess with .NET 2.0 and I my feeling is that it’s not as wide used as it could (or should) be.   I am not going to talk here about necessarity and advantages of using iterator pattern when accessing custom sequences (just google it).   Let’s look at it from the clean code point of view. Let's see if it really helps us to keep our code understandable, reusable and testable.   Let’s say we want to iterate a tree and do something with it’s nodes, for instance calculate a sum of their values. So the most elegant way would be to build a recursive method performing a classic depth traversal returning the sum.           private int CalculateTreeSum(Node top)         {             int sumOfChildNodes = 0;             foreach (Node childNode in top.ChildNodes)             {                 sumOfChildNodes += CalculateTreeSum(childNode);             }             return top.Value + sumOfChildNodes;         }     “Do One Thing” Nevertheless it violates one of the most important rules “Do One Thing”. Our  method CalculateTreeSum does two things at the same time. It travels inside the tree and performs some computation – in this case calculates sum. Doing two things in one method is definitely a bad thing because of several reasons: ·          Understandability: Readability / refactoring ·          Reuseability: when overriding - no chance to override computation without copying iteration code and vice versa. ·          Testability: you are not able to test computation without constructing the tree and you are not able to test correctness of tree iteration.   I want to spend some more words on this last issue. How do you test the method CalculateTreeSum when it contains two in one: computation & iteration? The only chance is to construct a test tree and assert the result of the method call, in our case the sum against our expectation. And if the test fails you do not know wether was the computation algorithm wrong or was that the iteration? At the end to top it all off I tell you: according to Murphy’s Law the iteration will have a bug as well as the calculation. Both bugs in a combination will cause the sum to be accidentally exactly the same you expect and the test will PASS. J   Ok let’s use yield! That’s why it is generally a very good idea not to mix but isolate “things”. Ok let’s use yield!           private int CalculateTreeSumClean(Node top)         {             IEnumerable<Node> treeNodes = GetTreeNodes(top);             return CalculateSum(treeNodes);         }             private int CalculateSum(IEnumerable<Node> nodes)         {             int sumOfNodes = 0;             foreach (Node node in nodes)             {                 sumOfNodes += node.Value;             }             return sumOfNodes;         }           private IEnumerable<Node> GetTreeNodes(Node top)         {             yield return top;             foreach (Node childNode in top.ChildNodes)             {                 foreach (Node currentNode in GetTreeNodes(childNode))                 {                     yield return currentNode;                 }             }         }   Two methods does not know anything about each other. One contains calculation logic another jut the iteration logic. You can relpace the tree iteration algorithm from depth traversal to breath trevaersal or use stack or visitor pattern instead of recursion. This will not influence your calculation logic. And vice versa you can relace the sum with product or do whatever you want with node values, the calculateion algorithm is not aware of beeng working on some tree or graph.  How about not using yield? Now let’s ask the question – what if we do not have yield operator? The brief look at the generated code gives us an answer. The compiler generates a 150 lines long class to implement the iteration logic.       [CompilerGenerated]     private sealed class <GetTreeNodes>d__0 : IEnumerable<Node>, IEnumerable, IEnumerator<Node>, IEnumerator, IDisposable     {         ...        150 Lines of generated code        ...     }   Often we compromise code readability, cleanness, testability, etc. – to reduce number of classes, code lines, keystrokes and mouse clicks. This is the human nature - we are lazy. Knowing and using such a sexy construct like yield, allows us to be lazy, write very few lines of code and at the same time stay clean and do one thing in a method. That's why I generally welcome using staff like that.   Note: The above used recursive depth traversal algorithm is possibly the compact one but not the best one from the performance and memory utilization point of view. It was taken to emphasize on other primary aspects of this post.

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  • Checking whether php script was resolved by optimal way

    - by user2135931
    Can anybody give some advise how to check the arbitrary php code on optimal solution. For example I create a simple algorithm and sent it on the special resource. After proccessing my algorithm this resource give me result whether my code is nice. If no it give me some advice and tell what is wrong (maybe I forgot check devision by zero etc). I looked for php code analyzer but could't find any variants. Maybe someone give me a resource where I can research this problem. Thanks in advance!

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  • Can anyone help solve this complex algorithmic problem?

    - by Locaaaaa
    I got this question in an interview and I was not able to solve it. You have a circular road, with N number of gas stations. You know the ammount of gas that each station has. You know the ammount of gas you need to GO from one station to the next one. Your car starts with 0. The question is: Create an algorithm, to know from which gas station you must start driving. As an exercise to me, I would translate the algorithm to C#.

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  • Narrow-phase collision detection algorithms

    - by Marian Ivanov
    There are three phases of collision detection. Broadphase: It loops between all objecs that can interact, false positives are allowed, if it would speed up the loop. Narrowphase: Determines whether they collide, and sometimes, how, no false positives Resolution: Resolves the collision. The question I'm asking is about the narrowphase. There are multiple algorithms, differing in complexity and accuracy. Hitbox intersection: This is an a-posteriori algorithm, that has the lowest complexity, but also isn't too accurate, Color intersection: Hitbox intersection for each pixel, a-posteriori, pixel-perfect, not accuratee in regards to time, higher complexity Separating axis theorem: This is used more often, accurate for triangles, however, a-posteriori, as it can't find the edge, when taking last frame in account, it's more stable Linear raycasting: A-priori algorithm, useful for semi-realistic-looking physics, finds the intersection point, even more accurate than SAT, but with more complexity Spline interpolation: A-priori, even more accurate than linear rays, even more coplexity. There are probably many more that I've forgot about. The question is, in when is it better to use SAT, when rays, when splines, and whether there is anything better.

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  • How can i get latency when using Game Center?

    - by Freddy
    I'm pretty new to network programming. Basically I'm using game center for making a relatively simple iPhone game using Game-center p2p. However i'm now working on a algorithm to improve the multiplayer performance. But, I need to know how long it took for a package to travel from one device to the another device (latency) for the algorithm to work good. As for now, I have solved the problem by sending a double with time interval since 1970 in the package and then I compare it with the time at the other device. However I have heard that the NSDate methods is connected to the internet, which also will cause latency so the time interval would not be perfectly correct. What is the ideal way to check for how long it take for a package to be sent?

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  • Application qos involving priority and bandwidth

    - by Steve Peng
    Our manager wants us to do applicaiton qos which is quite different from the well-known system qos. We have many services of three types, they have priorites, the manager wants to suspend low priority services requests when there are not enough bandwidth for high priority services. But if the high priority services requests decrease, the bandwidth for low priority services should increase and low priority service requests are allowed again. There should be an algorithm involving priority and bandwidth. I don't know how to design the algorithm, is there any example on the internet? Somebody can give suggestion? Thanks. UPDATE All these services are within a same process. We are setting the maximum bandwidth for the three types of services via ports of services via TC (TC is the linux qos tool whose name means traffic control).

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  • find second smallest element in Fibonacci Heap

    - by Longeyes
    I need to describe an algorithm that finds the second smallest element in a Fibonacci-Heap using the Operations: Insert, ExtractMin, DecreaseKey and GetMin. The last one is an algorithm previously implemented to find and return the smallest element of the heap. I thought I'd start by extracting the minimum, which results in its children becoming roots. I could then use GetMin to find the second smallest element. But it seems to me that I'm overlooking other cases because I don't know when to use Insert and DecreaseKey, and the way the question is phrased seems to suggest I should need them.

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  • How can calculus and linear algebra be useful to a system programmer?

    - by Victor
    I found a website saying that calculus and linear algebra are necessary for System Programming. System Programming, as far as I know, is about osdev, drivers, utilities and so on. I just can't figure out how calculus and linear algebra can be helpful on that. I know that calculus has several applications in science, but in this particular field of programming I just can't imagine how calculus can be so important. The information was on this site: http://www.wikihow.com/Become-a-Programmer Edit: Some answers here are explaining about algorithm complexity and optimization. When I made this question I was trying to be more specific about the area of System's Programming. Algorithm complexity and optimization can be applied to any area of programming not just System's Programming. That may be why I wasn't able to came up with such thinking at the time of the question.

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  • Change spring bean properties at configuration time

    - by Nick Gerakines
    In a spring servlet xml file, I'm using org.springframework.scheduling.quartz.SchedulerFactoryBean to regularly fire a set of triggers. <bean class="org.springframework.scheduling.quartz.SchedulerFactoryBean"> <property name="triggers"> <list> <ref local="AwesomeTrigger" /> <ref local="GreatTrigger" /> <ref local="FantasticTrigger"/> </list> </property> </bean> The issue is that in different environments, I don't want certain triggers firing. Is there a way to include some sort of configuration or variable defined either in my build.properties for the environment or in a spring custom context properties file that assists the bean xml to determine which triggers should be included in the list? That way, for example, AwesomeTrigger would be called in development but not qa.

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  • How can I store post rankings that change with time?

    - by Daniel Fein
    I'm trying to learn how to code a website algorithm like Reddit.com where there are thousands of posts that need to be ranked. Their ranking algorithm works like this (you don't have to read it, its more of a general question that I have): http://amix.dk/blog/post/19588 Right now I have posts stored in a database, I record their dates and they each have an upvotes and downvotes field so I'm storing their records. I want to figure out how do you store their rankings? When specific posts have ranking values, but they change with time, how could you store their rankings? If they aren't stored, do you rank every post every time a user loads the page? When would you store the posts? Do you run a cron job to automatically give every post a new value every x minutes? Do you store their value? Which is temporary. Maybe, until that post reaches its minimum score and is forgotten?

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  • How does a BSP tree work for Z sorting?

    - by Jenko
    I'm developing a 3D engine in software, and so I must compute Z sorting manually. I'm currently using the painters algorithm to sort triangles and then drawing them back-to-front. This causes artifacts that I'm trying to correct. Would using a dynamic BSP-tree ensure "correct Z sorting" of triangles? Why? Because the bounding volumes of triangles would be similar? Since I would have a single "world" BSP tree, would I have to remove and re-add any moved/scaled/rotated object into the tree? Is it possible to add triangles into a BSP tree without the expensive cutting process? Why do you need to cut triangles on the axis planes anyway? Is it faster to traverse a BSP tree from any angle, than to sort all tris each draw like the painters algorithm?

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  • Create association between informations

    - by Andrea Girardi
    I deployed a project some days ago that allow to extract some medical articles using the results of a questionnaire completed by a user. For instance, if I reply on questionnaire I'm affected by Diabetes type 2 and I'm a smoker, my algorithm extracts all articles related to diabetes bubbling up all articles contains information about Diabetes type 2 and smoking. Basically we created a list of topic and, for every topic we define a kind of "guideline" that allows to extract and order informations for a user. I'm quite sure there are some better way to put on relationship two content but I was not able to find them on network. Could you suggest my a model, algorithm or paper to better understand this kind of problem and that helps me to find a faster, and more accurate way to extract information for an user?

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  • Channelling an explosion along a narrow passage

    - by finnw
    I am simulating explosions in a 2D maze game. If an explosion occurs in an open area, it covers a circular region (this is the easy bit.) However if an explosion occurs in a narrow passage (i.e narrower than the blast range) then it should be "compressed", so that it goes further and also it should go around corners. Ultimately, unless it is completely boxed-in, then it should cover a constant number of pixels, spreading in whatever direction is necessary to reach this total area. I have tried using a shortest-path algorithm to pick the nearest N pixels to the origin avoiding walls, but the effect is exaggerated - the blast travels around corners too easily, making U-turns even when there is a clear path in another direction. I don't know whether this is realistic or not but it is counter-intuitive to players who assume that they can hide around a corner and the blast will take the path of least resistance in a different direction. Is there a well-known (and fast) algorithm for this?

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  • Continuous Collision Detection Techniques

    - by Griffin
    I know there are quite a few continuous collision detection algorithms out there , but I can't find a list or summary of different 2D techniques; only tutorials on specific algorithms. What techniques are out there for calculating when different 2D bodies will collide and what are the advantages / disadvantages of each? I say techniques and not algorithms because I have not yet decided on how I will store different polygons which might be concave or even have holes. I plan to make a decision on this based on what the algorithm requires (for instance if an algorithm breaks down a polygon into triangles or convex shapes I will simply store the polygon data in this form).

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  • Is Java much harder to "tweak" for performance compared with C/C++?

    - by user997112
    Does the "magic" of the JVM hinder the influence a programmer has over micro-optimisations in Java? I recently read in C++ sometimes the ordering of the data members can provide optimizations (granted, in the microsecond environment) and I presumed a programmer's hands are tied when it comes to squeezing performance from Java? I appreciate a decent algorithm provides greater speed-gains, but once you have the correct algorithm is Java harder to tweak due to the JVM control? If not, could people give examples of what tricks you can use in Java (besides simple compiler flags).

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  • What top companies look for in an Interview?

    - by Geek
    Basically I am looking for what is it that you goof up and you are out from the remaining process ? Are elimination rounds a fair way to judge a person ? Anyone can have a bad hour :-( Should you code the best possible or should you get the Algorithm right ? I generally first code a workable solution and then work on it till a level I think it looks beautiful to me. Is this a wrong approach ? Recently I had a telephonic interview in which I was asked to write a variation of Level Order traversal in 20 minutes. I could get the Algorithm and working code in 20 minutes but couldn't get the Exception handling and the coding convention right, after which I didn't hear back from them :-( All the other questions in the interview went very well this was the only thing which was 'not upto the mark'. Needless to say I like the company and will apply again but want to get it right the next time :-) Please share your experiences and valuable suggestions.

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  • Storing documents in DMS

    - by Shaza
    I need your opinions and suggestions about storing documents in a DMS system. I think the DMS should save its own copies of the documents, not their original path on the disk. So, the DMS should have its own space to write on. But what about the way they're stored? Do they have their own extension different from the original one? What about the algorithm that stores them? What about the algorithm that retrieves them? What do you suggest??

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  • Looking for a non-cryptographic hash function that returns a single character

    - by makerofthings7
    Suppose I have a dictionary of ASCII words stored in uppercase. I also want to save those words into separate files so that the total word count of each file is approximately the same. By simply looking at the word I need to know which file it should be in (if it's there at all). Duplicate words should go into the same file and overwrite the last one. My first attempt at solving this problem is to use .NET's object.GetHashCode() function and .Trim() to get one of the "random" characters that pop up. I asked a similar question here If I only use one character of object.GetHashCode() I would get a hash code character of A..Z or 0..9. However saving the result of GetHashCode to disk is a no-no so I need a substitute. Question: What algorithm (or subset of an algorithm) is appropriate for pigeonholing strings into a single character or range of characters (Like hex 0..F offers 16 chars)? Real world usage: I'll use this answer to modify the Partition key used in Azure Table storage as described here

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  • Tiled perlin/value noise texture with (2^n)+1 size

    - by tobi
    Actually what I have in mind is value noise I think, but what I am going to ask applies to both of them. It is known that if you want to produce tiled texture by using the perlin/value noise, the size of the texture should be specified as the power of 2 (2^n). Without any modifications to the algorithm when you use the size of (2^n)+1 the texture cannot be tiled anymore, so I am wondering whether it is possible (by modifying the algorithm somehow) to generate such tiling texture with the size of (2^n)+1. The article (from which I have my implementation) is here: http://devmag.org.za/2009/04/25/perlin-noise/ I am aware that I can produce texture with 2^n size and just copy twice the last column/row from the ends to make it (2^n)+1, but I don't want to, because such repetitions are visible too much.

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  • Is there a 'design pattern' type listing of common algorithms?

    - by KevinM1
    Is there a 'design pattern' styled listing of common/popular algorithms anywhere? Specifically, something that has a similar format along the lines of: Algorithm Name: e.g., Quick Sort, Bubble Sort, etc. Problem: A description of the stereotypical problem the algorithm is supposed to address Description: Description of the solution Implementation: Code examples of the solution Big O Rating: Self-explanatory Similar Algorithms: Algorithms that address the same problem in different ways, or similar problems I really like the GoF design pattern listing style, and I think it would help me learn various algorithms better/easier if I could find a resource that was similar in terms of organization.

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