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• Effective way to calculate a similarity percentage between data sets

  - by UltraVi01

  I am currently working with User objects -- each of which have many Goal objects. The Goal objects are not User specific, that is, Users can share the same Goal. I am attempting to fashion a way to calculate a "similarity percentage" between two Users... (i.e., taking into account how many Goals they share as well as how many Goals they do not share) Does anyone have experience with this type of situation? I am using Grails with Mysql if that is helpful. Thanks

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• How are PID's generated?

  - by Helltone

  How are *nix PID's generated and how do they get recycled?

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• Classical task-scheduling assignment

  - by Bruno

  I am working on a flight scheduling app (disclaimer: it's for a college project, so no code answers, please). Please read this question w/ a quantum of attention before answering as it has a lot of peculiarities :( First, some terminology issues: You have planes and flights, and you have to pair them up. For simplicity's sake, we'll assume that a plane is free as soon as the flight using it prior lands. Flights are seen as tasks: They have a duration They have dependencies They have an expected date/time for beginning Planes can be seen as resources to be used by tasks (or flights, in our terminology). Flights have a specific type of plane needed. e.g. flight 200 needs a plane of type B. Planes obviously are of one and only one specific type, e.g., Plane Airforce One is of type C. A "project" is the set of all the flights by an airline in a given time period. The functionality required is: Finding the shortest possible duration for a said project The earliest and latest possible start for a task (flight) The critical tasks, with basis on provided data, complete with identifiers of preceding tasks. Automatically pair up flights and planes, so as to get all flights paired up with a plane. (Note: the duration of flights is fixed) Get a Gantt diagram with the projects scheduling, in which all flights begin as early as possible, showing all previously referred data graphically (dependencies, time info, etc.) So the questions is: How in the world do I achieve this? Particularly: We are required to use a graph. What do the graph's edges and nodes respectively symbolise? Are we required to discard tasks to achieve the critical tasks set? If you could also recommend some algorithms for us to look up, that'd be great.

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• assignment vs std::swap and merging and keeping duplicates in seperate object

  - by rubenvb

  Say I have two std::set<std::string>s. The first one, old_options, needs to be merged with additional options, contained in new_options. I can't just use std::merge (well, I do, but not only that) because I also check for doubles and warn the user about this accordingly. To this effect, I have void merge_options( set<string> &old_options, const set<string> &new_options ) { // find duplicates and create merged_options, a stringset containing the merged options // handle duplicated the way I want to // ... old_options = merged_options; } Is it better to use std::swap( merged_options, old_options ); or the assignment I have? Is there a better way to filter duplicates and return the merged set than consecutive calls to std::set_intersection and std::set_union to detect dupes and merge the sets? I know it's slower than one traversal and doing both at once, but these sets are small (performance is not critical) and I trust the Standard more than I trust myself.

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• How does the Amazon Recommendation feature work?

  - by Rachel

  What technology goes in behind the screens of Amazon recommendation technology? I believe that Amazon recommendation is currently the best in the market, but how do they provide us with such relevant recommendations? Recently, we have been involved with similar recommendation kind of project, but would surely like to know about the in and outs of the Amazon recommendation technology from a technical standpoint. Any inputs would be highly appreciated. Update: This patent explains how personalized recommendations are done but it is not very technical, and so it would be really nice if some insights could be provided. From the comments of Dave, Affinity Analysis forms the basis for such kind of Recommendation Engines. Also here are some good reads on the Topic Demystifying Market Basket Analysis Market Basket Analysis Affinity Analysis Suggested Reading: Data Mining: Concepts and Technique

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• Fast, Vectorizable method of taking floating point number modulus of special primes?

  - by caffiend

  Is there a fast method for taking the modulus of a floating point number? With integers, there are tricks for Mersenne primes, so that its possible to calculate y = x MOD 2^31 without needing division. Can any similar tricks be applied for floating point numbers? Preferably, in a way that can be converted into vector/SIMD operations, or moved into GPGPU code. The primes I'm interested in would be 2^7 and 2^31, although if there are more efficient ones for floating point numbers, those would be welcome.

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• Need help with basic optimization problem

  - by ??iu

  I know little of optimization problems, so hopefully this will be didactic for me: rotors = [1, 2, 3, 4...] widgets = ['a', 'b', 'c', 'd' ...] assert len(rotors) == len(widgets) part_values = [ (1, 'a', 34), (1, 'b', 26), (1, 'c', 11), (1, 'd', 8), (2, 'a', 5), (2, 'b', 17), .... ] Given a fixed number of widgets and a fixed number of rotors, how can you get a series of widget-rotor pairs that maximizes the total value where each widget and rotor can only be used once?

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• Given a word, how do I get the list of all words, that differ by one letter?

  - by user187809

  Let's say I have the word "CAT". These words differ from "CAT" by one letter (not the full list) CUT CAP PAT FAT COT etc. Is there an elegant way to generate this? Obviously, one way to do it is through brute force. pseduo code: while (0 to length of word) while (A to Z) replace one letter at a time, and check if the resulting word is a valid word If I had a 10 letter word, the loop would run 26 * 10 = 260 times. Is there a better, elegant way to do this?

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• Check if a string substitution rule will ever generate another string.

  - by Mgccl

  Given two strings S and T of same length. Given a set of replacement rules, that find substring A in S and replace it with string B. A and B have the same length. Is there a sequence of rule application, such that it make string S into string T? I believe there is no better way to answer this than try every single rule in every single state. Which would be exponential time. But I don't know if there are better solutions to it.

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• I know my Before Tax Pay and my After Tax Pay, how can I work out how much I get taxed?

  - by Pete

  I've been entering some data into an Excel spreadsheet to work out my monthly earnings, etc. and was wondering how I can I find out how much I'm getting taxed? Say this is my current spreadsheet: Hours Worked 37.5 39.5 37.5 30 Hourly Rate $25 $25 $25 $25 Before Tax 937.50 987.50 937.50 750.00 After Tax 260.00 276.00 260.00 ??? How can I use this known data to work out my After Tax pay for the 4th column? :/

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• Find the centroid of a polygon with weighted vertices

  - by Calle Kabo

  Hi, I know how to find the centroid (center of mass) of a regular polygon. This assumes that every part of the polygon weighs the same. But how do I calculate the centroid of a weightless polygon (made from aerogel perhaps :), where each vertex has a weight? Simplified illustration of what I mean using straight line: 5kg-----------------5kg ^center of gravity 10kg---------------5kg ^center of gravity offset du to weight of vertices Of course, I know how to calculate the center of gravity on a straight line with weighted vertices, but how do I do it on a polygon with weighted vertices? Thanks for your time!

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• generic binary Search in c#

  - by Pro_Zeck

  Below is my Generic Binary Search it works ok with the intgers type array it finds all the elements in it . But the Problem Arises when i use a string array to find any string data. It runs ok for the first index and last index elements but i cant find the middle elements. Stringarray = new string[] { "b", "a", "ab", "abc", "c" }; public static void BinarySearch<T>(T[] array, T searchFor, Comparer<T> comparer) { int high, low, mid; high = array.Length - 1; low = 0; if (array[0].Equals(searchFor)) Console.WriteLine("Value {0} Found At Index {1}",array[0],0); else if (array[high].Equals(searchFor)) Console.WriteLine("Value {0} Found At Index {1}", array[high], high); else { while (low <= high) { mid = (high + low) / 2; if (comparer.Compare(array[mid], searchFor) == 0) { Console.WriteLine("Value {0} Found At Index {1}", array[mid], mid); break; } else { if (comparer.Compare(searchFor, array[mid]) > 0) high = mid + 1; else low = mid + 1; } } if (low > high) { Console.WriteLine("Value Not Found In the Collection"); } } }

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• How to compute palindrome from a stream of characters in sub-linear space/time?

  - by wrick

  I don't even know if a solution exists or not. Here is the problem in detail. You are a program that is accepting an infinitely long stream of characters (for simplicity you can assume characters are either 1 or 0). At any point, I can stop the stream (let's say after N characters were passed through) and ask you if the string received so far is a palindrome or not. How can you do this using less sub-linear space and/or time.

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• Time Complexities of recursive algorithms

  - by Peter

  Whenever I see a recursive solution, or I write recursive code for a problem, it is really difficult for me to figure out the time complexity, in most of the cases I just say its exponential? How is it exponential actually? How people say it is 2^n, when it is n!, when it is n^n or n^k. I have some questions in mind, let say find all permutations of a string (O(n!)) find all sequences which sum up to k in an array (exponential, how exactly do I calculate). Find all subsets of size k whose sum is 0 (will k come somewhere in complexity , it should come right?). Can any1 help me how to calculate the exact complexity of such questions, I am able to wrote code for them , but its hard understanding the exact time complexity.

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• Is there a website to lookup already common code functions?

  - by pinnacler

  I'm sitting here writing a function that I'm positive has been written before, somewhere on earth. It's just too common to have not been attempted, and I'm wondering why I can't just go to a website and search for a function that I can then copy and paste into my project in 2 seconds, instead of wasting my day reinventing the wheel. Sure there are certain libraries you can use, but where do you find these libraries and when they are absent, is there a site like I'm describing?

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• [C++] std::string manipulation: whitespace, "newline escapes '\'" and comments #

  - by rubenvb

  Kind of looking for affirmation here. I have some hand-written code, which I'm not shy to say I'm proud of, which reads a file, removes leading whitespace, processes newline escapes '\' and removes comments starting with #. It also removes all empty lines (also whitespace-only ones). Any thoughts/recommendations? I could probably replace some std::cout's with std::runtime_errors... but that's not a priority here :) const int RecipeReader::readRecipe() { ifstream is_recipe(s_buffer.c_str()); if (!is_recipe) cout << "unable to open file" << endl; while (getline(is_recipe, s_buffer)) { // whitespace+comment removeLeadingWhitespace(s_buffer); processComment(s_buffer); // newline escapes + append all subsequent lines with '\' processNewlineEscapes(s_buffer, is_recipe); // store the real text line if (!s_buffer.empty()) v_s_recipe.push_back(s_buffer); s_buffer.clear(); } is_recipe.close(); return 0; } void RecipeReader::processNewlineEscapes(string &s_string, ifstream &is_stream) { string s_temp; size_t sz_index = s_string.find_first_of("\\"); while (sz_index <= s_string.length()) { if (getline(is_stream,s_temp)) { removeLeadingWhitespace(s_temp); processComment(s_temp); s_string = s_string.substr(0,sz_index-1) + " " + s_temp; } else cout << "Error: newline escape '\' found at EOF" << endl; sz_index = s_string.find_first_of("\\"); } } void RecipeReader::processComment(string &s_string) { size_t sz_index = s_string.find_first_of("#"); s_string = s_string.substr(0,sz_index); } void RecipeReader::removeLeadingWhitespace(string &s_string) { const size_t sz_length = s_string.size(); size_t sz_index = s_string.find_first_not_of(" \t"); if (sz_index <= sz_length) s_string = s_string.substr(sz_index); else if ((sz_index > sz_length) && (sz_length != 0)) // "empty" lines with only whitespace s_string.clear(); } Some extra info: the first s_buffer passed to the ifstream contains the filename, std::string s_buffer is a class data member, so is std::vector v_s_recipe. Any comment is welcome :)

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• How do LL(*) parsers work?

  - by freezer878

  I cannot find any complete description about LL(*) parser, such as ANTLR, on Internet. I'm wondering what is the difference between an LL(k) parser and an LL(*) one and why they can't support left-recusrive grammars despite their flexibility.

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• How is load balancing in big systems implemented?

  - by uther-lightbringer

  Hello, I'm wondering how is implemented load balancing in realy big applications like google or facebook. I know that in normal scenario there may be machine dedicated to this task, but I would like to know how is it resolved in realy big aplication with hundreds of thousans people accessing it in any given time. I am just wondering how exactly when one types google.com will that request find its way to concrete computer (are there multiple load balancers? and how is it set up and implemented that user's request will find the way to concrete balancer out of many others). I will realy appreciate if someone enlightens me this issue, thank you.

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• Determine coordinates of rotated rectangle

  - by MathieuK

  I'm creating an utility application that should detect and report the coordinates of the corners of a transparent rectangle (alpha=0) within an image. So far, I've set up a system with Javascript + Canvas that displays the image and starts a floodfill-like operation when I click inside the transparent rectangle in the image. It correctly determines the bounding box of the floodfill operation and as such can provide me the correct coordinates. Here's my implementation so-far: http://www.scriptorama.nl/image/ (works in recent Firefox / Safari ). However, the bounding box approach breaks down then the transparent rectangle is rotated (CW or CCW) as the resulting bounding box no longer properly represents the proper width and height. I've tried to come up with a few alternatives to detect to corners, but have not been able to think up a proper solution. So, does anyone have any suggestions on how I might approach this so I can properly detect the coordinates of 4 corners of the rotated rectangle?

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• How can I solve the Log Pile wooden puzzle with a computer program?

  - by craig1410

  Can anyone suggest how to solve the Log Pile wooden puzzle using a computer program? See here to visualise the puzzle: http://www.puzzlethis.co.uk/products/madcow/the_log_pile.htm The picture only shows some of the pieces. The full set of 10 pieces are configured as follows with 1 representing a peg, -1 representing a hole and 0 representing neither a peg nor a hole. -1,1,0,-1,0 1,0,1,0,0 1,-1,1,0,0 -1,-1,0,0,-1 -1,1,0,1,0 0,1,0,0,1 1,0,-1,0,-1 0,-1,0,1,0 0,0,-1,1,-1 1,0,-1,0,0 The pieces can be interlocked in two layers of 5 pieces each with the top layer at 90 degrees to the bottom layer as shown in the above link. I have already created a solution to this problem myself using Java but I feel that it was a clumsy solution and I am interested to see some more sophisticated solutions. Feel free to either suggest a general approach or to provide a working program in the language of your choice. My approach was to use the numeric notation above to create an array of "Logs". I then used a combination/permutation generator to try all possible arrangements of the Logs until a solution was found where all the intersections equated to zero (ie. Peg to Hole, Hole to Peg or Blank to Blank). I used some speed-ups to detect the first failed intersection for a given permutation and move on to the next permutation. I hope you find this as interesting as I have. Thanks, Craig.

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• Finding patterns in Puzzle games.

  - by José Joel.

  I was wondering, which are the most commonly used algorithms applied to finding patterns in puzzle games conformed by grids of cells. I know that depends of many factors, like the kind of patterns You want to detect, or the rules of the game...but I wanted to know which are the most commonly used algorithms in that kind of problems... For example, games like columns, bejeweled, even tetris. I also want to know if detecting patterns by "brute force" ( like , scanning all the grid trying to find three adyacent cells of the same color ) is significantly worst that using particular algorithms in very small grids, like 4 X 4 for example ( and again, I know that depends of the kind of game and rules ...) Which structures are commonly used in this kind of games ?

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• How to optimize this Python code?

  - by RandomVector

  def maxVote(nLabels): count = {} maxList = [] maxCount = 0 for nLabel in nLabels: if nLabel in count: count[nLabel] += 1 else: count[nLabel] = 1 #Check if the count is max if count[nLabel] > maxCount: maxCount = count[nLabel] maxList = [nLabel,] elif count[nLabel]==maxCount: maxList.append(nLabel) return random.choice(maxList) nLabels contains a list of integers. The above function returns the integer with highest frequency, if more than one have same frequency then a randomly selected integer from them is returned. E.g. maxVote([1,3,4,5,5,5,3,12,11]) is 5

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• Python - Compress Ascii String

  - by n0idea

  I'm looking for a way to compress an ascii-based string, any help? I need also need to decompress it. I tried zlib but with no help. What can I do to compress the string into lesser length? code: def compress(request): if request.POST: data = request.POST.get('input') if is_ascii(data): result = zlib.compress(data) return render_to_response('index.html', {'result': result, 'input':data}, context_instance = RequestContext(request)) else: result = "Error, the string is not ascii-based" return render_to_response('index.html', {'result':result}, context_instance = RequestContext(request)) else: return render_to_response('index.html', {}, context_instance = RequestContext(request))

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• Auto scale and rotate images

  - by Dave Jarvis

  Given: two images of the same subject matter; the images have the same resolution, colour depth, and file format; the images differ in size and rotation; and two lists of (x, y) co-ordinates that correlate the images. I would like to know: How do you transform the larger image so that it visually aligns to the second image? (Optional.) What are the minimum number of points needed to get an accurate transformation? (Optional.) How far apart do the points need to be to get an accurate transformation? The transformation would need to rotate, scale, and possibly shear the larger image. Essentially, I want to create (or find) a program that does the following: Input two images (e.g., TIFFs). Click several anchor points on the small image. Click the several corresponding anchor points on the large image. Transform the large image such that it maps to the small image by aligning the anchor points. This would help align pictures of the same stellar object. (For example, a hand-drawn picture from 1855 mapped to a photograph taken by Hubble in 2000.) Many thanks in advance for any algorithms (preferably Java or similar pseudo-code), ideas or links to related open-source software packages.

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• List of Big-O for PHP functions?

  - by Kendall Hopkins

  After using PHP for a while now, I've noticed that not all PHP built in functions as fast as expected. Consider the below two possible implementations of a function that finds if a number is prime using a cached array of primes. //very slow for large $prime_array $prime_array = array( 2, 3, 5, 7, 11, 13, .... 104729, ... ); $result_array = array(); foreach( $array_of_number => $number ) { $result_array[$number] = in_array( $number, $large_prime_array ); } //still decent performance for large $prime_array $prime_array => array( 2 => NULL, 3 => NULL, 5 => NULL, 7 => NULL, 11 => NULL, 13 => NULL, .... 104729 => NULL, ... ); foreach( $array_of_number => $number ) { $result_array[$number] = array_key_exists( $number, $large_prime_array ); } This is because in_array is implemented with a linear search O(n) which will linearly slow down as $prime_array grows. Where the array_key_exists function is implemented with a hash lookup O(1) which will not slow down unless the hash table gets extremely populated (in which case it's only O(logn)). So far I've had to discover the big-O's via trial and error, and occasionally looking at the source code. Now for the question... I was wondering if there was a list of the theoretical (or practical) big O times for all* the PHP built in functions. *or at least the interesting ones For example find it very hard to predict what the big O of functions listed because the possible implementation depends on unknown core data structures of PHP: array_merge, array_merge_recursive, array_reverse, array_intersect, array_combine, str_replace (with array inputs), etc.

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