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• problems with studying algorithms

  - by rookie

  hello everyone, I'm currently studying computer science at the Institute, and I have some problems with course which is named Algorithms, I've just begun to study it, but I'm already feeling, that I'm going to fail it, my problem is that while understanding different algorithms on graphs I need to keep in my mind a lot of info, and usually I can't do it, I forget some points of the exercise or can't proceed to final result, I'm very desperate about it cause I like programming very much. Did somebody feel the same while studying in the University? thanks in advance for any help P.S. I began to program only two years ago, may it be the problem?

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• Randomly generate directed graph on a grid

  - by Talon876

  I am trying to randomly generate a directed graph for the purpose of making a puzzle game similar to the ice sliding puzzles from Pokemon. This is essentially what I want to be able to randomly generate: http://bulbanews.bulbagarden.net/wiki/Crunching_the_numbers:_Graph_theory. I need to be able to limit the size of the graph in an x and y dimension. In the example given in the link, it would be restricted to an 8x4 grid. The problem I am running into is not randomly generating the graph, but randomly generating a graph, which I can properly map out in a 2d space, since I need something (like a rock) on the opposite side of a node, to make it visually make sense when you stop sliding. The problem with this is that sometimes the rock ends up in the path between two other nodes or possibly on another node itself, which causes the entire graph to become broken. After discussing the problem with a few people I know, we came to a couple of conclusions that may lead to a solution. Including the obstacles in the grid as part of the graph when constructing it. Start out with a fully filled grid and just draw a random path and delete out blocks that will make that path work. The problem then becomes figuring out which ones to delete to avoid introducing an additional, shorter path. We were also thinking a dynamic programming algorithm may be beneficial, though none of us are too skilled with creating dynamic programming algorithms from nothing. Any ideas or references about what this problem is officially called (if it's an official graph problem) would be most helpful. Here are some examples of what I have accomplished so far by just randomly placing blocks and generating the navigation graph from the chosen start/finish. The idea (as described in the previous link) is you start at the green S and want to get to the green F. You do this by moving up/down/left/right and you continue moving in the direction chosen until you hit a wall. In these pictures, grey is a wall, white is the floor, and the purple line is the minimum length from start to finish, and the black lines and grey dots represented possible paths. Here are some bad examples of randomly generated graphs: http://i.stack.imgur.com/9uaM6.png Here are some good examples of randomly generated (or hand tweaked) graphs: i.stack.imgur.com/uUGeL.png (can't post another link, sorry) I've also seemed to notice the more challenging ones when actually playing this as a puzzle are ones which have lots of high degree nodes along the minimum path.

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• Sort algorithms that work on large amount of data

  - by Giorgio

  I am looking for sorting algorithms that can work on a large amount of data, i.e. that can work even when the whole data set cannot be held in main memory at once. The only candidate that I have found up to now is merge sort: you can implement the algorithm in such a way that it scans your data set at each merge without holding all the data in main memory at once. The variation of merge sort I have in mind is described in this article in section Use with tape drives. I think this is a good solution (with complexity O(n x log(n)) but I am curious to know if there are other (possibly faster) sorting algorithms that can work on large data sets that do not fit in main memory. EDIT Here are some more details, as required by the answers: The data needs to be sorted periodically, e.g. once in a month. I do not need to insert a few records and have the data sorted incrementally. My example text file is about 1 GB UTF-8 text, but I wanted to solve the problem in general, even if the file were, say, 20 GB. It is not in a database and, due to other constraints, it cannot be. The data is dumped by others as a text file, I have my own code to read this text file. The format of the data is a text file: new line characters are record separators. One possible improvement I had in mind was to split the file into files that are small enough to be sorted in memory, and finally merge all these files using the algorithm I have described above.

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• [C++] Write connected components of a graph using Boost Graph

  - by conradlee

  I have an file that is a long list of weighted edges, in the following form node1_id node2_id weight node1_id node3_id weight and so on. So one weighted edge per line. I want to load this file into boost graph and find the connected components in the graph. Each of these connected components is a subgraph. For each of these component subgraphs, I want to write the edges in the above-described format. I want to do all this using boost graph. This problem is in principle simple, it's just I'm not sure how to implement it neatly because I don't know my way around Boost Graph. I have already spent some hours and have code that will find the connected components, but my version is surely much longer and more complicated that necessary---I'm hoping there's a boost-graph ninja out there that can show me the right, easy way.

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• Scene Graph for Deferred Rendering Engine

  - by Roy T.

  As a learning exercise I've written a deferred rendering engine. Now I'd like to add a scene graph to this engine but I'm a bit puzzled how to do this. On a normal (forward rendering engine) I would just add all items (All implementing IDrawable and IUpdateAble) to my scene graph, than travel the scene-graph breadth first and call Draw() everywhere. However in a deferred rendering engine I have to separate draw calls. First I have to draw the geometry, then the shadow casters and then the lights (all to different render targets), before I combine them all. So in this case I can't just travel over the scene graph and just call draw. The way I see it I either have to travel over the entire scene graph 3 times, checking what kind of object it is that has to be drawn, or I have to create 3 separate scene graphs that are somehow connected to each other. Both of these seem poor solutions, I'd like to handle scene objects more transparent. One other solution I've thought of was traveling trough the scene graph as normal and adding items to 3 separate lists, separating geometry, shadow casters and lights, and then iterating these lists to draw the correct stuff, is this better, and is it wise to repopulate 3 lists every frame?

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• Where to find algorithms work?

  - by Misha

  The funnest parts of my projects have been the back-end algorithms work. I have worked on projects where I implemented Gaussian Mixture models, a Remez algorithm and a few Monte Carlo schemes. I loved figuring out how these processes worked and tuning them when they didn't. I recently graduated and my problem lies in the work I was able to find. The only jobs I have found, with my Electrical Engineering degree, are for writing user applications. Tasks such as fashioning web interfaces or front-ends for hardware devices. When I speak with potential employers about my interests they say they have no work of the sort. Where does one find work that involves implementing these kind of schemes?

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• What do I need to know about Data Structures and Algorithms in the "real" world

  - by Ray T Champion

  I just finished the data structures and algorithms course in school , I took it during the summer so 6wks course vs a 16 wk course during the regular semester. So not only was the course hard but it was really really really fast. My question is what do I need to know about data structures in the real world? I understand what they do and how they work, for the most part, but I had a real tough time coding them , I wouldn't be able to write the code for a binary tree class or a balanced tree class from scratch .... Is that bad? should I retake it , or is knowledge of how they work sufficient, without being able to write the classes from scratch?

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• Display Graph using Boost Graph Library

  - by TheTSPSolver

  Can anyone please tell me that once I've created a graph using Boost Graph library, how can I display that graph? My biggest concern is that the edge weights are coming from an exernal data source over the network. And I need to be able to display the edgeweights live as they get updated.

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• Architecture for Social Graph data that has a Time Frame Associated?

  - by Jay Stevens

  I am adding some "social" type features to an existing application. There are a limited # of node & edge types. Overall the data itself is relatively small (50,000 - 70,000 for each type of node) there will be a number of edges (relationships) between them (almost all directional). This, I know, is relatively easy to represent with an SDF store (such as BrightstarDB) or something like Microsoft's Trinity (or really many of the noSQL options). The thing that, I think, makes this a unique use case is that each relationship will have a timeframe associated with it (start and end dates). Right now, I'm thinking of just storing this in a relational structure and dealing with the headaches of "traversing the graph", but I'm looking for suggestions on a better approach (both in terms of data structure and server): Column ================ From_Node_ID Relationship To_Node_ID StartDate EndDate Any suggestions or thoughts are welcomed.

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• Stereo images rectification and disparity: which algorithms?

  - by alessandro.francesconi

  I'm trying to figure out what are currently the two most efficent algorithms that permit, starting from a L/R pair of stereo images created using a traditional camera (so affected by some epipolar lines misalignment), to produce a pair of adjusted images plus their depth information by looking at their disparity. Actually I've found lots of papers about these two methods, like: "Computing Rectifying Homographies for Stereo Vision" (Zhang - seems one of the best for rectification only) "Three-step image recti?cation" (Monasse) "Rectification and Disparity" (slideshow by Navab) "A fast area-based stereo matching algorithm" (Di Stefano - seems a bit inaccurate) "Computing Visual Correspondence with Occlusions via Graph Cuts" (Kolmogorov - this one produces a very good disparity map, with also occlusion informations, but is it efficient?) "Dense Disparity Map Estimation Respecting Image Discontinuities" (Alvarez - toooo long for a first review) Anyone could please give me some advices for orienting into this wide topic? What kind of algorithm/method should I treat first, considering that I'll work on a very simple input: a pair of left and right images and nothing else, no more information (some papers are based on additional, pre-taken, calibration infos)? Speaking about working implementations, the only interesting results I've seen so far belongs to this piece of software, but only for automatic rectification, not disparity: http://stereo.jpn.org/eng/stphmkr/index.html I tried the "auto-adjustment" feature and seems really effective. Too bad there is no source code...

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• How to fit a custom graph to the boost graph library template?

  - by Michael

  I'm rusty on C++ templates and I'm using the boost graph library (a fatal combination). I've searched the web and can't find any direct instructions on how to take a custom graph structure and fit enough of it to BGL (boost graph library) that I can use boosts graph traversing algorithms. Anyone familiar enough with the library to help me out?

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• C# graph library to be used from Unity3D

  - by Heisenbug

  I'm looking for a C# graph library to be used inside Unity3D script. I'm not looking for pathfinding libraries (I know there are good one available). I could consider using a path finding library only if it gives me direct access to underlying graph classes (I need nodes and edges, and classic graph algorithms) The only product I've seen that seems intersting is QuickGraph. I have the following question: Is it possible to use QuickGraph inside Unity3d? If yes. Is this a good idea? Does it have any drawbacks? Is it a quite fast and well written/supported library? Does anyone has ever used it? Are available other C# graph library that can be easily integrated in Unity3d?

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• Scene Graph as Object Container?

  - by Bunkai.Satori

  Scene graph contains game nodes representing game objects. At a first glance, it might seem practical to use Scene Graph as physical container for in game objects, instead of std::vector< for example. My question is, is it practical to use Scene Graph to contain the game objects, or should it be used only to define scene objects/nodes linkages, while keepig the objects stored in separate container, such as std::vector<?

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• Splitting Graph into distinct polygons in O(E) complexity

  - by Arthur Wulf White

  If you have seen my last question: trapped inside a Graph : Find paths along edges that do not cross any edges How do you split an entire graph into distinct shapes 'trapped' inside the graph(like the ones described in my last question) with good complexity? What I am doing now is iterating over all edges and then starting to traverse while always taking the rightmost turn. This does split the graph into distinct shapes. Then I eliminate all the excess shapes (that are repeats of previous shapes) and return the result. The complexity of this algorithm is O(E^2). I am wondering if I could do it in O(E) by removing edges I already traversed previously. My current implementation of that returns unexpected results.

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• Facebook Graph API authentication in canvas app and track session

  - by cdpnet

  Short question is: how can i use graph api oauth redirects mechanism to authenticate user and save retrieved access_token and also use javascript SDK when needed (the problem is javascript SDK will have different access_token when initialized). I have initially setup my facebook iframe canvas app, with single sign on. This works well with graph api, as I am able to use access_token saved by facebook's javascript when it detects sessionchange(user logged in). But, I want to rather not do single sign-on. But, use graph api redirect and force user to send to a permissions dialog. But, if he has already given permissions, I shouldn't redirect user. How to handle this? Another question: I have done graph api redirects for authentication and have retrieved access_token also. But then, what if I want to use javascript call FB.ui to do stream.Publish? I think it will use it's own access_token which is set during FB.init and detecting session. So, I am looking for some path here. How to use graph api for authentication and also use facebook's javascript SDK when needed. P.S. I'm using ASP .NET MVC 2. I have an authentication filter developed, which needs to detect the user's authentication state and redirect.(currently it does this to graph api authorize url)

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• Trust metrics and related algorithms

  - by Nick Gerakines

  I'm trying to learn more about trust metrics (including related algorithms) and how user voting, ranking and rating systems can be wired to stiffle abuse. I've read abstract articles and papers describing trust metrics but haven't seen any actual implementations. My goal is to create a system that allows users to vote on other users and the content of other users and with those votes and related meta-data, determine if those votes can be applied to a users level or popularity. Have you used or seen some sort of trust system within a social graph? How did it work and what were its areas of strength and weaknesses?

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• Color schemes generation - theory and algorithms

  - by daniel.sedlacek

  Hi I will be generating charts and diagrams and I am looking for some theory on color schemes and algorithm examples. Example questions: How to generate complementary or analogous colors? How to generate pastel, cold and warm colors? How to generate any number of random but distinct colors? How to translate all that to the hex triplet (web color)? My implementation will be in AS3 but any examples in metacode are welcome.

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• Order of learning sort algorithms

  - by user619818

  I have already studied bubblesort, insertion sort and selection sort and can implement them in C pretty much from knowledge of the algorithm. I want to go on to learn shellsort, merge sort, heapsort and quicksort, which I guess are a lot harder to understand. What order should I take these other sort algos? I am assuming a simpler sort algo helps learn a more complex one. Don't mind taking on some others if it helps the learning process.

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• Functional programming and stateful algorithms

  - by bigstones

  I'm learning functional programming with Haskell. In the meantime I'm studying Automata theory and as the two seem to fit well together I'm writing a small library to play with automata. Here's the problem that made me ask the question. While studying a way to evaluate a state's reachability I got the idea that a simple recursive algorithm would be quite inefficient, because some paths might share some states and I might end up evaluating them more than once. For example, here, evaluating reachability of g from a, I'd have to exclude f both while checking the path through d and c: So my idea is that an algorithm working in parallel on many paths and updating a shared record of excluded states might be great, but that's too much for me. I've seen that in some simple recursion cases one can pass state as an argument, and that's what I have to do here, because I pass forward the list of states I've gone through to avoid loops. But is there a way to pass that list also backwards, like returning it in a tuple together with the boolean result of my canReach function? (although this feels a bit forced) Besides the validity of my example case, what other techniques are available to solve this kind of problems? I feel like these must be common enough that there have to be solutions like what happens with fold* or map. So far, reading learnyouahaskell.com I didn't find any, but consider I haven't touched monads yet. (if interested, I posted my code on codereview)

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• Prefer algorithms to hand-written loops?

  - by FredOverflow

  Which of the following to you find more readable? The hand-written loop: for (std::vector<Foo>::const_iterator it = vec.begin(); it != vec.end(); ++it) { bar.process(*it); } Or the algorithm invocation: #include <algorithm> #include <functional> std::for_each(vec.begin(), vec.end(), std::bind1st(std::mem_fun_ref(&Bar::process), bar)); I wonder if std::for_each is really worth it, given such a simple example already requires so much code. What are your thoughts on this matter?

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• How do I learn algorithms and data structures?

  - by sushil bharwani

  this is in continuation to my previous question where i asked is it necessary to learn algorithm and datastructures. I feel yes it is. Now when i work in a enviornment where i wont ever get the chance to learn it by experimenting or practically or in any assignment. What is the right approach like the right books, right kind of problems, right kind of resources that i can go through to give six months or a year or two to learn it. And also mould my mind in a way that it can relate problems to datastructures and algos.

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• Algorithms for pairing a rating system to an assignment queue

  - by blunders

  Attempting to research how to allow a group of people to effectively rank a set of objects (each group member will have contributed one object to the group), and then assign each member an object that's not their own based on: Their ratings of the objects, Their objects rating, and The object remaining to be assigned. Idea is to attempt to assign objects to people based on the groups rating of their contribution to the group relative to other member's contribution, the the personal preferences expressed via the ratings. Any suggestions for: Further research, Refining the statement of the problem/solution, or A solution.

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• Semantic algorithms

  - by Mythago

  I have a more theoretical than practical question. I'll start with an example - when I get an email and open it on my iPad, there is a feature, which recognizes the timestamp from the text and offers me to create an event in the calendar. Simply told, I want to know theoretically how it's done - I believe it's some kind of semantic parsing, and I would like if someone could point me to some resources, where I can read more about this.

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• Structuring Access Control In Hierarchical Object Graph

  - by SB2055

  I have a Folder entity that can be Moderated by users. Folders can contain other folders. So I may have a structure like this: Folder 1 Folder 2 Folder 3 Folder 4 I have to decide how to implement Moderation for this entity. I've come up with two options: Option 1 When the user is given moderation privileges to Folder 1, define a moderator relationship between Folder 1 and User 1. No other relationships are added to the db. To determine if the user can moderate Folder 3, I check and see if User 1 is the moderator of any parent folders. This seems to alleviate some of the complexity of handling updates / moved entities / additions under Folder 1 after the relationship has been defined, and reverting the relationship means I only have to deal with one entity. Option 2 When the user is given moderation privileges to Folder 1, define a new relationship between User 1 and Folder 1, and all child entities down to the grandest of grandchildren when the relationship is created, and if it's ever removed, iterate back down the graph to remove the relationship. If I add something under Folder 2 after this relationship has been made, I just copy all Moderators into the new Entity. But when I need to show only the top-level Folders that a user is Moderating, I need to query all folders that have a parent folder that the user does not moderate, as opposed to option 1, where I just query any items that the user is moderating. Thoughts I think it comes down to determining if users will be querying for all parent items more than they'll be querying child items... if so, then option 1 seems better. But I'm not sure. Is either approach better than the other? Why? Or is there another approach that's better than both? I'm using Entity Framework in case it matters.

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• Make my website dynamically loaded data available to Facebook Open Graph Object Scrapper

  - by fvaliquette

  Here is the design of my web site: The user enter myWebsite.com/a/1 .htaccess rules redirect to myWebsite.com/b Now the JavaScript ExtJS library is loading. Extracting the value from the URL (in this case it is “1”) Loading ./xml/1.xml From 1.xml setting the Open Graph data (Title, type, image, etc) Loading data that will be shown to the user from 1.xml into the website. My question is: How can I make the Open Graph data available to Facebook? Facebook do not to load my ExtJS JavaScript Library before extracting the Open Graph Object values from the HTML. Is there an easy solution to this problem? The only solutions I found is to make statics web pages or dynamically pages rendered on the server side but I would like to avoid these since my web page implementation is already finished and I would like to avoid re working on it.

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