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• mystified by qr.Q(): what is an orthonormal matrix in "compact" form?

  - by gappy

  R has a qr() function, which performs QR decomposition using either LINPACK or LAPACK (in my experience, the latter is 5% faster). The main object returned is a matrix "qr" that contains in the upper triangular matrix R (i.e. R=qr[upper.tri(qr)]). So far so good. The lower triangular part of qr contains Q "in compact form". One can extract Q from the qr decomposition by using qr.Q(). I would like to find the inverse of qr.Q(). In other word, I do have Q and R, and would like to put them in a "qr" object. R is trivial but Q is not. The goal is to apply to it qr.solve(), which is much faster than solve() on large systems.

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• Which encoding (code page) is used for file names in ZIP archive under Mac OS x 10.6

  - by bao

  I have a zip library SharpZipLib which intended to work with ZIP archives using C#. It has parameter ICSharpCode.SharpZipLib.Zip.ZipConstants.DefaultCodePage which specifies encoding of file names in zip archive. I know that in Windows and in OSX different encodings are used to store file names. 1) Which encodings (code pages) are used in both? 2) How to determine programmatically which encoding is used? When I open in Win7 zip file packed under MacOS X, I see files with bad names (originally - cyrillic) and folder called __MACOSX, so I can say zip was prepared on Mac box. Any other way? What about other UNIX like systems?

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• Whats the difference between Paxos and W+R>=N in Cassandra?

  - by user1128016

  Dynamo-like databases (e.g. Cassandra) provide ability to enforce consistency by means of quorum, i.e. a number of synchronously written replicas (W) and a number of replicas to read (R) should be chosen in such a way that W+RN where N is a replication factor. On the other hand, PAXOS-based systems like Zookeeper are also used as a consistent fault-tolerant storage. What is the difference between these two approaches? Does PAXOS provide guarantees that are not provided by W+RN schema?

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• Creating/compling .net data class within an application

  - by MrTelly

  Is there a pattern, Xml structure, architecture technique we can use to create simple data holder class code that we can deserialise to/from and do that at runtime? We're in the early design stage of a .Net project and one of our goals is to make the resulting system extensible through configuration without needing a developer. We need to support new sources of data, typcially delivered as Xml messages. Currently we convert/deserialise the messages into simple classes and then use an already existing language which can manipulate those classes as we need. That works well when you have a developer to map the Xml to simple class, create the class and then write the deserialisation, but it's not extensible for for an administrator. Our target user audience is high end DBA and/or network admin - people who can handle Xml but may not know C#.

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• How is the ">" operator implemented (on 32 bit integers)?

  - by Ron Klein

  Let's say that the environment is x86. How do compilers compile the "" operator on 32 bit integers. Logically, I mean. Without any knowledge of Assembly. Let's say that the high level language code is: int32 x, y; x = 123; y = 456; bool z; z = x > y; What does the compiler do for evaluating the expression x > y? Does it perform something like (assuming that x and y are positive integers): w = sign_of(x - y); if (w == 0) // expression is 'false' else if (w == 1) // expression is 'true' else // expression is 'false' Is there any reference for such information?

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• What are the uses of svn copy?

  - by nav.jdwdw

  Example: $ svn copy foo.txt bar.txt A bar.txt When would you use this technique, and why? Will this command (taken from svn's "red book") creates a copy of <foo.txt> while preserving the history of it to be shared with <bar.txt>? If I'm changing <bar.txt>, what will happen to <foo.txt>? What are the equivalents to this in other modern systems (Clearcase, Accurev, Perforce)? Clarification: Let me emphasize the point I'm searching for: Is this kind of branching out on a file level? What happens if you use it in the same branch, i.e. create a copy of a file and than start changing that new file. all in the same branch? I understand that it is also used for tagging but what is interesting me is what to expect when performing <svn copy> On the file level

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• Calculating terrain height in 3d-space

  - by Jonas B

  Hi I'm diving into 3d programming a bit and am currently learning by writing a procedural terrain generator that generates terrain based on a heightmap. I would also want to implement some physics and my first attempt at terrain collision was by simply checking the current position vs the heightmap. This however wont work well against small objects as you'd have to calculate the height by taking the heightdifference of the nearest vertices of the object and doing this every colision check is pretty slow. Beleive me I tried googling for it but there's simply so much crap and millions of blogs posting ripped-of newbie tutorials everywhere with basically no real information on the subject, I can't find anything that explains it or even names any generally used techniques. I'm not asking for code or a complete solution, but if anyone knows a particular technique good for calculating a high-res heightmap out of the already generated and smoothed terrain I would be very happy as I could look into it further when I know what I'm looking for. Thanks

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• Linux based MS Office thumbnail generation

  - by El Yobo

  I've been taken on board to work on a PHP based web application. One part of the application generates thumbnail images for MS Office documents on demand, and it uses MS Office + the VeryPDF docprint utility to do this. Because of this one requirement, the system is running on Windows Server 2003 + IIS. I would prefer to have the system running on a Linux server, rather than MS, as I have far more experience in administering Linux systems than Windows and we have no other in-house technical staff. Does anyone know a way to handle the document conversion using native Linux software? I would love something PHP native, but am willing to look outside that if necessary. Thanks for your suggestions.

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• PPTP VPN connection always connects successfully in Mac OS X/windows but drops after 2 minute in iPad or it doesn't connect al all!

  - by Farshid

  Dear Users, I've a big problem with using iPad's VPN connection. When I use the same VPN connection (PPTP) configuration parameters in my Mac OS X Snow Leopard or my windows XP (Bootcamp), it works flawlessly but when I define the same connection in my iPad, connection drops in about 1 or 2 minutes (some times) and most of the time, it Doesn't Connect at all. I'm using iPad 3G with iOS 4.2.1 and my iPad is not jail-broken. I've tried so hard to make it work in an stable manner but not found any solution yet. (I should have put encryption level to No-Encryption in Mac and Windows otherwise it won't connect in my desktop operating systems. It is a host-related issue and I'm binding to that). I'm sure there is no technical problem in my iPad's hardware and this problem also exists in my friend's iPad too. Our devices work perfectly with 3G and Wifi connections. Your answers are really appreciated.

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• How can a large number of developers write software without either a cumbersome process or poor qual

  - by Mark Robinson

  I work at a company with hundreds of people writing software for essentially the same product. The quality of the software has to be high because so many people depend on it (not least the developers themselves). Because of this every major issue has resulted in a new check - either automated or manual. As a result the process of delivering software is becoming ever more burdensome. So that requires more developers which... well you can see it is a vicious circle. We now have a problem with releasing software quickly - the lead time even to change one line of code for a very serious issue is at least one day. What techniques do you use to speed up the delivery of software in a large organization, while still maintaining software quality?

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• Time order of messages

  - by Aiden Bell

  Read (skimmed enough to get coding) through Erlang Programming and Programming Erlang. One question, which is as simple as it sounds: If you have a process Pid1 on machine m1 and a billion million messages are sent to Pid1, are messages handled in parallel by that process (I get the impression no) and(answered below) is there any guarantee of order when processing messages? ie. Received in order sent? If so, how is clock skew handled in high traffic situations for ordering? Coming from the whole C/Thread pools/Shared State background ... I want to get this concrete. I understand distributing an application, but want to ensure the 'raw bones' are what I expect before building processes and distributing workload. Also, am I right in thinking the whole world is currently flicking through Erlang texts ;)

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• What Database to distribute as part of a C# app backend

  - by jez

  I am planning on writing and commercialising a C# app which will store data in an underlying database I use MySQL on my environment for my own development and this is what I would have used to write the application for myself (no need to use FK - MyISAM engine would be fine). I am concerned about how easy it will be to distribute the app together with the database engine. Would using MySQL allow me for easy packaging of the app for a "one-click" install on the client side? (ie I do not want them to have to install MySQL by themselves) and also is it feasible from a licensing point of view? Are there other Database systems which would make the process more straight-forward

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• How to set the HomeScreen image without using method HomeScreen.setBackgroundImage(url)?

  - by Richa

  Hello, Can anyone help me out as I am looking for a method that helps me to set the wallpaper using the images from the Pictures folder on Blackberry Curve 8310. I found one method HomeScreen.setBackground(url), but it doen't works on Blackberry Curve. I am using Operating system 4.5 and this method is useable with operating systems 4.7 & above So, i want to know is their any method for blackberry curve to set the wallpaper or is their any operating system that supports the above method? Kindly reply Thanks a lot Best Regards, Richa Bhatia

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• GPU Computing - # of GPUs supported

  - by TehTypoKing

  I currently have a desktop with 6 GPUs ( 3x HD 5970s ) in non-crossfire mode. Unfortunately, it seems that Windows 7 64bit only supports up to 4 GPUs. I have not been able to find a reliable source to deny or confirm this. If windows 7 has this limitation, is there a Linux flavor that supports more than 4 GPUs? In-case you are wondering, this is not for gaming but high-speed single precision computing. With this current setup ( if I can find 6gpu support ) I am looking to reach 13.8 Teraflops. Also, my motherboard does support 3 16x pci-xpress gen2 slots... and I have a 1500w powersupply plugged into a 20amp outlet. Windows is able to detect all 6 cores.. although, 2 of which displays the warning "Drivers failed to load". To recap: - Can windows support 6 GPUs? - If not, does Linux? Thank you.

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• What's the best way to match a query to a set of keywords?

  - by Ryan Detzel

  Pretty much what you would assume Google does. Advertisers come in and big on keywords, lets say "ipod", "ipod nano", "ipod 60GB", "used ipod", etc. Then we have a query, "I want to buy an ipod nano" or "best place to buy used ipods" what kind of algorithms and systems are used to match those queries to the keyword set. I would imagine that some of those keyword sets are huge, 100k keywords made up of one or more actual words. on top of that queries can be 1-n words as well. Any thoughts, links to wikipedia I can start reading? From what I know already I would use some stemmed hash in disk(CDB?) and a bloom filter to check to see if I should even go to disk.

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• Curious: Could LLVM be used for Infocom z-machine code, and if so how? (in general)

  - by jonhendry2

  Forgive me if this is a silly question, but I'm wondering if/how LLVM could be used to obtain a higher performance Z-Machine VM for interactive fiction. (If it could be used, I'm just looking for some high-level ideas or suggestions, not a detailed solution.) It might seem odd to desire higher performance for a circa-1978 technology, but apparently Z-Machine games produced by the modern Inform 7 IDE can have performance issues due to the huge number of rules that need to be evaluated with each turn. Thanks! FYI: The Z-machine architecture was reverse-engineered by Graham Nelson and is documented at http://www.inform-fiction.org/zmachine/standards/z1point0/overview.html

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• How do I set a property to the output of a command in msbuild/xbuild

  - by Nils

  In msbuild/xbuild I'd like to have a "libPath" property which can be ovveridden on the commandline using /p:libpath="/path/to/all/libs". But when this property is undefined I want to call pkg-config --retrieve-Path somePackage to get the current systems path. I thought like here I need the output of a command to be stored in a Property. The command always returns one line of output. I have tryied something like <PropertyGroup> <LibPath /> </PropertyGroup> <Task ....> <Exec Command="pkg-config --retrieve-Path somePackage" Condition="$(LibPath)' == ''"> <OutputTaskParameter="output" PropertyName="LibPath" /> </Exec> </Task> But that didn't work.

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• Empty or "flush" a file descriptor without read()?

  - by Teddy

  (Note: This is not a question of how to flush a write(). This is the other end of it, so to speak.) Is it possible to empty a file descriptor that has data to be read in it without having to read() it? You might not be interested in the data, and reading it all would therefore waste space and cycles you might have better uses for. If it is not possible in POSIX, do any operating systems have any non-portable ways to do this? UPDATE: Please note that I'm talking about file descriptors, not streams.

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• What is the best book in client server programming ?

  - by Nayef

  What is the best "new" book in client server programming with C#? I mean principles of inter-network architecture and communication protocols. Open systems and interoperability. Case studies of particular protocols from network layer and above. Socket programming. Remoting. Selected examples of networked client-server applications such as e-mail, news, file-transfer, HTTP. Client-Server Programming Project(s). Using APIs. Software tools and environments.

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• How do you design a database to allow fast multicolumn searching?

  - by Fletcher Moore

  I am creating a real estate search from RETS data, but this is a general question. When you have a variety of columns that you would like the user to be able to filter their search result by, how do you optimize this? For example, http://www.charlestonrealestateguide.com/listings.php has 16 or so optional filters. Granted, he only has up to 11,000 entries (I have the same data), but I don't imagine the search is performed with just a giant WHERE AND AND AND ... clause. Or is this typically accomplished with one giant multicolumn index? Newegg, Amazon, and countless others also have cool & fast filtering systems for large amounts of data. How do they do it? And is there a database optimization reason for the tendency to provide ranges instead of empty inputs, or is that merely for user convenience?

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• "SQL Server does not exist or access denied." upon VB.Net Deployment

  - by JasonMHirst

  Have built a small VB.Net app in VS2010. The connection string to the SQL Server is as follows: <connectionStrings> <add name="IWSR_DataDownloadWizard.My.MySettings.sqlConnection" connectionString="Provider=SQLOLEDB.1;Data Source=SQLServer;Integrated Security=SSPI;Initial Catalog=IWSROL" providerName="System.Data.OleDb" /> </connectionStrings> Within the VS2010 environment AND on my machine, the connection works perfectly, however when I deploy to clients I get an error message of: [DBNETLIB][ConnectionOpen (Connect()).]SQL Server does not exist or access denied. I've tried to search the net for solutions, have seen a few instances of people saying to NOT use the Server Name but the IP Address instead (this doesn't work) and some saying to remove the "Provider=SQLOLEDB.1", again this doesn't work. Can anyone suggest a solution please? Further information: Development System is Windows7 using VS2010 Deployment systems are a combination of Windows XP, Windows 2000 and Windows 7 SQL Server is SQL2000 (soon to be SQL2005). TIA

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• How can I detect if a file is binary (non-text) in python?

  - by grieve

  How can I tell if a file is binary (non-text) in python? I am searching through a large set of files in python, and keep getting matches in binary files. This makes the output look incredibly messy. I know I could use grep -I, but I am doing more with the data than what grep allows for. In the past I would have just searched for characters greater than 0x7f, but utf8 and the like make that impossible on modern systems. Ideally the solution would be fast, but any solution will do.

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• How should my team decide between 3-tier and 2-tier architectures?

  - by j0rd4n

  My team is discussing the future direction we take our projects. Half the team believes in a pure 3-tier architecture while the other half favors a 2-tier architecture. Project Assumptions: Enterprise business applications Business logic needed between user and database Data validation necessary Service-oriented (prefer RESTful services) Multi-year maintenance plan Support hundreds of users 3-tier Team Favors: Persistant layer <== Domain layer <== UI layer Service boundary between at least persistant layer and domain layer. Domain layer might have service boundary between it. Translations between each layer (clean DTO separation) Hand roll persistance unless we can find creative yet elegant automation 2-tier Team Favors: Entity Framework + WCF Data Service layer <== UI layer Business logic kept in WCF Data Service interceptors Minimal translation between layers - favor faster coding So that's the high-level argument. What considerations should we take into account? What experiences have you had with either approach?

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• Security / Protecting code in JavaScript

  - by Evans

  With all the recent hype about JavaScript and HTML5 replacing Flash, I wanted to know - How would it be possible to protect client-side js code? Of course, it is possible to obfuscate it, but that would only make it a little harder. Also, for games which submit high scores to the server, wouldn't it be incredibly easy to modify those scores before they are sent to the server? I know even Flash files can be decompiled, but they can be obfuscated and flash decompilation is not as easy as modifying data in JS - could be done easily using a plugin such as Firebug. I'd like to know everyone's views on this.

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• Script Speed vs Memory Usage

  - by Doug Neiner

  I am working on an image generation script in PHP and have gotten it working two ways. One way is slow but uses a limited amount of memory, the second is much faster, but uses 6x the memory . There is no leakage in either script (as far as I can tell). In a limited benchmark, here is how they performed: -------------------------------------------- METHOD | TOTAL TIME | PEAK MEMORY | IMAGES -------------------------------------------- One | 65.626 | 540,036 | 200 Two | 20.207 | 3,269,600 | 200 -------------------------------------------- And here is the average of the previous numbers (if you don't want to do your own math): -------------------------------------------- METHOD | TOTAL TIME | PEAK MEMORY | IMAGES -------------------------------------------- One | 0.328 | 540,036 | 1 Two | 0.101 | 3,269,600 | 1 -------------------------------------------- Which method should I use and why? I anticipate this being used by a high volume of users, with each user making 10-20 requests to this script during a normal visit. I am leaning toward the faster method because though it uses more memory, it is for a 1/3 of the time and would reduce the number of concurrent requests.

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