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• Evaluating a function at a particular value in parallel

  - by Gaurav Kalra

  Hi. The question may seem vague, but let me explain it. Suppose we have a function f(x,y,z ....) and we need to find its value at the point (x1,y1,z1 .....). The most trivial approach is to just replace (x,y,z ...) with (x1,y1,z1 .....). Now suppose that the function is taking a lot of time in evaluation and I want to parallelize the algorithm to evaluate it. Obviously it will depend on the nature of function, too. So my question is: what are the constraints that I have to look for while "thinking" to parallelize f(x,y,z...)? If possible, please share links to study.

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• Algorithm for Source Control System?

  - by Michael Stum

  I need to write a simple source control system and wonder what algorithm I would use for file differences? I don't want to look into existing source code due to license concerns. I need to have it licensed under MPL so I can't look at any of the existing systems like CVS or Mercurial as they are all GPL licensed. Just to give some background, I just need some really simple functions - binary files in a folder. no subfolders and every file behaves like it's own repository. No Metadata except for some permissions. Overall really simple stuff, my single concern really is how to store only the differences of a file from revision to revision without wasting too much space but also without being too inefficient (Maybe store a full version every X changes, a bit like Keyframes in Videos?)

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• Counting problem: possible sudoko tables?

  - by Sorush Rabiee

  Hi, I'm working on a sudoko solver. my method is using a game tree and explore possible permutations for each set of digits by DFS Algorithm. in order to analyzing problem, i want to know what is the count of possible valid and invalid sudoko tables? - a 9*9 table that have 9 one, 9 two, ... , 9 nine. (this isn't exact duplicate by this question) my solution is: 1- First select 9 cells for 1s: (*) 2- and like (1) for other digits (each time, 9 cells will be deleted from remaining available cells): C(81-9,9) , C(81-9*2,9) .... = 3- finally multiply the result by 9! (permutation of 123456789 in (*)) this is not equal to accepted answer of this question but problems are equivalent. what did i do wrong?

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• Finding recurrence relations of an algorithm

  - by Roarke

  I'm reading my algorithms text book, and I'm reading about recurrence relations and finding the algorithms big O complexity. I run across this line "In the case of the merge-sort algorithm, we get the recurrence equation: t(n) = b if n < 2 = 2t(n/2) +bn if n >= 2 for b > 0 my response was "how the heck did we know that?!?!" So i'm wondering if there is a systematic approach, or just a logical way of getting these recurrence relations from the algorithms can some one explain where the b and the two 2's come from?

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• Algorithm for Search page

  - by Geetha

  Hi All, I am creating a search page where we can find the product by entering the text. ex: Brings on the night. My query bring the records which contain atleast word from this. Needs: 1. First row should contains the record with the given sentence. 2. Second most matching....etc How to achieve this. Is there any algorithm for this. It will be more helpful if anyone share your idea. Geetha

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• How to improve performance

  - by Ram

  Hi, In one of mine applications I am dealing with graphics objects. I am using open source GPC library to clip/merge two shapes. To improve accuracy I am sampling (adding multiple points between two edges) existing shapes. But before displaying back the merged shape I need to remove all the points between two edges. But I am not able to find an efficient algorithm that will remove all points between two edges which has same slope with minimum CPU utilization. Currently all points are of type PointF Any pointer on this will be a great help.

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• List of generic algorithms and data structures

  - by Jake Petroules

  As part of a library project, I want to include a plethora of generic algorithms and data structures. This includes algorithms for searching and sorting, data structures like linked lists and binary trees, path-finding algorithms like A*... the works. Basically, any generic algorithm or data structure you can think of that you think might be useful in such a library, please post or add it to the list. Thanks! (NOTE: Because there is no single right answer I've of course placed this in community wiki... and also, please don't suggest algorithms which are too specialized to be provided by a generic library) The List: Data structures AVL tree B-tree B*-tree B+-tree Binary tree Binary heap Binary search tree Linked lists Singly linked list Doubly linked list Stack Queue Sorting algorithms Binary tree sort Bubble sort Heapsort Insertion sort Merge sort Quicksort Selection sort Searching algorithms

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• Task management algorithm in C#

  - by silverwizz

  Hi guys, i am looking for efficient task management fo C# what i mean by task management is executing pre-defined interval time of task. Example: task a needs to be run every 1 mins task b needs to be run every 3 mins task c needs to be run every 5 mins these tasks can be added and removed in arbitary time... And the task that i mentioned can be 100000 or more... The task will bw executed forever until it is removed... Do u guys familiar with this kind of algorithm? I am thinking to implement in either c# or php.... Thanks

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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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• Scale image to completely fill bounding box

  - by Larsenal

  For instance, if I need to fill a bounding box that is 100px wide by 50px tall, the following input images would have the following behavior: 200w x 200h gets scaled down 50% and 25% gets chopped off the top and bottom. 200w x 100h gets scaled down 50% with no cropping. 100w x 200h gets is not scaled, but 75px get chopped off top and bottom. This seems like it'd be a common resizing function, but I haven't been able to track down an example of the algorithm. Will accept answer in any language including pseudo code. A link to a page with the answer is great too!

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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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• Rerversing AND Bitwise.

  - by Benjamin

  Hey all, Here's the following algorithm: int encryption(int a, int b) { short int c, c2; uint8_t d; c = a ^ b; c2 = c; d = 0; while(c) { c &= c - 1; d++; } return d; } How can I find which variable a and b I should send in that function to decide of the output value of d? In other words, how can I reverse the algoritm to let's say if I want d=11?

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• Factorial Algorithms in different languages

  - by Brad Gilbert

  I want to see all the different ways you can come up with, for a factorial subroutine, or program. The hope is that anyone can come here and see if they might want to learn a new language. Ideas: Procedural Functional Object Oriented One liners Obfuscated Oddball Bad Code Polyglot Basically I want to see an example, of different ways of writing an algorithm, and what they would look like in different languages. Please limit it to one example per entry. I will allow you to have more than one example per answer, if you are trying to highlight a specific style, language, or just a well thought out idea that lends itself to being in one post. The only real requirement is it must find the factorial of a given argument, in all languages represented. Be Creative! Recommended Guideline: # Language Name: Optional Style type - Optional bullet points Code Goes Here Other informational text goes here I will ocasionally go along and edit any answer that does not have decent formatting.

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• JQuery/JavaScript div tag "containment" approach/algorithm?

  - by Pete Alvin

  Background: I've created an online circuit design application where div tags are containers that contain smaller div containers and so forth. Question: For any particular div tag I need to quickly identify if it contains other div tags (that may in turn contain other div tags). I've searched JQuery and I don't see any built-in routine for this. Does anyone know of an algorithm that's quicker than O(n^2)? Seems like I have to walk the list of div tags in an outer loop (n) and have an inner loop (another n) to compare against all other div tags and do a "containment test" (position, width, height), building a list of contained div tags. That's n-squared. Then I have to build a list of all nested div tags by concatenating contained lists. So the total would be O(n^2)+n. There must be a better way?

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• Nesting maximum amount of shapes on a surface

  - by Fuu

  In industry, there is often a problem where you need to calculate the most efficient use of material, be it fabric, wood, metal etc. So the starting point is X amount of shapes of given dimensions, made out of polygons and/or curved lines, and target is another polygon of given dimensions. I assume many of the current CAM suites implement this, but having no experience using them or of their internals, what kind of computational algorithm is used to find the most efficient use of space? Can someone point me to a book or other reference that discusses this topic?

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• Distance from a point to a polygon

  - by clwen

  I am trying to determine the distance from a point to a polygon in 2D space. The point can be inside or outside the polygon; The polygon can be convex or concave. If the point is within the polygon or outside the polygon with a distance smaller than a user-defined constant d, the procedure should return True; False otherwise. I have found a similar question: Distance from a point to a polyhedron or to a polygon. However, the space is 2D in my case and the polygon can be concave, so it's somehow different from that one. I suppose there should be a method simpler than offsetting the polygon by d and determining it's inside or outside the polygon. Any algorithm, code, or hints for me to google around 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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• Find point which sum of distances to set of other points is minimal

  - by Pawel Markowski

  I have one set (X) of points (not very big let's say 1-20 points) and the second (Y), much larger set of points. I need to choose some point from Y which sum of distances to all points from X is minimal. I came up with an idea that I would treat X as a vertices of a polygon and find centroid of this polygon, and then I will choose a point from Y nearest to the centroid. But I'm not sure whether centroid minimizes sum of its distances to the vertices of polygon, so I'm not sure whether this is a good way? Is there any algorithm for solving this problem? Points are defined by geographical coordinates.

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• Determine if a day of week and time fall in a custom weekend period

  - by Y Low

  I've been trying to come up with a method, that given a day of week and time of day, it will return if it falls under a custom weekend period. The weekend period is saved in 4 variables as follows: The day of week when it starts (0-6) The time of day when it starts (0-23) The day of week when it ends (0-6) The time of day when it ends (0-23) Based on these fields, the weekend period can be anything the user selected (even if it is well beyond a regular weekend) I've been banging this one for awhile but to no avail. I'm trying this in delphi but any language will do, as I'm looking for the algorithm and not the implementation.

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• How to find all clusters of forest on map?

  - by Damir

  How to find all clusters of forest on map ? I have simple class cell like (Type is enum {RIVER, FOREST,GRASS,HILL} class Cell{ public: Type type; int x; int y }; and map like vector<Cell> grid. Can anyone suggest me algorithm to create list<list<Cell>> clusters where list contains FOREST cells in same cluster (cluster are set of connected cells - connection can be in eight direction:up,down,left,right,up_right,up_left,down_left,down_right)? I need to find all clusters of forest on map and put every single cluster in list<Cell>.

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• Reverse Breadth First traversal in C#

  - by Ngu Soon Hui

  Anyone has a ready implementation of the Reverse Breadth First traversal algorithm in C#? By Reverse Breadth First traversal , I mean instead of searching a tree starting from a common node, I want to search the tree from the bottom and gradually converged to a common node. Let's see the below figure, this is the output of a Breadth First traversal : In my reverse breadth first traversal , 9,10,11 and 12 will be the first few nodes found ( the order of them are not important as they are all first order). 5, 6, 7 and 8 are the second few nodes found, and so on. 1 would be the last node found. Any ideas or pointers? Edit: Change "Breadth First Search" to "Breadth First traversal" to clarify the question

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• Writing a post search algorithm.

  - by MdaG

  I'm trying to write a free text search algorithm for finding specific posts on a wall (similar kind of wall as Facebook uses). A user is suppose to be able to write some words in a search field and get hits on posts that contain the words; with the best match on top and then other posts in decreasing order according to match score. I'm using the edit distance (Levenshtein) "e(x, y) = e" to calculate the score for each post when compared to the query word "x" and post word "y" according to: score(x, y) = 2^(2 - e)(1 - min(e, |x|) / |x|) Each word in a post contributes to the total score for that specific post. This approach seems to work well when the posts are of roughly the same size, but sometime certain large posts manages to rack up score solely on having a lot of words in them while in practice not being relevant to the query. Am I approaching this problem in the wrong way or is there some way to normalize the score that I haven't thought of?

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• Is there a technical way to speed up a general program above current PC speed limit?

  - by Maksee

  Let's imagine I developed a Windows console application implementing some algorithm calculating something. Let's say it doesn't use any threads, just straightforward linear approach with ifs, loops and so on. Is there any technical way to make if run it 100x times faster than on the most advanced current PC? For example one of the way would be to run it on a super computer that emulates i386 faster than any of the existing PCs. But in this case the question what computer and does it really have ability to emulate Windows. In other words, is there real examples of such approach? Although in general it looks useless, but if there is a way, one could develop some program on his general home computer and pay for running it much faster on some other hardware. I suppose that this question could be asked on superuser.com, but since there are possible specific with such things as assembler instructions or threads, I thought that stackoverflow.com is better

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• Algorithm for calculating definite integrals with bounds at infinity

  - by mbac32768

  Suppose I have an integral that's bounded on one (or both) end by (-)infinity. AFAICT, I can't analytically solve this problem, it takes brute force (e.g. using a Left Riemann Sum). I'm having trouble generalizing the algorithm so that it sets the proper subdivisions; I'll either do far too much work to calculate something trivial, or not do nearly enough and have huge aliasing errors. Answering in any language is cool, but maybe someone with better google-fu can end this quickly. :) Is what I'm looking for as impossible as trying to measure the British coastline?

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• Element point map for html5 canvas element, need algorithm

  - by Artiom Chilaru

  I'm currently working on a pure html 5 canvas implementation of the "flying tag cloud sphere", which many of you have undoubtedly seen as a flash object in some pages. The tags are drawn fine, and the performance is satisfactory, but there's one thing in the canvas element that's kind of breaking this idea: you can't identify the objects that you've drawn on a canvas, as it's just a simple flat "image".. What I have to do in this case is catch the click event, and try to "guess" which element was clicked. So I have to have some kind of matrix, which stores a link to a tag object for each pixel on the canvas, AND I have to update this matrix on every redraw. Now this sounds incredibly inefficient, and before I even start trying to implement this, I want to ask the community - is there some "well known" algorithm that would help me in this case? Or maybe I'm just missing something, and the answer is right behind the corner? :)

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