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  • The sieve of Eratosthenes in F#

    - by IVlad
    I am interested in an implementation of the sieve of eratosthenes in purely functional F#. I am interested in an implementation of the actual sieve, not the naive functional implementation that isn't really the sieve, so not something like this: let rec PseudoSieve list = match list with | hd::tl -> hd :: (PseudoSieve <| List.filter (fun x -> x % hd <> 0) tl) | [] -> [] The second link above briefly describes an algorithm that would require the use of a multimap, which isn't available in F# as far as I know. The Haskell implementation given uses a map that supports an insertWith method, which I haven't seen available in the F# functional map. Does anyone know a way to translate the given Haskell map code to F#, or perhaps knows of alternative implementation methods or sieving algorithms that are as efficient and better suited for a functional implementation or F#?

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  • How to unit test internals (organization) of a data structure?

    - by Herms
    I've started working on a little ruby project that will have sample implementations of a number of different data structures and algorithms. Right now it's just for me to refresh on stuff I haven't done for a while, but I'm hoping to have it set up kind of like Ruby Koans, with a bunch of unit tests written for the data structures but the implementations empty (with full implementations in another branch). It could then be used as a nice learning tool or code kata. However, I'm having trouble coming up with a good way to write the tests. I can't just test the public behavior as that won't necessarily tell me about the implementation, and that's kind of important here. For example, the public interfaces of a normal BST and a Red-Black tree would be the same, but the RB Tree has very specific data organization requirements. How would I test that?

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  • I need to stop execution during recursive algorithm

    - by Shaza
    Hey, I have a problem in choosing the right method to accomplish my goal. I'm working on Algorithms teaching system, I'm using C#. I need to divide my algorithm into steps, each step will contain a recursion. I have to stop execution after each step, user can then move to the next step(next recursion) using a button in my GUI. After searching, threads was the right choice, but I found several methods: (Thread.sleep/interrupt): didn't work, my GUI freezed !! (Suspend/Resume): I've read that it's a bad idea to use. (Waithandles): still reading about them. (Monitor wait/resume). I don't have much time to try and read all previous methods, please help me in choosing the best method that fits my system.Any suggestions are extremal welcomed.

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  • How do you go from an abstract project description to actual code?

    - by Jason
    Maybe its because I've been coding around two semesters now, but the major stumbling block that I'm having at this point is converting the professor's project description and requirements to actual code. Since I'm currently in Algorithms 101, I basically do a bottom-up process, starting with a blank whiteboard and draw out the object and method interactions, then translate that into classes and code. But now the prof has tossed interfaces and abstract classes into the mix. Intellectually, I can recognize how they work, but am stubbing my toes figuring out how to use these new tools with the current project (simulating a web server). In my professors own words, mapping the abstract description to Java code is the real trick. So what steps are best used to go from English (or whatever your language is) to computer code? How do you decide where and when to create an interface, or use an abstract class?

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  • Using EXR images in OpenCV

    - by Nestor
    Hi there! I'm currently working on a project with OpenEXR and I would like to implement some Blob detection algorithms. To do this I figured that I could use OpenCV as it says in the documentation that it can open OpenEXR format files. I have all the libraries installed and working as I've been doing other things. I open a simple jpg file with openCV cvLoadImage. It works fine. But when i try to open any .exr file it doesn't seam to like it. I get a gray window where there should be the image display. Has anyone done any tests with OpenCV and OpenEXR libraries working together? Have they worked for you? What do you think? Thanks.

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  • String from Httpresponse not passing full value.

    - by Shekhar_Pro
    HI i am in desperate need for help here, I am making a web request and getting a json string with Response.ContentLenth=2246 but when i parse it in a string it gives only few 100 characters, i traked it down that it is only getting values less than 964. strings length is still 2246 but remaining values are just (\0) null characters. Its also giving an error Unterminated string passed in. (2246): at following line FacebookFeed feed = sr.Deserialize<FacebookFeed>(data); It works fine if the response stream contains characters less than 964 chars. Following is the extract from the full code error encountered in last line. System.Web.Script.Serialization.JavaScriptSerializer sr = new System.Web.Script.Serialization.JavaScriptSerializer(); System.Net.HttpWebRequest req = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(@"https://graph.facebook.com/100000570310973_181080451920964"); req.Method = "GET"; System.Net.HttpWebResponse res = (System.Net.HttpWebResponse)req.GetResponse(); byte[] resp = new byte[(int)res.ContentLength]; res.GetResponseStream().Read(resp, 0, (int)res.ContentLength); string data = Encoding.UTF8.GetString(resp); FacebookFeed feed = sr.Deserialize<FacebookFeed>(data); error given is Unterminated string passed in. (2246): {"id":"100000570310973_1810804519209........ (with rest of data in the string data including null chars) following is the shape of classes used in my code: public class FacebookFeed { public string id { get; set; } public NameIdPair from { get; set; } public NameIdPair to { get; set; } public string message { get; set; } public Uri link{get;set;} public string name{get; set;} public string caption { get; set; } public Uri icon { get; set; } public NameLinkPair[] actions { get; set; } public string type { get; set; } public NameIdPair application { get; set; } //Mentioned in Graph API as attribution public DateTime created_time { get; set; } public DateTime updated_time { get; set; } public FacebookPostLikes likes { get; set; } } public class NameIdPair { public string name { get; set; } public string id { get; set; } } public class NameLinkPair { public string name { get; set; } public Uri link{get; set;} } public class FacebookPostLikes { public NameIdPair[] data { get; set; } public int count { get; set; } }

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  • Placement of defensive structures in a game

    - by Martin
    I am working on an AI bot for the game Defcon. The game has cities, with varying populations, and defensive structures with limited range. I'm trying to work out a good algorithm for placing defence towers. Cities with higher populations are more important to defend Losing a defence tower is a blow, so towers should be placed reasonably close together Towers and cities can only be placed on land So, with these three rules, we see that the best kind of placement is towers being placed in a ring around the largest population areas (although I don't want an algorithm just to blindly place a ring around the highest area of population, sometime there might be 2 sets of cities far apart, in which case the algorithm should make 2 circles, each one half my total towers). I'm wondering what kind of algorithms might be used for determining placement of towers?

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  • Problem drawing a polygon on data clusters in MATLAB

    - by Hossein
    Hi, I have some data points which I have devided into them into some clusters with some clustering algorithms as the picture below:(it might takes some time for the image to appear) Each color represents different cluster. I have to draw polygons around each cluster. I use convhull for this reason. But as you can see the polygon for the red cluster is very big and covers a lot of areas, which is not the one I am looking for. I need to draw lines(ploygons) exactly around my data sets. For example in the picture above I want a polygon that is drawn exactly the same(and around) as the red cluster with the 3 branches. In other words, in this case I need a polygon with 3 branches to cover my red clusters not that big polygon that covers the whole area. Can anyone help me with this? Please Note that the solution should be general, because the clusters will change in each run of the algorithm, so it needs to be in a way that is general.

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  • C#: Efficiently search a large string for occurences of other strings

    - by Jon
    Hi, I'm using C# to continuously search for multiple string "keywords" within large strings, which are = 4kb. This code is constantly looping, and sleeps aren't cutting down CPU usage enough while maintaining a reasonable speed. The bog-down is the keyword matching method. I've found a few possibilities, and all of them give similar efficiency. 1) http://tomasp.net/articles/ahocorasick.aspx -I do not have enough keywords for this to be the most efficient algorithm. 2) Regex. Using an instance level, compiled regex. -Provides more functionality than I require, and not quite enough efficiency. 3) String.IndexOf. -I would need to do a "smart" version of this for it provide enough efficiency. Looping through each keyword and calling IndexOf doesn't cut it. Does anyone know of any algorithms or methods that I can use to attain my goal?

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  • [WM6] Owner of a component. What exactly is this for?

    - by kornelijepetak
    I am developing an app for Windows Mobile 6 and there is a CameraCaptureDialog class that enables me to call a camera app from my own application. The class has an Owner property that most examples on the internet set to "this". cam.Owner = this; What exactly does this do? I've seen a similar scenario with Windows Forms components/controls by setting the Parent control. In that case I guess it's used only for layout algorithms, but other than that, is there any useful purpose? I don't see how setting an owner of the CameraCaptureDialog can (and actually does not) make any difference (at least not a visible one). Any insight would be appreciated.

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  • What is the typical method to separate connected letters in a word using OCR

    - by Maysam
    I am very new to OCR and almost know nothing about the algorithms used to recognize words. I am just getting familiar to that. Could anybody please advise on the typical method used to recognize and separate individual characters in connected form (I mean in a word where all letters are linked together)? Forget about handwriting, supposing the letters are connected together using a known font, what is the best method to determine each individual character in a word? When characters are written separately there is no problem, but when they are joined together, we should know where every single character starts and ends in order to go to the next step and match them individually with a letter. Is there any known algorithm for that?

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  • Is Work Stealing always the most appropriate user-level thread scheduling algorithm?

    - by Il-Bhima
    I've been investigating different scheduling algorithms for a thread pool I am implementing. Due to the nature of the problem I am solving I can assume that the tasks being run in parallel are independent and do not spawn any new tasks. The tasks can be of varying sizes. I went immediately for the most popular scheduling algorithm "work stealing" using lock-free deques for the local job queues, and I am relatively happy with this approach. However I'm wondering whether there are any common cases where work-stealing is not the best approach. For this particular problem I have a good estimate of the size of each individual task. Work-stealing does not make use of this information and I'm wondering if there is any scheduler which will give better load-balancing than work-stealing with this information (obviously with the same efficiency). NB. This question ties up with a previous question.

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  • Dealing with passwords securely

    - by Krt_Malta
    Hi I have a Java web service and a Java web client making use of this service. One of the functions is to create a new user account. My two concerns are: How will I send the user's password securely from the client. How will I store the user's password securely on the server. How can I achieve these? I know the theory basically behind security, security algorithms etc but can anyone give me some advice on how I should go about in coding? Could anyone point me to some good (and if possible not complicated) examples to follow since I found some examples on the Internet very contorted? Thanks a lot and regards, Krt_Malta

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  • What are some C++ Standard Library usage best practices?

    - by apphacker
    I'm learning C++ and the book I'm reading (The C++ Programming Language) says to not reinvent the wheel, to rely on the standard libraries. In C, I often end up creating a linked list, and link list iteration over and over again (maybe I'm doing that wrong not sure), so the ideas of containers available in C++, and strings, and algorithms really appeal to me. However I have read a little online, and heard some criticisms from my friends and coworkers about STL, so I thought I maybe I'd pick some brains here. What are some best practices for using STL, and what lessons have you learned about STL?

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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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  • bluetooth emulators/simulators

    - by scatman
    what are the best bluetooth emulators/simulators? i have a project for the security course, and i chose bluetooth as my topic. so i need to create encryption algorithms using a bluetooth simulator and test them. i have already searched for the best emulaters, and the results were google anroid, and NS2. is there any emulator better? the languages i prefer to program with are c , c++ , and c# java is also not a problem but i prefer any of the above languages.

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  • Determine if a string contains only alphanumeric characters (or a space)

    - by dreamlax
    I'm learning C++ and I am writing a function that determines whether a string contains only alphanumeric characters and spaces. I suppose I am effectively testing whether it matches the regular expression ^[[:alnum:] ]+$ but without using regular expressions. I have seen a lot of algorithms revolve around iterators, so I tried to find a solution that made use of iterators, and this is what I have: #include <algorithm> static inline bool is_not_alnum_space(char c) { return !(isalpha(c) || isdigit(c) || (c == ' ')); } bool string_is_valid(const std::string &str) { return find_if(str.begin(), str.end(), is_not_alnum_space) == str.end(); } Is there a better solution, or a “more C++” way to do this?

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  • Algorithm to match natural text in mail

    - by snøreven
    I need to separate natural, coherent text/sentences in emails from lists, signatures, greetings and so on before further processing. example: Hi tom, last monday we did bla bla, lore Lorem ipsum dolor sit amet, consectetur adipisici elit, sed eiusmod tempor incidunt ut labore et dolore magna aliqua. list item 2 list item 3 list item 3 Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquid x ea commodi consequat. Quis aute iure reprehenderit in voluptate velit regards, K. ---line-of-funny-characters-####### example inc. 33 evil street, london mobile: 00 234534/234345 Ideally the algorithm would match only the bold parts. Is there any recommended approach - or are there even existing algorithms for that problem? Should I try approximate regular expressions or more statistical stuff based on number of punctation marks, length and so on?

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  • best method of turning millions of x,y,z positions of particles into visualisation

    - by Griff
    I'm interested in different algorithms people use to visualise millions of particles in a box. I know you can use Cloud-In-Cell, adaptive mesh, Kernel smoothing, nearest grid point methods etc to reduce the load in memory but there is very little documentation on how to do these things online. i.e. I have array with: x,y,z 1,2,3 4,5,6 6,7,8 xi,yi,zi for i = 100 million for example. I don't want a package like Mayavi/Paraview to do it, I want to code this myself then load the decomposed matrix into Mayavi (rather than on-the-fly rendering) My poor 8Gb Macbook explodes if I try and use the particle positions. Any tutorials would be appreciated.

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  • Fastest way of converting a quad to a triangle strip?

    - by Tina Brooks
    What is the fastest way of converting a quadrilateral (made up of foyr x,y points) to a triangle strip? I'm well aware of the general triangulation algorithms that exist, but I need a short, well optimized algorithm that deals with quadrilaterals only. My current algorithm does this, which works for most quads but still gets the points mixed up for some: #define fp(f) bounds.p##f /* Sort four points in ascending order by their Y values */ point_sort4_y(&fp(1), &fp(2), &fp(3), &fp(4)); /* Bottom two */ if (fminf(-fp(1).x, -fp(2).x) == -fp(2).x) { out_quad.p1 = fp(2); out_quad.p2 = fp(1); } else { out_quad.p1 = fp(1); out_quad.p2 = fp(2); } /* Top two */ if (fminf(-fp(3).x, -fp(4).x) == -fp(3).x) { out_quad.p3 = fp(3); out_quad.p4 = fp(4); } else { out_quad.p3 = fp(4); out_quad.p4 = fp(3); }

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  • Algorithm for non-contiguous netmask match

    - by Gianluca
    Hi, I have to write a really really fast algorithm to match an IP address to a list of groups, where each group is defined using a notation like 192.168.0.0/252.255.0.255. As you can see, the bitmask can contain zeros even in the middle, so the traditional "longest prefix match" algorithms won't work. If an IP matches two groups, it will be assigned to the group containing most 1's in the netmask. I'm not working with many entries (let's say < 1000) and I don't want to use a data structure requiring a large memory footprint (let's say 1-2 MB), but it really has to be fast (of course I can't afford a linear search). Do you have any suggestion? Thanks guys. UPDATE: I found something quite interesting at http://www.cse.usf.edu/~ligatti/papers/grouper-conf.pdf, but it's still too memory-hungry for my utopic use case

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  • Is there a name for the technique of using base-2 numbers to encode a list of unique options?

    - by Lunatik
    Apologies for the rather vague nature of this question, I've never been taught programming and Google is rather useless to a self-help guy like me in this case as the key words are pretty ambiguous. I am writing a couple of functions that encode and decode a list of options into a Long so they can easily be passed around the application, you know this kind of thing: 1 - Apple 2 - Orange 4 - Banana 8 - Plum etc. In this case the number 11 would represent Apple, Orange & Plum. I've got it working but I see this used all the time so assume there is a common name for the technique, and no doubt all sorts of best practice and clever algorithms that are at the moment just out of my reach.

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  • Remove from a std::set<shared_ptr<T>> by T*

    - by Autopulated
    I have a set of shared pointers: std::set<boost::shared_ptr<T>> set; And a pointer: T* p; I would like to efficiently remove the element of set equal to p, but I can't do this with any of the members of set, or any of the standard algorithms, since T* is a completely different type to boost::shared_ptr<T>. A few approaches I can think of are: somehow constructing a new shared_ptr from the pointer that won't take ownership of the pointed to memory (ideal solution, but I can't see how to do this) wrapping / re-implementing shared_ptr so that I can do the above just doing my own binary search over the set Help!

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  • What is it about Fibonacci numbers?

    - by Ian Bishop
    Fibonacci numbers have become a popular introduction to recursion for Computer Science students and there's a strong argument that they persist within nature. For these reasons, many of us are familiar with them. They also exist within Computer Science elsewhere too; in surprisingly efficient data structures and algorithms based upon the sequence. There are two main examples that come to mind: Fibonacci heaps which have better amortized running time than binomial heaps. Fibonacci search which shares O(log N) running time with binary search on an ordered array. Is there some special property of these numbers that gives them an advantage over other numerical sequences? Is it a density quality? What other possible applications could they have? It seems strange to me as there are many natural number sequences that occur in other recursive problems, but I've never seen a Catalan heap.

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  • What's the best way to match a query to a set of keywords?

    - by Ryan Detzel
    Pretty much what you would assume Google does. Advertisers come in and big on keywords, lets say "ipod", "ipod nano", "ipod 60GB", "used ipod", etc. Then we have a query, "I want to buy an ipod nano" or "best place to buy used ipods" what kind of algorithms and systems are used to match those queries to the keyword set. I would imagine that some of those keyword sets are huge, 100k keywords made up of one or more actual words. on top of that queries can be 1-n words as well. Any thoughts, links to wikipedia I can start reading? From what I know already I would use some stemmed hash in disk(CDB?) and a bloom filter to check to see if I should even go to disk.

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