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• Initialize array in O(1) -- how is this trick called?

  - by user946850

  There is this pattern that trades performance of array access against the need to iterate it when clearing it. You keep a generation counter with each entry, and also a global generation counter. The "clear" operation increases the generation counter. On each access, you compare local vs. global generation counters; if they differ, the array has been reset. This has come up in StackOverflow recently, but I don't remember if this trick has an official name. Does it? One use case is Dijkstra's algorithm if only a tiny subset of the nodes has to be relaxed, and if this has to be done repeatedly.

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• sum with matlab code

  - by user27089

  i need help it is only put sum in a code line if you can understand in below i have a code in this line below -> X(i,k+1) = X(i,k) + (( X(j,k)-X(i,k))); i want to change it as: X(i,k+1) = X(i,k) + sum(( X(j,k)-X(i,k))); briefly how can I put sum on the code line clear;rand('state', 1); global xLocation; N = 4; xLocation = rand(N,1); t=2;k=1;X=[N,t]; for i=1:N X(i,1)=xLocation(i); end while( k < t ) for i = 1 : N for j = 1 : N X(i,k+1) = X(i,k) + (( X(j,k)-X(i,k))); end end k = k + 1; end

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• Attempt at Merge Sort: Is this correct? [migrated]

  - by Beatrice

  I am trying to write a merge sort algo. I can't tell if this is actually a canonical merge sort. If I knew how to calculate the runtime I would give that a go. Does anyone have any pointers? Thanks. public static void main(String[] argsv) { int[] A = {2, 4, 5, 7, 1, 2, 3, 6}; int[] L, R; L = new int[A.length/2]; R = new int[A.length/2]; int i = 0, j = 0, k; for (k = 0; k < A.length; k++) { if (k < A.length/2) { L[i] = A[k]; i++; } else { R[j] = A[k]; j++; } } i = 0; j = 0; for (k = 0; k < A.length; k++) { System.out.println(i + " " + j + " " + k); if (i < L.length && j < R.length) { if (L[i] < R[j]) { A[k] = L[i]; i++; } else { A[k] = R[j]; j++; } } } }

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• Where to find algorithms for standard math functions?

  - by dsimcha

  I'm looking to submit a patch to the D programming language standard library that will allow much of std.math to be evaluated at compile time using the compile-time function evaluation facilities of the language. Compile-time function evaluation has several limitations, the most important ones being: You can't use assembly language. You can't call C code or code for which the source is otherwise unavailable. Several std.math functions violate these and compile-time versions need to be written. Where can I get information on good algorithms for computing things such as logarithms, exponents, powers, and trig functions? I prefer just high level descriptions of algorithms to actual code, for two reasons: To avoid legal ambiguity and the need to make my code look "different enough" from the source to make sure I own the copyright. I want simple, portable algorithms. I don't care about micro-optimization as long as they're at least asymptotically efficient. Edit: D's compile time function evaluation model allows floating point results computed at compile time to differ from those computed at runtime anyhow, so I don't care if my compile-time algorithms don't give exactly the same result as the runtime version as long as they aren't less accurate to a practically significant extent.

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• Is programming or computer science in general, all about algorithms?

  - by wakandan

  As a grad student, I find it more and more common for prestigious companies (like Google, Facebook, Microsoft,...) to put algorithm questions in their test and interviews. A few startups I applied to also asked about algorithms. I wonder if algorithms fluency is the most important thing for software developer in those companies? If the answer being yes, what are the best method or resources for one to learn & practice about algorithms effectively? I can't seem to get interested in solving seemingly too complicated problems found in most textbook or websites. Though easily understand basic algorithms (like quicksort, bubblesort,...), I find it immensely difficult to remember and reuse them later. Thanks. P/S: If you ask me what I like, it's building good softwares to solve users' problems innovatively. I suppose that does not necessarily mean the software has to be very complicated.

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• What algorithms did Dijkstra develop?

  - by baris_a

  I recently asked a question about one of the Dijkstra’s algorithms (shunting-yard). But almost everyone thought "Dijkstra's algorithm" meant his shortest path algorithm. What other algorithms has Dijkstra developed?

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• Dijkstra’s algorithms - a complete list

  - by baris_a

  Hi guys, I have recently asked a question about one of the Dijkstra’s algorithms. But, almost everyone thought it was shortest path. Therefore, I opened this post to gather all the algorithms that were invented by Dijkstra. Please add any if you know. Thanks in advance. 1 ) Shunting-yard algorithm

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• Techniques and algorithms for precise and efficient graph plotting

  - by Viet

  I'd like to plot mathematical functions and I'm looking for techniques and algorithms for precise and efficient plotting. 2D plotting should suffice. I focus on techniques and algorithms, not libraries or languages. Thanks in advance!

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• OOP vs PP for algorithms

  - by Ilian

  Which paradigm is better for design and analysis of algorithms? Which is faster? Because I have a subject called Design and Analysis of Algorithms in university and have a time limit for programs. Is OOP slower than Procedure programming? Or the time difference is not big?

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• Helper library for distributed algorithms programming?

  - by anon

  When you code a distributed algorithm, do you use any library to model abstract things like processor, register, message, link, etc.? Is there any library that does that? I'm thinking about e.g. self-stabilizing algorithms, like self-stabilizing minimum spanning-tree algorithms.

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• Amazing families of algorithms over implicit graphs

  - by Diego de Estrada

  Dynamic programming is, almost by definition, to find a shortest/longest path on an implicit dag. Every DP algorithm just does this. An Holographic algorithm can be loosely described as something that counts perfect matchings in implicit planar graphs. So, my question is: are there any other families of algorithms that use well-known algorithms over implicit graphs to achieve a considerable speedup?

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• Algorithms behind load-balancers?

  - by Vimvq1987

  I need to study about load-balancers, such as Network Load Balancing, Linux Virtual Server, HAProxy,...There're somethings under-the-hood I need to know: What algorithms/technologies are used in these load-balancers? Which is the most popular? most effective? I expect that these algorithms/technologies will not be too complicated. Are there some resources written about them? Thank you very much for your help.

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• graph algorithms

  - by davit-datuashvili

  now one ask please help me to write a few graph algorithms for example http://en.wikipedia.org/wiki/Dijkstra%27s_algorithm there is given such graph my problem is that i want implement graph algorithms on arrays can anybody help me to imlement ddijkstra algorithm on array i want to see one example because it is difficulty for me to understand this pseudocodes which is in internet i mean classes edges and so on please help me

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• how to measure running time of algorithms in python

  - by Bunny Rabbit

  How can i mesure and compare the running times of my algorithms written in python .Also point me to a nice algorithms site/forum like stackoverflow if you can.

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• Sync Algorithms

  - by Kristopher Johnson

  Are there any good references out there for sync algorithms? I'm interested in algorithms that synchronize the following kinds of data between multiple users: calendars documents lists and outlines I'm not just looking for synchronization of contents of directories a la rsync; I am interested in merging the data within individual files.

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• Algorithms behind FarmVille Game

  - by Vadi

  What are all the algorithms involved in Farmville game, specifically I am interested in drawing trees that has fruits based on user's activities. I am into a project which has a specific need to draw a tree-type image in SVG. I am not sure how to go about the algorithms to define the tree and based on certain business rules the leafs in the tree grows etc., I think you get the idea. Farmville is just an example I took to explain. Any help is greatly appreciated..

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• LaTeX package for writing algorithms

  - by lmsasu

  Hi all, which package would you use for writing algorithms in LaTeX? I know about algorithm, algorithmic and algorithms packages. Thanks

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• Algorithms after load-balancer?

  - by Vimvq1987

  I need to study about load-balancers, such as Network Load Balancing, Linux Virtual Server, HAProxy,...There're somethings under-the-hood I need to know: What algorithms/technologies are used in these load-balancers? Which is the most popular? most effective? I expect that these algorithms/technologies will not be too complicated. Are there some resources written about them? Thank you very much for your help.

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• Is there a shorthand term for O(n log n)?

  - by jemfinch

  We usually have a single-word shorthand for most complexities we encounter in algorithmic analysis: O(1) == "constant" O(log n) == "logarithmic" O(n) == "linear" O(n^2) == "quadratic" O(n^3) == "cubic" O(2^n) == "exponential" We encounter algorithms with O(n log n) complexity with some regularity (think of all the algorithms dominated by sort complexity) but as far as I know, there's no single word we can use in English to refer to that complexity. Is this a gap in my knowledge, or a real gap in our English discourse on computational complexity?

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• Using local classes with STL algorithms

  - by David Rodríguez - dribeas

  I have always wondered why you cannot use locally defined classes as predicates to STL algorithms. In the question: Approaching STL algorithms, lambda, local classes and other approaches, BubbaT mentions says that 'Since the C++ standard forbids local types to be used as arguments' Example code: int main() { int array[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; std::vector<int> v( array, array+10 ); struct pair : public std::unary_function<int,bool> { bool operator()( int x ) { return !( x % 2 ); } }; std::remove_if( v.begin(), v.end(), pair() ); // error } Does anyone know where in the standard is the restriction? What is the rationale for disallowing local types?

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• Recursive vs. Iterative algorithms

  - by teehoo

  I'm implementing the Euclidian algorithm for finding the GCD (Greatest Common Divisor) of two integers. Two sample implementations are given: Recursive and Iterative. http://en.wikipedia.org/wiki/Euclidean_algorithm#Implementations My Question: In school I remember my professors talking about recursive functions like they were all the rage, but I have one doubt. Compared to an iterative version don't recursive algorithms take up more stack space and therefore much more memory? Also, because calling a function requires uses some overhead for initialization, aren't recursive algorithms more slower than their iterative counterpart?

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• Choice of programming language for learning data structures and algorithms

  - by bguiz

  Which programming language would you recommend to learn about data structures and algorithms in? Considering the follwing: Personal experience Language features (pointers, OO, etc) Suitability for learning DS & A concepts I ask because there are some books out there that are programming language-agnostic (written from a Mathematical perspective, and use pseudocode). If I learn from one of these I would like to work out the algorithms in a chosen language. Then, there are other books which introduce DS & A concepts with examples in a particular programming laguage - and I would follow these examples as well. Either way, I have to choose a language, and I would like to stick to one throughout. Which one best fits the bill.

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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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• Bouncycastle encryption algorithms not provided

  - by David Read

  I'm trying to use BouncyCastle with android to implement ECDH and EL Gamal. I've added the bouncycastle jar file (bcprov-jdk16-144.jar) and written some code that works with my computers jvm however when I try and port it to my android application it throws: java.security.NoSuchAlgorithmException: KeyPairGenerator ECDH implementation not found A sample of the code is: Security.addProvider(new org.bouncycastle.jce.provider.BouncyCastleProvider()); java.security.KeyPairGenerator keyGen = org.bouncycastle.jce.provider.asymmetric.ec.KeyPairGenerator.getInstance("ECDH", "BC"); ECGenParameterSpec ecSpec = new ECGenParameterSpec("prime192v1"); keyGen.initialize(ecSpec, SecureRandom.getInstance("SHA1PRNG")); KeyPair pair = keyGen.generateKeyPair(); PublicKey pubk = pair.getPublic(); PrivateKey prik = pair.getPrivate(); I then wrote a simple program to see what encryption algorithms are available and ran it on my android emulator and on my computers jvm the code was: Set<Provider.Service> rar = new org.bouncycastle.jce.provider.BouncyCastleProvider().getServices(); Iterator<Provider.Service> ir = rar.iterator(); while(ir.hasNext()) System.out.println(ir.next().getAlgorithm()); On android I do not get any of the EC algorithms while ran normally on my computer it's fine. I'm also getting the following two errors when compiling for a lot of the bouncy castle classes: 01-07 17:17:42.548: INFO/dalvikvm(1054): DexOpt: not resolving ambiguous class 'Lorg/bouncycastle/asn1/ASN1Encodable;' 01-07 17:17:42.548: DEBUG/dalvikvm(1054): DexOpt: not verifying 'Lorg/bouncycastle/asn1/ess/OtherSigningCertificate;': multiple definitions What am I doing wrong?

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• Suggestions of the easiest algorithms for some Graph operations

  - by Nazgulled

  Hi, The deadline for this project is closing in very quickly and I don't have much time to deal with what it's left. So, instead of looking for the best (and probably more complicated/time consuming) algorithms, I'm looking for the easiest algorithms to implement a few operations on a Graph structure. The operations I'll need to do is as follows: List all users in the graph network given a distance X List all users in the graph network given a distance X and the type of relation Calculate the shortest path between 2 users on the graph network given a type of relation Calculate the maximum distance between 2 users on the graph network Calculate the most distant connected users on the graph network A few notes about my Graph implementation: The edge node has 2 properties, one is of type char and another int. They represent the type of relation and weight, respectively. The Graph is implemented with linked lists, for both the vertices and edges. I mean, each vertex points to the next one and each vertex also points to the head of a different linked list, the edges for that specific vertex. What I know about what I need to do: I don't know if this is the easiest as I said above, but for the shortest path between 2 users, I believe the Dijkstra algorithm is what people seem to recommend pretty often so I think I'm going with that. I've been searching and searching and I'm finding it hard to implement this algorithm, does anyone know of any tutorial or something easy to understand so I can implement this algorithm myself? If possible, with C source code examples, it would help a lot. I see many examples with math notations but that just confuses me even more. Do you think it would help if I "converted" the graph to an adjacency matrix to represent the links weight and relation type? Would it be easier to perform the algorithm on that instead of the linked lists? I could easily implement a function to do that conversion when needed. I'm saying this because I got the feeling it would be easier after reading a couple of pages about the subject, but I could be wrong. I don't have any ideas about the other 4 operations, suggestions?

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