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• Algorithm for sentence analysis and tokenization

  - by Andrea Nagar

  I need to analyze a document and compile statistics as to how many times each a sequence of words is used (so the analysis is not on single words but of batch of recurring words). I read that compression algorithms do something similar to what I want - creating dictionaries of blocks of text with a piece of information reporting its frequency. It should be something similar to http://www.codeproject.com/KB/recipes/Patterns.aspx Do you have anything written in C#?

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• Reverse regular expressions to generate data

  - by Anton Gogolev

  In one of the StackOverflow Podcasts (the one where guys were discussing data generation for testing DBs -- either #11 or #12), Jeff mentioned something like "reverse regular expressions", which are used exactly for that purpose: given a regex, produce a string which will eventually match said regex. What is the correct term for this whole concept? Is this a well-known concept?

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• Effecient data structure design

  - by Sway

  Hi there, I need to match a series of user inputed words against a large dictionary of words (to ensure the entered value exists). So if the user entered: "orange" it should match an entry "orange' in the dictionary. Now the catch is that the user can also enter a wildcard or series of wildcard characters like say "or__ge" which would also match "orange" The key requirements are: * this should be as fast as possible. * use the smallest amount of memory to achieve it. If the size of the word list was small I could use a string containing all the words and use regular expressions. however given that the word list could contain potentially hundreds of thousands of enteries I'm assuming this wouldn't work. So is some sort of 'tree' be the way to go for this...? Any thoughts or suggestions on this would be totally appreciated! Thanks in advance, Matt

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• Finding the maximum weight subsequence of an array of positive integers?

  - by BeeBand

  I'm tring to find the maximum weight subsequence of an array of positive integers - the catch is that no adjacent members are allowed in the final subsequence. The exact same question was asked here, and a recursive solution was given by MarkusQ. He provides an explanation, but can anyone help me understand how he has expanded the function? How does this solution take into consideration non-adjacent members?

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• Count Occurence of Needle String in Haystack String, most optimally?

  - by Taranfx

  The Problem is simple Find "ABC" in "ABCDSGDABCSAGAABCCCCAAABAABC" Here is the solution I propose, I'm looking for any solutions that might be better than this one. public static void main(String[] args) { String haystack = "ABCDSGDABCSAGAABCCCCAAABAABC"; String needle = "ABC"; char [] needl = needle.toCharArray(); int needleLen = needle.length(); int found=0; char hay[] = haystack.toCharArray(); int index =0; int chMatched =0; for (int i=0; i<hay.length; i++){ if (index >= needleLen || chMatched==0) index=0; System.out.print("\nchar-->"+hay[i] + ", with->"+needl[index]); if(hay[i] == needl[index]){ chMatched++; System.out.println(", matched"); }else { chMatched=0; index=0; if(hay[i] == needl[index]){ chMatched++; System.out.print("\nchar->"+hay[i] + ", with->"+needl[index]); System.out.print(", matched"); }else continue; } if(chMatched == needleLen){ found++; System.out.println("found. Total ->"+found); } index++; } System.out.println("Result Found-->"+found); } It took me a while creating this one. Can someone suggest a better solution (if any) P.S. Drop the sysouts if they look messy to you.

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• Where can I find soft-multiply and divide algorithms?

  - by srking

  I'm working on a micro-controller without hardware multiply and divide. I need to cook up software algorithms for these basic operations that are a nice balance of compact size and efficiency. My C compiler port will employ these algos, not the the C developers themselves. My google-fu is so far turning up mostly noise on this topic. Can anyone point me to something informative? I can use add/sub and shift instructions. Table lookup based algos might also work for me, but I'm a bit worried about cramming so much into the compiler's back-end...um, so to speak. Thanks!

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• Find a repeated numbers out of 3 boxes

  - by james1

  I have 3 boxes, each box contain 10 piece of numbered paper (1 - 10) but there is a number the same in all 3 boxes eg: box1 has number 4 and box2 has number 4 and box3 also has number 4. How to find that repeated number in java with an efficient/fastest way possible?

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• Whats the best data-structure for storing 2-tuple (a, b) which support adding, deleting tuples and c

  - by bhups

  Hi So here is my problem. I want to store 2-tuple (key, val) and want to perform following operations: - keys are strings and values are Integers - multiple keys can have same value - adding new tuples - updating any key with new value (any new value or updated value is greater than the previous one, like timestamps) - fetching all the keys with values less than or greater than given value - deleting tuples. Hash seems to be the obvious choice for updating the key's value but then lookups via values will be going to take longer (O(n)). The other option is balanced binary search tree with key and value switched. So now lookups via values will be fast (O(lg(n))) but updating a key will take (O(n)). So is there any data-structure which can be used to address these issues? Thanks.

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• A balanced binary search tree which is also a heap

  - by saeedn

  I'm looking for a data structure where each element in it has two keys. With one of them the structure is a BST and looking at the other one, data structure is a heap. With a little search, I found a structure called Treap. It uses the heap property with a random distribution on heap keys to make the BST balanced! What I want is a Balanced BST, which can be also a heap. The BST in Treap could be unbalanced if I insert elements with heap Key in the order of my choice. Is there such a data structure?

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• Simple/Basic steganography algorithms and methods

  - by tomp

  What are the basic and simpliest steganography algorithms and methods? I mean the steganography applied to images. How does simple program that hides data to images work? How does the program recognize the encrypted message in image without the source image? What are the main techniques used?

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• Writing shorter code/algorithms, is more efficient (performance)?

  - by Carlos

  After coming across the code golf trivia around the site it is obvious people try to find ways to write code and algorithms as short as the possibly can in terms of characters, lines and total size, even if that means writing something like: n=input() while n>1:n=(n/2,n*3+1)[n%2];print n So as a beginner I start to wonder whether size actually matters :D. It is obviously a very subjective question highly dependent on the actual code being used, but what is the rule of thumb in the real world. In the case that size wont matter, how come then we don't focus more on performance rather than size?

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• How to detect how far the object on photo is from another objects on that photo?

  - by Ole Jak

  So for example we have real life photo. how to get (relativly to image dimentions for example) the distance from wall to girls, from girls to trees if all we know ts this picture? Any papers with algorithms or Open Source programs doing this would be appreciated. So How to detect how far the object on photo is from another objects on that photo?

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• Find common nodes from two linked lists using recursion

  - by Dan

  I have to write a method that returns a linked list with all the nodes that are common to two linked lists using recursion, without loops. For example, first list is 2 - 5 - 7 - 10 second list is 2 - 4 - 8 - 10 the list that would be returned is 2 - 10 I am getting nowhere with this.. What I have been think of was to check each value of the first list with each value of the second list recursively but the second list would then be cut by one node everytime and I cannot compare the next value in the first list with the the second list. I hope this makes sense... Can anyone help?

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• How would you write a program to find the shortest pangram in a list of words?

  - by jonathanasdf

  Given a list of words which contains the letters a-z at least once, how would you write a program to find the shortest pangram counted by number of characters (not counting spaces) as a combination of the words? Since I am not sure whether short answers exist, this is not code golf, but rather just a discussion of how you would approach this. However, if you think you can manage to write a short program that would do this, then go ahead, and this might turn into code golf :)

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• Methodologies or algorithms for filling in missing data

  - by tbone

  I am dealing with datasets with missing data and need to be able to fill forward, backward, and gaps. So, for example, if I have data from Jan 1, 2000 to Dec 31, 2010, and some days are missing, when a user requests a timespan that begins before, ends after, or encompasses the missing data points, I need to "fill in" these missing values. Is there a proper term to refer to this concept of filling in data? Imputation is one term, don't know if it is "the" term for it though. I presume there are multiple algorithms & methodologies for filling in missing data (use last measured, using median/average/moving average, etc between 2 known numbers, etc. Anyone know the proper term for this problem, any online resources on this topic, or ideally links to open source implementations of some algorithms (C# preferably, but any language would be useful)

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• What is the context switching time ?

  - by srikanth rongali

  I have a doubt in context switching. In multi threading, when the context switching occurs, what will be the time between two context switches? Is it fixed time interval?

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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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• 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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• An efficient way to compute mathematical constant e

  - by Cinnamon

  Hello, The standard representation of constant e as the sum of the infinite series is very inefficient for computation, because of many division operations. So are there any alternative ways to compute the constant efficiently? Thanks!

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• Are there Adaptive Replacement Cache patent-free alternatives?

  - by aleccolocco

  An open source high-performance project I'm working on needs to keep a cache of parsed/compiled files. A plain LRU or a plain LFU wouldn't fit. Plain LRU wouldn't work as there will be remote batch/spider processes hitting the service regularly. Plain LFU wouldn't work because content will age. ARC seems like the perfect solution but since IBM holds patents to it at least one open source project dropped it. Are there any (good enough) alternatives? EDIT: I'm not looking for exactly the same thing, just something that could handle those two situations. Perhaps some simple strategy with timestamps and sources. There have to be many programmers who faced this situation before. That's why the "good enough" bit.

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• Javascript Number Random Scrambler

  - by stjowa

  Hi, I need a Javascript random number scrambler for my website. Seems simple, but I can not figure out how to do it. Can anyone help me out? I have the following array of numbers: 1 2 3 4 5 6 7 8 9 I would like to be able to have these numbers scrambled randomly. Like the following: 3 6 4 2 9 5 1 8 7 or 4 1 7 3 5 9 2 6 8 So, specifically, I would like a function that takes in an array of numbers (1 - n) and then returns that same array of numbers - scrambled randomly with different calls to the function. Maybe a noob function, but can't seem to figure it out. Thanks!

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• How to find rectangle of difference between two images

  - by Simon Brown

  I have two images the same size. What is the best way to find the rectangle in which they differ. Obviously I could go through the image 4 times in different directions, but i'm wondering if there's an easier way. Example:

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• Fast dot product for a very special case

  - by psihodelia

  Given a vector X of size L, where every scalar element of X is from a binary set {0,1}, it is to find a dot product z=dot(X,Y) if vector Y of size L consists of the integer-valued elements. I suggest, there must exist a very fast way to do it. Let's say we have L=4; X[L]={1, 0, 0, 1}; Y[L]={-4, 2, 1, 0} and we have to find z=X[0]*Y[0] + X[1]*Y[1] + X[2]*Y[2] + X[3]*Y[3] (which in this case will give us -4). It is obvious that X can be represented using binary digits, e.g. an integer type int32 for L=32. Then, all what we have to do is to find a dot product of this integer with an array of 32 integers. Do you have any idea or suggestions how to do it very fast?

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• Why can't RB-Tree be a list?

  - by Alex

  Hey everyone. I have a problem with the rb-trees. according to wikipedia, rb-tree needs to follow the following: A node is either red or black. The root is black. (This rule is used in some definitions and not others. Since the root can always be changed from red to black but not necessarily vice-versa this rule has little effect on analysis.) All leaves are black. Both children of every red node are black. Every simple path from a given node to any of its descendant leaves contains the same number of black nodes. As we know, an rb-tree needs to be balanced and has the height of O(log(n)). But, if we insert an increasing series of numbers (1,2,3,4,5...) and theoretically we will get a tree that will look like a list and will have the height of O(n) with all its nodes black, which doesn't contradict the rb-tree properties mentioned above. So, where am I wrong?? thanks.

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• Operations on bytes in C#

  - by Hooch

  Hello. I'm writing application to control LEDS on LPT. I have everything working except this. This is one small function. I have sth like that: I want to build function that will take two argument and return one number: In actual code those binary numers will be in hex. I put them there like that so that it's easier for you to visualize it. Example1: arg1 = 1100 1100 arg2 = 1001 0001 retu = 0100 1100 Example2: arg1 = 1111 1111 arg2 = 0001 0010 retu = 1110 1101 Example3: arg1 = 1111 0000 arg2 = 0010 0010 retu = 1101 0000 I have no idea how this function should look like. I want it to be as fast as possible. I'll call this function 200 times per second.

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