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  • Explain BFS and DFS in terms of backtracking

    - by HH
    Wikipedia about DFS Depth-first search (DFS) is an algorithm for traversing or searching a tree, tree structure, or graph. One starts at the root (selecting some node as the root in the graph case) and explores as far as possible along each branch before backtracking. So is BFS? "an algorithm that choose a starting node, checks all nodes -- backtracks --, chooses the shortest path, chose neighbour nodes -- backtracks --, chose the shortest path -- finally finds the optimal path because of traversing each path due to continuos backtracking. Regex, find's pruning -- backtracking? The term backtracking confuseses due to its variety of use. UNIX find's pruning an SO-user explained with backtracking. Regex Buddy uses the term "catastrophic backtracking" if you do not limit the scope of your Regexes. It seems to be too wide umbrella-term. So: how do you define "Backtracking" GRAPH-theoretically? what is "backtracking" in BFS and DFS?

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  • Matrix inversion in OpenCL

    - by buchtak
    Hi, I am trying to accelerate some computations using OpenCL and part of the algorithm consists of inverting a matrix. Is there any open-source library or freely available code to compute lu factorization (lapack dgetrf and dgetri) of matrix or general inversion written in OpenCL or CUDA? The matrix is real and square but doesn't have any other special properties besides that. So far, I've managed to find only basic blas matrix-vector operations implementations on gpu. The matrix is rather small, only about 60-100 rows and cols, so it could be computed faster on cpu, but it's used kinda in the middle of the algorithm, so I would have to transfer it to host, calculate the inverse, and then transfer the result back on the device where it's then used in much larger computations.

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  • Made an interview mistake. Should I try to correct after the fact?

    - by AT Developer
    Ever been in a situation where you were in an interview, and realized immediately afterwards (after the nervousness wore off) that you did something wrong? I had a phone interview today. I was asked an n-ary tree problem, and coded an algorithm that used a space overhead, then a different algorithm with no space overhead. However, my solution was inefficient, since I traversed the tree top-down rather than bottom-up. The interviewer said I did a good job, but I'm still wondering if he noticed and marked down for my choice of implementation. Should I follow up with an email correcting myself, or just let it and avoid making things worse?

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  • PBKDF2-HMAC-SHA1

    - by Jason
    To generate a valid pairwise master key for a WPA2 network a router uses the PBKDF2-HMAC-SHA1 algorithm. I understand that the sha1 function is performed 4096 times to derive the PMK, however I have two questions about the process. Excuse the pseudo code. 1) How is the input to the first instance of the SHA1 function formatted? SHA1("network_name"+"network_name_length"+"network_password") Is it formatted in that order, is it the hex value of the network name, length and password or straight ASCII? Then from what I gather the 160 bit digest received is fed straight into another round of hashing without any additional salting. Like this: SHA1("160bit digest from last round of hashing") Rise and repeat. 2) Once this occurs 4096 times 256 bits of the output is used as the pairwise master key. What I don't understand is that if SHA1 produces 160bit output, how does the algorithm arrive at the 256bits required for a key? Thanks for the help.

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  • How to use sudo with WinSCP and ProFTPd?

    - by Gaia
    I need to run the SFTP fileserver binary as root, but direct root login is not allowed. In WinSCP, if I use "default" on SFTP server protocol option everything works as expected. Following the instructions to sudo in WinSCP, I tried using "sudo /usr/sbin/proftpd" (works on the command line without any prompts) but it brings up "Cannot initialize SFTP protocol. Is the host running a SFTP server?" How to use sudo with WinSCP and ProFTPd? WinSCP 4.3.7 GUI Protocol: SFTP-3 CentOS 6.2 Webmin/Virtualmin (Current Version) PS: only cert based login is allowed . 2012-06-17 11:05:56.998 -------------------------------------------------------------------------- . 2012-06-17 11:05:56.998 WinSCP Version 4.3.7 (Build 1679) (OS 6.1.7601 Service Pack 1) . 2012-06-17 11:05:56.998 Configuration: HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\ . 2012-06-17 11:05:56.999 Login time: Sunday, June 17, 2012 11:05:56 AM . 2012-06-17 11:05:56.999 -------------------------------------------------------------------------- . 2012-06-17 11:05:56.999 Session name: KVM1 (Modified stored session) . 2012-06-17 11:05:57.047 Host name: mykvm.com (Port: 22) . 2012-06-17 11:05:57.048 User name: adminuser (Password: No, Key file: Yes) . 2012-06-17 11:05:57.048 Tunnel: No . 2012-06-17 11:05:57.048 Transfer Protocol: SFTP (SCP) . 2012-06-17 11:05:57.048 Ping type: -, Ping interval: 30 sec; Timeout: 15 sec . 2012-06-17 11:05:57.048 Proxy: none . 2012-06-17 11:05:57.048 SSH protocol version: 2; Compression: Yes . 2012-06-17 11:05:57.048 Bypass authentication: No . 2012-06-17 11:05:57.048 Try agent: Yes; Agent forwarding: No; TIS/CryptoCard: No; KI: Yes; GSSAPI: No . 2012-06-17 11:05:57.048 Ciphers: aes,blowfish,3des,WARN,arcfour,des; Ssh2DES: No . 2012-06-17 11:05:57.048 SSH Bugs: -,-,-,-,-,-,-,-,- . 2012-06-17 11:05:57.048 SFTP Bugs: -,- . 2012-06-17 11:05:57.048 Return code variable: Autodetect; Lookup user groups: Yes . 2012-06-17 11:05:57.048 Shell: default . 2012-06-17 11:05:57.048 EOL: 0, UTF: 2 . 2012-06-17 11:05:57.048 Clear aliases: Yes, Unset nat.vars: Yes, Resolve symlinks: Yes . 2012-06-17 11:05:57.048 LS: ls -la, Ign LS warn: Yes, Scp1 Comp: No . 2012-06-17 11:05:57.048 Local directory: default, Remote directory: home, Update: No, Cache: Yes . 2012-06-17 11:05:57.048 Cache directory changes: Yes, Permanent: Yes . 2012-06-17 11:05:57.048 DST mode: 1 . 2012-06-17 11:05:57.048 -------------------------------------------------------------------------- . 2012-06-17 11:05:57.113 Looking up host "mykvm.com" . 2012-06-17 11:05:57.132 Connecting to xxx.xxx.128.59 port 22 . 2012-06-17 11:05:57.499 Server version: SSH-2.0-OpenSSH_5.3 . 2012-06-17 11:05:57.499 Using SSH protocol version 2 . 2012-06-17 11:05:57.499 We claim version: SSH-2.0-WinSCP_release_4.3.7 . 2012-06-17 11:05:57.679 Server supports delayed compression; will try this later . 2012-06-17 11:05:57.679 Doing Diffie-Hellman group exchange . 2012-06-17 11:05:58.077 Doing Diffie-Hellman key exchange with hash SHA-1 . 2012-06-17 11:05:58.498 Host key fingerprint is: . 2012-06-17 11:05:58.498 ssh-rsa 2048 bd:e4:34:b1:d4:69:d6:4e:e4:26:04:8b:b7:b3:de:c3 . 2012-06-17 11:05:58.498 Initialised AES-256 SDCTR client->server encryption . 2012-06-17 11:05:58.498 Initialised HMAC-SHA1 client->server MAC algorithm . 2012-06-17 11:05:58.498 Initialised AES-256 SDCTR server->client encryption . 2012-06-17 11:05:58.498 Initialised HMAC-SHA1 server->client MAC algorithm . 2012-06-17 11:05:58.922 Reading private key file "D:\id_rsa.ppk" ! 2012-06-17 11:05:58.924 Using username "adminuser". . 2012-06-17 11:05:59.550 Offered public key . 2012-06-17 11:05:59.743 Offer of public key accepted ! 2012-06-17 11:05:59.743 Authenticating with public key "masterkey for admin" . 2012-06-17 11:05:59.764 Prompt (3, SSH key passphrase, , Passphrase for key "masterkey for admin": ) . 2012-06-17 11:06:02.938 Sent public key signature . 2012-06-17 11:06:03.352 Access granted . 2012-06-17 11:06:03.352 Initiating key re-exchange (enabling delayed compression) . 2012-06-17 11:06:03.765 Doing Diffie-Hellman group exchange . 2012-06-17 11:06:03.955 Doing Diffie-Hellman key exchange with hash SHA-1 . 2012-06-17 11:06:04.410 Initialised AES-256 SDCTR client->server encryption . 2012-06-17 11:06:04.410 Initialised HMAC-SHA1 client->server MAC algorithm . 2012-06-17 11:06:04.410 Initialised zlib (RFC1950) compression . 2012-06-17 11:06:04.410 Initialised AES-256 SDCTR server->client encryption . 2012-06-17 11:06:04.410 Initialised HMAC-SHA1 server->client MAC algorithm . 2012-06-17 11:06:04.410 Initialised zlib (RFC1950) decompression . 2012-06-17 11:06:04.839 Opened channel for session . 2012-06-17 11:06:05.247 Started a shell/command . 2012-06-17 11:06:05.253 -------------------------------------------------------------------------- . 2012-06-17 11:06:05.253 Using SFTP protocol. . 2012-06-17 11:06:05.253 Doing startup conversation with host. > 2012-06-17 11:06:05.259 Type: SSH_FXP_INIT, Size: 5, Number: -1 . 2012-06-17 11:06:05.354 Server sent command exit status 0 . 2012-06-17 11:06:05.354 Disconnected: All channels closed * 2012-06-17 11:06:05.380 (ESshFatal) Connection has been unexpectedly closed. Server sent command exit status 0. * 2012-06-17 11:06:05.380 Cannot initialize SFTP protocol. Is the host running a SFTP server?

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  • files build execution order

    - by Mahesh
    Hi, I have a data structure which is as given below: class File { public string Value { get; set; } public File[] Dependencies { get; set; } public bool Change { get; private set; } public File(string value,File[] dependencies) { Value = value; Dependencies = dependencies; Change = false; } } Basically, this data structure follows a typical build execution of files. Each File has a value and a list of dependencies which is again of type File. Every file is exposed with a property called Change which tells whether the file is changed or not. I brainstormed to form a algorithm which goes through all these files and build in an order( i.e typical build process ) but haven't got a better algorithm. Can anyone throw some light on this? Thanks a lot. Mahesh

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  • fuzzy implementaion for capture specific strings

    - by kasun-456
    I am going to develop a web crawler using java to capture hotel room prices from hotel websites. In this case i want to capture room price with the room type and the meal type, so my algorithm should intelligent for that. as an example: Room type: Delux Meal type: HalfBoad price : $20.00 The main problem is room prices can be in different different ways in different different hotel sites. so my algorithm should independent from hotel sites. I am plan to use above room types and meal types as a fuzzy sets and compare the words in webpage with above fuzzy sets using a suitable membership function. any one experienced with this??? or have an Idea for my problem??

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  • Image 8-connectivity without excessive branching?

    - by shoosh
    I'm writing a low level image processing algorithm which needs to do alot of 8-connectivity checks for pixels. For every pixel I often need to check the pixels above it, below it and on its sides and diagonals. On the edges of the image there are special cases where there are only 5 or 3 neighbors instead of 8 neighbors for a pixels. The naive way to do it is for every access to check if the coordinates are in the right range and if not, return some default value. I'm looking for a way to avoid all these checks since they introduce a large overhead to the algorithm. Are there any tricks to avoid it altogether?

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  • Discrete and Continuous Classifier on Sparse Data

    - by Chris S
    I'm trying to classify an example, which contains discrete and continuous features. Also, the example represents sparse data, so even though the system may have been trained on 100 features, the example may only have 12. What would be the best classifier algorithm to use to accomplish this? I've been looking at Bayes, Maxent, Decision Tree, and KNN, but I'm not sure any fit the bill exactly. The biggest sticking point I've found is that most implementations don't support sparse data sets and both discrete and continuous features. Can anyone recommend an algorithm and implementation (preferably in Python) that fits these criteria? Libraries I've looked at so far include: Orange (Mostly academic. Implementations not terribly efficient or practical.) NLTK (Also academic, although has a good Maxent implementation, but doesn't handle continuous features.) Weka (Still researching this. Seems to support a broad range of algorithms, but has poor documentation, so it's unclear what each implementation supports.)

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  • Blog Engine .NET with XML Data Storage How is it so fast ?

    - by Kubi
    I am looking for a way to embed a blog engine into my own application and I am too curious about Blog Engine algorithm. This may not be the correct place to ask but, How is that possible to store blog entry data in an XML file like BlogEngine.Net with Default Configuration. It must be getting slower everyday while the file is getting larger and larger. I am wondering the algorithm behind that. Is it loading with a different way ? Or Am I wrong with the time estimation ? I know it is open source but I thought it would be better to see a discussion here for some others might be thinking the same and this thread can be a reference.

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  • [Java] Flood fill using a stack

    - by dafero
    Hello to everyone :), I am using the recursive Flood fill algorithm in Java to fill some areas of a image. With very small images it works fine, but when de image becomes larger the JVM gives me a Stack Over Flow Error. That's the reason why I have to reimplement the method using a Flood Fill with my own stack. (I read that's the best way to do it in these kind of cases) Can anyone explain me how to code it? (if you don't have the code at hand, with the pseudo-code of the algorithm will be fine) I've read a lot in the Internet but I haven't understood it very well. Thanks!

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  • Practical non-Turing-complete languages?

    - by Kyle Cronin
    Nearly all programming languages used are Turing Complete, and while this affords the language to represent any computable algorithm, it also comes with its own set of problems. Seeing as all the algorithms I write are intended to halt, I would like to be able to represent them in a language that guarantees they will halt. Regular expressions used for matching strings and finite state machines are used when lexing, but I'm wondering if there's a more general, broadly language that's not Turing complete? edit: I should clarify, by 'general purpose' I don't necessarily want to be able to write all halting algorithms in the language (I don't think that such a language would exist) but I suspect that there are common threads in halting proofs that can be generalized to produce a language in which all algorithms are guaranteed to halt. There's also another way to tackle this problem - eliminate the need for theoretically infinite memory. Once you limit the amount of memory the machine is allowed, the number of states the machine is in is finite and countable, and therefore you can determine if the algorithm will halt (by not allowing the machine to move into a state it's been in before).

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  • Shortest-path algorithms which use a space-time tradeoff?

    - by Chris Mounce
    I need to find shortest paths in an unweighted, undirected graph. There are algorithms which can find a shortest path between two nodes, but this can take time. There are also algorithms for computing shortest paths for all pairs of nodes in the graph, but storing such a lookup table would take lots of disk space. What I'm wondering: Is there an algorithm which offers a space-time tradeoff that's somewhere between these two extremes? In other words, is there a way to speed up a shortest-path search, while using less disk space than would be occupied by an all-pairs shortest-path table? I know there are ways to efficiently store lookup tables for this problem, and I already have a couple of ideas for speeding up shortest-path searches using precomputed data. But I don't want to reinvent the wheel if there's already some established algorithm that solves this problem.

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  • Java: confirm method Binary division and find remainder is correct?

    - by cadwag
    I am parsing binary files and have to implement a CRC algorithm to ensure the file is not corrupted. Problem is that I can't seem to get the binary math working when using larger numbers. The example I'm trying to get working: BigInteger G = new BigInteger("11001", 2); BigInteger M = new BigInteger("1110010000", 2); BigInteger R = M.remainder(G); I am expecting: R = "0101" But I am getting: R = "1100" I am assuming the remainder of 0101 is correct since it is given to me in this book I am using as a reference for the CRC algorithm (it's not based in Java), but I can't seem to get it working. I can get small binary calculations to work that I have solved by hand, but not the larger ones. I'll admit that I haven't worked the larger ones by hand yet, that is my next step, but I wanted to see if someone could point out a glaring flaw I have in my code. Can anyone confirm or deny that my methodology is correct? Thanks

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  • Dispelling the UIImage imageNamed: FUD

    - by Roger Nolan
    I see a lot of people saying imageNamed is bad but equal numbers of people saying the performance is good - especially when rendering UITableViews. See this SO question for example or this article on iPhoneDeveloperTips.com UIImage's imageNamed method used to leak so it was best avoided but has been fixed in recent releases. I'd like to understand the caching algorithm better in order to make a reasoned decision about where I can trust the system to cache my images and where I need to go the extra mile and do it myself. My current basic understanding is that it's a simple NSMutableDictionary of UIImages referenced by filename. It gets bigger and when memory runs out it gets a lot smaller. For example, does anyone know for sure that the image cache behind imageNamed does not respond to didReceiveMemoryWarning? It seems unlikely that Apple would not do this. If you have any insight into the caching algorithm, please post it here.

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  • How can it be impossible to "decrypt" an MD5 hash?

    - by Rob
    I was reading a question about MD5, and it made me remember something that boggles me. Very simple question, and I'm sorry if it's not a good one. I just can't understand how you convert something to one thing using some algorithm, and there being no way to convert it back using the algorithm in reverse. So how is this possible? Also, since multiple strings can create the same MD5 hash, due to it being less data than the input string, how would any other hashing system be any better?

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  • Bejeweled Blitz - How does it assert there is always a move?

    - by EvilTeach
    I have been playing Bejeweled Blitz for a while now. Yes, it is an addiction. In thinking about the game, I have observed that on some boards, the bottom runs dry (no moves) leaving only the top part of the board playable. Frequently that part of the board drys up, and one is left with moves in area cleared by the last move. The board never runs completely dry, so clearly the program is doing some sorts of calculation that allows it to choose what to drop to prevent it from running dry. I have noticed in this 'mode' that it is very common for the algorithm to drop jewels which causes more non-dry area to appear in the horizontal area. Perhaps less frequent is a drop which seems designed to open up the bottom part of the board again. So my question is "How would one go about designing an algorithm guarantee that there is always a move available.?"

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  • Estimate gaussian (mixture) density from a set of weighted samples

    - by Christian
    Assume I have a set of weighted samples, where each samples has a corresponding weight between 0 and 1. I'd like to estimate the parameters of a gaussian mixture distribution that is biased towards the samples with higher weight. In the usual non-weighted case gaussian mixture estimation is done via the EM algorithm. Does anyone know an implementation (any language is ok) that permits passing weights? If not, does anyone know how to modify the algorithm to account for the weights? If not, can some one give me a hint on how to incorporate the weights in the initial formula of the maximum-log-likelihood formulation of the problem? Thanks!

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  • SHA1 form .exe file in C#

    - by alibipl
    Hi! I hope that someone could help me with reading exe files in C# and create a SHA1 hash from it. I have tried to read from executable file using StreamReader and BinaryReader. Then using built-in SHA1 algorithm I tried to create a hash but without success. The algorithm results for StreamReader was "AEUj+Ppo5QdHoeboidah3P65N3s=" and for BinaryReader was "rWXzn/CoLLPBWqMCE4qcE3XmUKw=". Can anyone help me to acheive SHA1 hash from exe file? Thx. BTW Sorry for my English ;)

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  • analysis Big Oh notation psuedocode

    - by tesshu
    I'm having trouble getting my head around algorithm analysis. I seem to be okay identifying linear or squared algorithms but am totally lost with nlogn or logn algorithms, these seem to stem mainly from while loops? Heres an example I was looking at: Algorithm Calculate(A,n) Input: Array A of size n t?0 for i?0 to n-1 do if A[i] is an odd number then Q.enqueue(A[i]) else while Q is not empty do t?t+Q.dequeue() while Q is not empty do t?t+Q.dequeue() return t My best guess is the for loop is executed n times, its nested while loop q times making NQ and the final while loop also Q times resulting in O(NQ +Q) which is linear? I am probably totally wrong. Any help would be much appreciated. thanks

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