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  • For a user who is not logged into Facebook, the like button gives an error in the pop up window (non

    - by dealsvista
    Hi, In order to make full use the Facebook like button, I am trying to switching from iframe-based like button to js-based like button. url: http://www.dealsvista.com Everything works fine for a user who is logged into Facebook. However, for a user who is not logged into Facebook, when he clicks the like button, there is a error in the popup window. Could someone point me to the right direction? I am really new to Facebook api and have no idea what is going on? Do I have to implement connect at the same time? Do I have to set up the connect call back url in my apps?

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  • How can I find columns which have non-null values?

    - by aartist
    I have many columns in oracle database and some new are added with values. I like to find out which columns have values other than 0 or null. So I am looking for column names for which some sort of useful values exists at least in one row. How do I do this? Update: This sounds very close. How do I modify this to suit my needs? select column_name, nullable, num_distinct, num_nulls from all_tab_columns where table_name = 'SOME_TABLE'

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  • Is there a way to catch an attempt to access a non existant property or method?

    - by Tor Valamo
    For instance this code: function stuff() { this.onlyMethod = function () { return something; } } // some error is thrown stuff().nonExistant(); Is there a way to do something like PHP's __call as a fallback from inside the object? function stuff() { this.onlyMethod = function () { return something; } this.__call__ = function (name, params) { alert(name + " can't be called."); } } // would then raise the alert "nonExistant can't be called". stuff().nonExistant();

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  • How to list library dependencies of a non-native binary?

    - by lvella
    When developing for native platform, I can use ldd to list all the shared libraries (.so files) a binary executable I build will try to load upon start-up. But when cross-compiling, I don't know how to get the same information. The ldd is not a normal binutils utility, like strip or ar, that can be built alongside gcc for cross compiling, but instead, it is a cryptic shell script that apparently can only run on native platform. So, using the cross-target binutils tools, is there any way to get a list of the dynamically linked dependency for of a foreign binary?

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  • How do you deal with non-logical people in the workplace?

    - by unrealtrip
    I ask this because you are probably the best audience I could think of. I program for a living, and it goes hand in hand with my personality type which is likely true for most of us, as such I live and work in the world of logic and logical decisions. A problem I have is in dealing with people that live and work in a world of emotional responses and reactions that typically make no sense, or have any real bearing on any given situation. What is the trick to dealing with these people? It is nothing but an act of futility, leading to utter exasperation dealing with people like this when attempting to get them to understand some pretty basic concepts. How do you do it?

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  • What PHP function(s) can I use to perform operations on non-integer timestamps?

    - by stephenhay
    Disclaimer, I'm not a PHP programmer, so you might find this question trivial. That's why I'm asking you! I've got this kind of timestamp: 2010-05-10T22:00:00 (That's Y-m-d) I would like to subtract, say, 10 days (or months, whatever) from this, and have my result be in the same format, i.e. 2010-04-30T22:00:00. What function(s) do I need to do this in PHP? Note: I'm using this to do a computed field in Drupal. The result will be the date that an e-mail is sent. Bonus question: If 2010-05-10T22:00:00 means "May 10, 2010 at 10pm", is there a timestamp equivalent of "May 10, 2010 (all day)"? Thanks everyone.

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  • Ignoring a xml Tag in the middle of the file in Regex (with non capturing group ?)

    - by schmirrwurst
    I have an xml with an embeded tag, and I would like to capture everthing but the FType Tags... in python regex. <xml> <EType> <E></E> <F></F> <FType><E1></E1><E2></E2></FType> <FType><E1></E1><E2></E2></FType> <FType><E1></E1><E2></E2></FType> <G></G> </EType> </xml> I tried : (?P<xml>.*(?=<FType>.*<FType>).*) But it give me everything ;-( I Expect : <xml> <EType> <E></E> <F></F> <G></G> </EType> </xml>

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  • Can I and should I use Eclipselink for non OR database interactions?

    - by Tim
    We are using Eclipselink for ORM, but we have a need for some more lightweight database interactions that are more equivalent to JDBC. My question is whether Eclipselink supports such an idiom and whether there are any advantages of it to straight JDBC. I can see one advantage being consistency and being able to use the existing connection handling. Others? Specifically, what I'm looking for is something equivalent to Hibernate's Native SQL Query.

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  • Call to a member function ... on a non-object

    - by jayceekay
    i have an object which is instantiated in an initialize file, which is called with every request. the name is right, so why is it telling me that oourls isn't an object and that redirectLoggedIn isn't its method? a var dump on oourls says NULL. but it's instantiated, and the backtrace at the bottom shows that it goes through initialization and instantiates it. pretty small snippet of code, here's the relevant bit: if($email) { global $session; $session->grantLogin($email); global $oourls; $oourls->redirectLoggedIn(); } else { return false; } and here's the output of debug_print_backtrace i threw in above the oourls method call because i'm completely confused: #0 accounts::verifyEmailRegisterAccount(37a6274c8f4bfa5c537b40e8e04d634a) called at [\public\includes\default\verifyemail.php:16] #1 require_once(\public\includes\default\verifyemail.php) called at [\support\php\ObjectOrientedURLs.class.php:48] #2 ObjectOrientedURLs->mhqqrVerifyemail(Array ([0] => 37a6274c8f4bfa5c537b40e8e04d634a)) #3 ReflectionMethod->invoke(ObjectOrientedURLs Object (), Array ([0] => 37a6274c8f4bfa5c537b40e8e04d634a)) called at [\support\php\ObjectOrientedURLs.class.php:280] #4 ObjectOrientedURLs->parseAndInvokeURL() called at [\support\php\ObjectOrientedURLs.class.php:255] #5 ObjectOrientedURLs->__construct() called at [\support\php\initialize.php:76] #6 require_once(\support\php\initialize.php) called at [\public\index.php:2]

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  • Non-public application uses GPL code — are we obliged to publish our source?

    - by Cimbit
    Real Example. I have a closed source server-side program for "internal purposes only" (no distribution or selling). I want to use "MYSQL C++ Connector" in my server application in my VPS (MYSQL C++ Connector is GPL). Do I have to make my application "GPL" and distribute my source code? Can I use the GPL Source Code in my closed source application. (There is no distribution or selling of my application — it is for internal purposes only.)

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  • Is there an HTML code that can make my background picture transparent and my text non-transparent?

    - by user1831312
    Okay so I've been typing some HTML code for a technology class that I need to satisfy for my Education major. This is what i have for my background: body { background-image:url('islandbeach.jpg'); background-repeat:repeat; background-position:center; background-attachment:fixed; background-size:cover; } Now, I want to make my background transparent or faded so I can see the text and the other image that I have. The background is too colorful to be able to see the words without having to squint. Are there any HTML codes that can do this for me? I am not a pro at this stuff, I've just been following everything my professor has told me to do so please explain stuff in baby steps if you do have an answer. Thank you so so much!

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  • How to find "y" values of the already estimated monotone function of the non-monotone regression curve corresponding to the original "x" points?

    - by parenthesis
    The title sounds complicated but that is what I am looking for. Focus on the picture. ## data x <- c(1.009648,1.017896,1.021773,1.043659,1.060277,1.074578,1.075495,1.097086,1.106268,1.110550,1.117795,1.143573,1.166305,1.177850,1.188795,1.198032,1.200526,1.223329,1.235814,1.239068,1.243189,1.260003,1.262732,1.266907,1.269932,1.284472,1.307483,1.323714,1.326705,1.328625,1.372419,1.398703,1.404474,1.414360,1.415909,1.418254,1.430865,1.431476,1.437642,1.438682,1.447056,1.456152,1.457934,1.457993,1.465968,1.478041,1.478076,1.485995,1.486357,1.490379,1.490719) y <- c(0.5102649,0.0000000,0.6360097,0.0000000,0.8692671,0.0000000,1.0000000,0.0000000,0.4183691,0.8953987,0.3442624,0.0000000,0.7513169,0.0000000,0.0000000,0.0000000,0.0000000,0.1291901,0.4936121,0.7565551,1.0085108,0.0000000,0.0000000,0.1655482,0.0000000,0.1473168,0.0000000,0.0000000,0.0000000,0.1875293,0.4918018,0.0000000,0.0000000,0.8101771,0.6853480,0.0000000,0.0000000,0.0000000,0.0000000,0.4068802,1.1061434,0.0000000,0.0000000,0.0000000,0.0000000,0.0000000,0.0000000,0.0000000,0.0000000,0.0000000,0.6391678) fit1 <- c(0.5102649100,0.5153380934,0.5177234836,0.5255544980,0.5307668662,0.5068087080,0.5071001179,0.4825657520,0.4832969250,0.4836378194,0.4842147729,0.5004039310,0.4987301366,0.4978800742,0.4978042478,0.4969807064,0.5086987191,0.4989497612,0.4936121200,0.4922210302,0.4904593166,0.4775197108,0.4757040857,0.4729265271,0.4709141776,0.4612406896,0.4459316517,0.4351338346,0.4331439717,0.4318664278,0.3235179189,0.2907908968,0.1665721429,0.1474035158,0.1443999345,0.1398517097,0.1153991839,0.1142140393,0.1022584672,0.1002410843,0.0840033244,0.0663669309,0.0629119398,0.0627979240,0.0473336492,0.0239237481,0.0238556876,0.0084990298,0.0077970954,0.0000000000,-0.0006598571) fit2 <- c(-0.0006598571,0.0153328298,0.0228511733,0.0652889427,0.0975108758,0.1252414661,0.1270195143,0.1922510501,0.2965234797,0.3018551305,0.3108761043,0.3621749370,0.4184150225,0.4359301495,0.4432114081,0.4493565757,0.4510158144,0.4661865431,0.4744926045,0.4766574718,0.4796937554,0.4834718810,0.4836125426,0.4839450098,0.4841092849,0.4877317306,0.4930561638,0.4964939389,0.4970089201,0.4971376528,0.4990394601,0.5005881678,0.5023814257,0.5052125977,0.5056691690,0.5064254338,0.5115481820,0.5117259449,0.5146054557,0.5149729419,0.5184178197,0.5211542908,0.5216215426,0.5216426533,0.5239797875,0.5273573222,0.5273683002,0.5293994824,0.5295130266,0.5306236672,0.5307303109) ## picture plot(x, y) ## red regression curve points(x, fit1, col=2); lines(x, fit1, col=2) ## blue monotonic curve to the regression points(min(x) + cumsum(c(0, rev(diff(x)))), rev(fit2), col="blue"); lines(min(x) + cumsum(c(0, rev(diff(x)))), rev(fit2), col="blue") ## "x" original point matches with the regression estimated point ## but not with the estimated (fit2=estimate) monotonic curve abline(v=1.223329, lty=2, col="grey") Focus on the dashed grey line. The idea is to get y value of the monotonic blue curve corresponding to x original value. The grey line should cross three points (the original one "black", the regression estimate "red", the adjusted regression estimate "blue"). Can we do this? Methodology: The object "fit2" is the output of the function rearrangement(). It is always monotonically increasing. library(Rearrangement) fit2 <- rearrangement(x=as.data.frame(x), y=fit1)

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  • Trying to update debian not working

    - by Sean
    As root i type this command apt-get update and get these error messages. > Err http://security.debian.org lenny/updates Release.gpg Could not resolve 'security.debian.org' Err http://security.debian.org lenny/updates/main Translation-en_US Could not resolve 'security.debian.org' Err http://security.debian.org lenny/updates/contrib Translation-en_US Could not resolve 'security.debian.org' Err http://security.debian.org lenny/updates/non-free Translation-en_US Could not resolve 'security.debian.org' Err http://www.backports.org lenny-backports Release.gpg Could not resolve 'www.backports.org' Err http://www.backports.org lenny-backports/main Translation-en_US Could not resolve 'www.backports.org' Err http://www.backports.org lenny-backports/contrib Translation-en_US Could not resolve 'www.backports.org' Err http://www.backports.org lenny-backports/non-free Translation-en_US Could not resolve 'www.backports.org' Err http://ftp.us.debian.org lenny Release.gpg Could not resolve 'ftp.us.debian.org' Err http://ftp.us.debian.org lenny/main Translation-en_US Could not resolve 'ftp.us.debian.org' Err http://ftp.us.debian.org lenny/contrib Translation-en_US Could not resolve 'ftp.us.debian.org' Err http://ftp.us.debian.org lenny/non-free Translation-en_US Could not resolve 'ftp.us.debian.org' Err http://http.us.debian.org stable Release.gpg Could not resolve 'http.us.debian.org' Err http://http.us.debian.org stable/main Translation-en_US Could not resolve 'http.us.debian.org' Err http://http.us.debian.org stable/contrib Translation-en_US Could not resolve 'http.us.debian.org' Err http://http.us.debian.org stable/non-free Translation-en_US Could not resolve 'http.us.debian.org' Reading package lists... Done W: Failed to fetch http://ftp.us.debian.org/debian/dists/lenny/Release.gpg Could not resolve 'ftp.us.debian.org' W: Failed to fetch http://ftp.us.debian.org/debian/dists/lenny/main/i18n/Translation-en_US.gz Could not resolve 'ftp.us.debian.org' W: Failed to fetch http://ftp.us.debian.org/debian/dists/lenny/contrib/i18n/Translation-en_US.gz Could not resolve 'ftp.us.debian.org' W: Failed to fetch http://ftp.us.debian.org/debian/dists/lenny/non-free/i18n/Translation-en_US.gz Could not resolve 'ftp.us.debian.org' W: Failed to fetch http://http.us.debian.org/debian/dists/stable/Release.gpg Could not resolve 'http.us.debian.org' W: Failed to fetch http://http.us.debian.org/debian/dists/stable/main/i18n/Translation-en_US.gz Could not resolve 'http.us.debian.org' W: Failed to fetch http://http.us.debian.org/debian/dists/stable/contrib/i18n/Translation-en_US.gz Could not resolve 'http.us.debian.org' W: Failed to fetch http://http.us.debian.org/debian/dists/stable/non-free/i18n/Translation-en_US.gz Could not resolve 'http.us.debian.org' W: Failed to fetch http://security.debian.org/dists/lenny/updates/Release.gpg Could not resolve 'security.debian.org' W: Failed to fetch http://security.debian.org/dists/lenny/updates/main/i18n/Translation-en_US.gz Could not resolve 'security.debian.org' W: Failed to fetch http://security.debian.org/dists/lenny/updates/contrib/i18n/Translation-en_US.gz Could not resolve 'security.debian.org' W: Failed to fetch http://security.debian.org/dists/lenny/updates/non-free/i18n/Translation-en_US.gz Could not resolve 'security.debian.org' W: Failed to fetch http://www.backports.org/debian/dists/lenny-backports/Release.gpg Could not resolve 'www.backports.org' W: Failed to fetch http://www.backports.org/debian/dists/lenny-backports/main/i18n/Translation-en_US.gz Could not resolve 'www.backports.org' W: Failed to fetch http://www.backports.org/debian/dists/lenny-backports/contrib/i18n/Translation-en_US.gz Could not resolve 'www.backports.org' W: Failed to fetch http://www.backports.org/debian/dists/lenny-backports/non-free/i18n/Translation-en_US.gz Could not resolve 'www.backports.org' W: Some index files failed to download, they have been ignored, or old ones used instead. W: You may want to run apt-get update to correct these problems This is on a dreamplug linux server. Configured so that my network starts on 192.168.1.2 and my router is port forwarding ssh to 192.168.1.6 to the server.

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  • Trouble with dns and debian update

    - by Sean
    I tried to update my debian dreamplug server with the command running as root apt-get update and recieved these errors. Err http://security.debian.org lenny/updates Release.gpg Could not resolve 'security.debian.org' Err htdtp://security.debian.org lenny/updates/main Translation-en_US Could not resolve 'security.debian.org' Err htdtp://security.debian.org lenny/updates/contrib Translation-en_US Could not resolve 'security.debian.org' Err htdtp://security.debian.org lenny/updates/non-free Translation-en_US Could not resolve 'security.debian.org' Err httdp://www.backports.org lenny-backports Releasegpg Could not resolve 'www.backports.org' Err httdp://www.backports.org lenny-backports/main Translation-en_US Could not resolve 'www.backports.org' Err httdp://www.backports.org lenny-backports/contrib Translation-en_US Could not resolve 'www.backports.org' Err httdp://www.backports.org lenny-backports/non-free Translation-en_US Could not resolve 'www.backports.org' Err httdp://ftp.us.debian.org lenny Release.gpg Could not resolve 'ftp.us.debian.org' Err httdp://ftp.us.debian.org lenny/main Translation-en_US Could not resolve 'ftp.us.debian.org' Err httdp://ftp.us.debian.org lenny/contrib Translation-en_US Could not resolve 'ftp.us.debian.org' Err httdp://ftp.us.debian.org lenny/non-free Translation-en_US Could not resolve 'ftp.us.debian.org' Err httdp://http.us.debian.org stable Release.gpg Could not resolve 'http.us.debian.org' Err htdtp://http.us.debian.org stable/main Translation-en_US Could not resolve 'http.us.debian.org' Err httdp://http.us.debian.org stable/contrib Translation-en_US Could not resolve 'http.us.debian.org' Err htdtp://http.us.debian.org stable/non-free Translation-en_US Could not resolve 'http.us.debian.org' Reading package lists... Done W: Failed to fetch ttp://ftp.us.debian.org/debian/dists/lenny/Release.gpg Could not resolve 'ftp.us.debian.org' W: Failed to fetch ttp://ftp.us.debian.org/debian/dists/lenny/main/i18n/Translation-en_US.gz Could not resolve 'ftp.us.debian.org' W: Failed to fetch ttp://ftp.us.debian.org/debian/dists/lenny/contrib/i18n/Translation-en_US.gz Could not resolve 'ftp.us.debian.org' W: Failed to fetch ttp://ftp.us.debian.org/debian/dists/lenny/non-free/i18n/Translation-en_US.gz Could not resolve 'ftp.us.debian.org' W: Failed to fetch ttp://http.us.debian.org/debian/dists/stable/Release.gpg Could not resolve 'http.us.debian.org' W: Failed to fetch ttp://http.us.debian.org/debian/dists/stable/main/i18n/Translation-en_US.gz Could not resolve 'http.us.debian.org' W: Failed to fetch ttp://http.us.debian.org/debian/dists/stable/contrib/i18n/Translation-en_US.gz Could not resolve 'http.us.debian.org' W: Failed to fetch ttp://http.us.debian.org/debian/dists/stable/non-free/i18n/Translation-en_US.gz Could not resolve 'http.us.debian.org' W: Failed to fetch ttp://security.debian.org/dists/lenny/updates/Release.gpg Could not resolve 'security.debian.org' W: Failed to fetch ttp://security.debian.org/dists/lenny/updates/main/i18n/Translation-en_US.gz Could not resolve 'security.debian.org' W: Failed to fetch ttp://security.debian.org/dists/lenny/updates/contrib/i18n/Translation-en_US.gz Could not resolve 'security.debian.org' W: Failed to fetch ttp://security.debian.org/dists/lenny/updates/non-free/i18n/Translation-en_US.gz Could not resolve 'security.debian.org' W: Failed to fetch ttp://www.backports.org/debian/dists/lenny-backports/Release.gpg Could not resolve 'www.backports.org' W: Failed to fetch ttp://www.backports.org/debian/dists/lenny-backports/main/i18n/Translation-en_US.gz Could not resolve 'www.backports.org' W: Failed to fetch ttp://www.backports.org/debian/dists/lenny-backports/contrib/i18n/Translation-en_US.gz Could not resolve 'www.backports.org' W: Failed to fetch ttp://www.backports.org/debian/dists/lenny-backports/non-free/i18n/Translation-en_US.gz Could not resolve 'www.backports.org' W: Some index files failed to download, they have been ignored, or old ones used instead. W: You may want to run apt-get update to correct these problems I am able to ping ip addresses but not namespaces. Can't seem to figure out the problem. My /etc/resolv.conf file contains nameserver 192.168.1.2 which is my router.

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  • Looking for Kiosk-style / camera store easy photo memory card to CD/DVD burning program for Windows-7 Notebook? For non techie user.

    - by Rob
    I'm looking for a Kiosk-style / camera shop easy photo memory card to CD/DVD burning program? For non technie user. The kind of system you see in a camera shop / store, e.g. in the UK, Jessops and Boots stores. This is for my Dad who is adept at general PC usage as a notebook owner, but would prefer something fairly simple. The task of burning photos to CD/DVD, in their original photo file .jpg form, i.e. NOT as CD or DVD video or slideshow, is what I'm looking for. I'm guessing this might be possible in Picasa, but all the options available might be superfluous and confusing. He could probably learn to use that but thought I would try simpler options first. Looking for something that guides the user through the steps/stages of the process, 'Wizard' style. Any suggestions? Platform: HP Windows 7 Home notebook with CD/DVD burner and SD memory card slot.

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  • Parallelism in .NET – Part 11, Divide and Conquer via Parallel.Invoke

    - by Reed
    Many algorithms are easily written to work via recursion.  For example, most data-oriented tasks where a tree of data must be processed are much more easily handled by starting at the root, and recursively “walking” the tree.  Some algorithms work this way on flat data structures, such as arrays, as well.  This is a form of divide and conquer: an algorithm design which is based around breaking up a set of work recursively, “dividing” the total work in each recursive step, and “conquering” the work when the remaining work is small enough to be solved easily. Recursive algorithms, especially ones based on a form of divide and conquer, are often a very good candidate for parallelization. This is apparent from a common sense standpoint.  Since we’re dividing up the total work in the algorithm, we have an obvious, built-in partitioning scheme.  Once partitioned, the data can be worked upon independently, so there is good, clean isolation of data. Implementing this type of algorithm is fairly simple.  The Parallel class in .NET 4 includes a method suited for this type of operation: Parallel.Invoke.  This method works by taking any number of delegates defined as an Action, and operating them all in parallel.  The method returns when every delegate has completed: Parallel.Invoke( () => { Console.WriteLine("Action 1 executing in thread {0}", Thread.CurrentThread.ManagedThreadId); }, () => { Console.WriteLine("Action 2 executing in thread {0}", Thread.CurrentThread.ManagedThreadId); }, () => { Console.WriteLine("Action 3 executing in thread {0}", Thread.CurrentThread.ManagedThreadId); } ); .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } Running this simple example demonstrates the ease of using this method.  For example, on my system, I get three separate thread IDs when running the above code.  By allowing any number of delegates to be executed directly, concurrently, the Parallel.Invoke method provides us an easy way to parallelize any algorithm based on divide and conquer.  We can divide our work in each step, and execute each task in parallel, recursively. For example, suppose we wanted to implement our own quicksort routine.  The quicksort algorithm can be designed based on divide and conquer.  In each iteration, we pick a pivot point, and use that to partition the total array.  We swap the elements around the pivot, then recursively sort the lists on each side of the pivot.  For example, let’s look at this simple, sequential implementation of quicksort: public static void QuickSort<T>(T[] array) where T : IComparable<T> { QuickSortInternal(array, 0, array.Length - 1); } private static void QuickSortInternal<T>(T[] array, int left, int right) where T : IComparable<T> { if (left >= right) { return; } SwapElements(array, left, (left + right) / 2); int last = left; for (int current = left + 1; current <= right; ++current) { if (array[current].CompareTo(array[left]) < 0) { ++last; SwapElements(array, last, current); } } SwapElements(array, left, last); QuickSortInternal(array, left, last - 1); QuickSortInternal(array, last + 1, right); } static void SwapElements<T>(T[] array, int i, int j) { T temp = array[i]; array[i] = array[j]; array[j] = temp; } Here, we implement the quicksort algorithm in a very common, divide and conquer approach.  Running this against the built-in Array.Sort routine shows that we get the exact same answers (although the framework’s sort routine is slightly faster).  On my system, for example, I can use framework’s sort to sort ten million random doubles in about 7.3s, and this implementation takes about 9.3s on average. Looking at this routine, though, there is a clear opportunity to parallelize.  At the end of QuickSortInternal, we recursively call into QuickSortInternal with each partition of the array after the pivot is chosen.  This can be rewritten to use Parallel.Invoke by simply changing it to: // Code above is unchanged... SwapElements(array, left, last); Parallel.Invoke( () => QuickSortInternal(array, left, last - 1), () => QuickSortInternal(array, last + 1, right) ); } This routine will now run in parallel.  When executing, we now see the CPU usage across all cores spike while it executes.  However, there is a significant problem here – by parallelizing this routine, we took it from an execution time of 9.3s to an execution time of approximately 14 seconds!  We’re using more resources as seen in the CPU usage, but the overall result is a dramatic slowdown in overall processing time. This occurs because parallelization adds overhead.  Each time we split this array, we spawn two new tasks to parallelize this algorithm!  This is far, far too many tasks for our cores to operate upon at a single time.  In effect, we’re “over-parallelizing” this routine.  This is a common problem when working with divide and conquer algorithms, and leads to an important observation: When parallelizing a recursive routine, take special care not to add more tasks than necessary to fully utilize your system. This can be done with a few different approaches, in this case.  Typically, the way to handle this is to stop parallelizing the routine at a certain point, and revert back to the serial approach.  Since the first few recursions will all still be parallelized, our “deeper” recursive tasks will be running in parallel, and can take full advantage of the machine.  This also dramatically reduces the overhead added by parallelizing, since we’re only adding overhead for the first few recursive calls.  There are two basic approaches we can take here.  The first approach would be to look at the total work size, and if it’s smaller than a specific threshold, revert to our serial implementation.  In this case, we could just check right-left, and if it’s under a threshold, call the methods directly instead of using Parallel.Invoke. The second approach is to track how “deep” in the “tree” we are currently at, and if we are below some number of levels, stop parallelizing.  This approach is a more general-purpose approach, since it works on routines which parse trees as well as routines working off of a single array, but may not work as well if a poor partitioning strategy is chosen or the tree is not balanced evenly. This can be written very easily.  If we pass a maxDepth parameter into our internal routine, we can restrict the amount of times we parallelize by changing the recursive call to: // Code above is unchanged... SwapElements(array, left, last); if (maxDepth < 1) { QuickSortInternal(array, left, last - 1, maxDepth); QuickSortInternal(array, last + 1, right, maxDepth); } else { --maxDepth; Parallel.Invoke( () => QuickSortInternal(array, left, last - 1, maxDepth), () => QuickSortInternal(array, last + 1, right, maxDepth)); } We no longer allow this to parallelize indefinitely – only to a specific depth, at which time we revert to a serial implementation.  By starting the routine with a maxDepth equal to Environment.ProcessorCount, we can restrict the total amount of parallel operations significantly, but still provide adequate work for each processing core. With this final change, my timings are much better.  On average, I get the following timings: Framework via Array.Sort: 7.3 seconds Serial Quicksort Implementation: 9.3 seconds Naive Parallel Implementation: 14 seconds Parallel Implementation Restricting Depth: 4.7 seconds Finally, we are now faster than the framework’s Array.Sort implementation.

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  • What happens to C# 4 optional parameters when compiling against 3.5?

    - by Bertrand Le Roy
    Here’s a method declaration that uses optional parameters: public Path Copy( Path destination, bool overwrite = false, bool recursive = false) Something you may not know is that Visual Studio 2010 will let you compile this against .NET 3.5, with no error or warning. You may be wondering (as I was) how it does that. Well, it takes the easy and rather obvious way of not trying to be too smart and just ignores the optional parameters. So if you’re compiling against 3.5 from Visual Studio 2010, the above code is equivalent to: public Path Copy( Path destination, bool overwrite, bool recursive) The parameters are not optional (no such thing in C# 3), and no overload gets magically created for you. If you’re building a library that is going to have both 3.5 and 4.0 versions, and you want 3.5 users to have reasonable overloads of your methods, you’ll have to provide those yourself, which means that providing a version with optional parameters for the benefit of 4.0 users is not going to provide that much value, except for the ability to provide named parameters out of order. I guess that’s not so bad… Providing all of the following overloads will compile against both 3.5 and 4.0: public Path Copy(Path destination)public Path Copy(Path destination, bool overwrite)public Path Copy( Path destination, bool overwrite = false, bool recursive = false)

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  • Is recursion really bad?

    - by dotneteer
    After my previous post about the stack space, it appears that there is perception from the feedback that recursion is bad and we should avoid deep recursion. After writing a compiler, I know that the modern computer and compiler are complex enough and one cannot automatically assume that a hand crafted code would out-perform the compiler optimization. The only way is to do some prototype to find out. So why recursive code may not perform as well? Compilers place frames on a stack. In additional to arguments and local variables, compiles also need to place frame and program pointers on the frame, resulting in overheads. So why hand-crafted code may not performance as well? The stack used by a compiler is a simpler data structure and can grow and shrink cleanly. To replace recursion with out own stack, our stack is allocated in the heap that is far more complicated to manage. There could be overhead as well if the compiler needs to mark objects for garbage collection. Compiler also needs to worry about the memory fragmentation. Then there is additional complexity: CPUs have registers and multiple levels of cache. Register access is a few times faster than in-CPU cache access and is a few 10s times than on-board memory access. So it is up to the OS and compiler to maximize the use of register and in-CPU cache. For my particular problem, I did an experiment to rewrite my c# version of recursive code with a loop and stack approach. So here are the outcomes of the two approaches:   Recursive call Loop and Stack Lines of code for the algorithm 17 46 Speed Baseline 3% faster Readability Clean Far more complex So at the end, I was able to achieve 3% better performance with other drawbacks. My message is never assuming your sophisticated approach would automatically work out better than a simpler approach with a modern computer and compiler. Gage carefully before committing to a more complex approach.

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  • Is it just me or is this a baffling tech interview question

    - by Matthew Patrick Cashatt
    Background I was just asked in a tech interview to write an algorithm to traverse an "object" (notice the quotes) where A is equal to B and B is equal to C and A is equal to C. That's it. That is all the information I was given. I asked the interviewer what the goal was but apparently there wasn't one, just "traverse" the "object". I don't know about anyone else, but this seems like a silly question to me. I asked again, "am I searching for a value?". Nope. Just "traverse" it. Why would I ever want to endlessly loop through this "object"?? To melt my processor maybe?? The answer according to the interviewer was that I should have written a recursive function. OK, so why not simply ask me to write a recursive function? And who would write a recursive function that never ends? My question: Is this a valid question to the rest of you and, if so, can you provide a hint as to what I might be missing? Perhaps I am thinking too hard about solving real world problems. I have been successfully coding for a long time but this tech interview process makes me feel like I don't know anything. Final Answer: CLOWN TRAVERSAL!!! (See @Matt's answer below) Thanks! Matt

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  • What is wrong with locking non-static fields? What is the correct way to lock a particular instance?

    - by smartcaveman
    Why is it considered bad practice to lock non-static fields? And, if I am not locking non-static fields, then how do I lock an instance method without locking the method on all other instances of the same or derived class? I wrote an example to make my question more clear. public abstract class BaseClass { private readonly object NonStaticLockObject = new object(); private static readonly object StaticLockObject = new object(); protected void DoThreadSafeAction<T>(Action<T> action) where T: BaseClass { var derived = this as T; if(derived == null) { throw new Exception(); } lock(NonStaticLockObject) { action(derived); } } } public class DerivedClass :BaseClass { private readonly Queue<object> _queue; public void Enqueue(object obj) { DoThreadSafeAction<DerivedClass>(x=>x._queue.Enqueue(obj)); } } If I make the lock on the StaticLockObject, then the DoThreadSafeAction method will be locked for all instances of all classes that derive from BaseClass and that is not what I want. I want to make sure that no other threads can call a method on a particular instance of an object while it is locked.

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